Export of leaf litter from forest siopes can affect soil organic matter (SOM) concentrations, soil nutrient pools, and consequently foliar nutrient concentrations in vegetation (Lang and Orndorff, 1984). Mechanisms which retain litter are especially important in maintaining soil fertility in montane forests, where a high proportion of the landscape is on steep slopes. Litter retained on slopes decomposes in situ and contributes to SOM and soil nutrients in these sites. In addition, litter mats protect surface soil from erosional losses of SOM and nutrients. (Cogo et al., 1984). This paper presents that binding of leaf litter by basidiomycetous fungi retards the export of organic matter from forests slopes.
Despite their often dangerous and unpredictable nature, landslides provide fascinating templates for studying how soil organisms, plants and animals respond to such destruction. The emerging field of landslide ecology helps us understand these responses, aiding slope stabilization and restoration and contributing to the progress made in geological approaches to landslide prediction and mitigation. Summarizing the growing body of literature on the ecological consequences of landslides, this book provides a framework for the promotion of ecological tools in predicting, stabilizing, and restoring biodiversity to landslide scars at both local and landscape scales. It explores nutrient cycling; soil development; and how soil organisms disperse, colonize and interact in what is often an inhospitable environment. Recognizing the role that these processes play in providing solutions to the problem of unstable slopes, the authors present ecological approaches as useful, economical and resilient supplements to landslide management.
This volume contains a comprehensive analysis of ecological gradients in the Luquillo Mountains of Puerto Rico. This tropical island setting comprises six ecological life zones and is ideal for studying environmental gradients given dramatic differences in temperature and precipitation that are associated with a rise in elevation from sea level to more than 1000 m over a distance of 10-15 km. Chapters in this volume cover climatic (e.g., precipitation and energy), abiotic (e.g., nutrients, carbon stores soil characteristics and biogeochemistry), and biotic (e.g., microbes, plants, and animal biodiversity) patterns and responses to gradients. These original and synthetic research findings should be of considerable interest to all concerned with understanding the importance of environmental gradients in molding the structure and functioning of ecological systems and to those dedicated to managing or conserving complex tropical ecosystems in light of global change.
Microbial communities respond to multiple abiotic and biotic factors that change along elevation gradients. We compare changes in microbial community composition in soil and review previous research on differential abundance of microbial functional groups along an elevation gradient in eastern Puerto Rico. Previous studies within the Luquillo Mountains showed that activity of methanogenic bacteria increased significantly with elevation, whereas diversity, abundance or activity decreased with elevation in 'slime molds', microbial nitrogen-fixing activity (nitrogenase), and abundance of basidiomycete fungi that degrade lignin in leaf litter. Our results, based on fatty acid (FA) composition and TRFLP analyses from a longer gradient (dry coastal forest to elfin rainforest) produced humped distributions for Shannon diversity of FA, fungal to bacteria (F:B) ratios, fungi, Myxomycetes, G- FA cy19:0 and sulfate reducing bacteria (SRB) 10Me18:0. Soil microbial communities differed significantly among forest types using ANCOVA. TRFLP were more frequently unique to forest types in fungi than bacteria, but we found unique and diverse sulfidogenic and crenarchaeal assemblages in some forest types, with highest diversity in high elevation palm and elfin forests. In multiple linear regression (MLR) models, soil moisture was predictive for all but Actinomycete FA abundance, and forest type contributed significantly to these same models for F:B ratios and all FA fractions except for G- SRB 10Me18:0, and G+ bacteria 15:0. F:B ratio peaked at mid-elevation, then declined with increasing moisture at higher elevation. Since most G- and G+ bacterial FA were positively related to soil pH in MLR models, lower pH in mid-elevation tabonuco forest soil may suppress bacteria and contribute to higher F:B ratios in this forest type.
Global change threatens ecosystems worldwide, and tropical systems with their high diversity and rapid development are of special concern. We can mitigate the impacts of change if we understand how tropical ecosystems respond to disturbance. For tropical forests and streams in Puerto Rico this book describes the impacts of, and recovery from, hurricanes, landslides, floods, droughts, and human disturbances in the Luquillo Mountains of Puerto Rico. These ecosystems recover quickly after natural disturbances, having been shaped over thousands of years by such events. Human disturbance, however, has longer-lasting impacts. Chapters are by authors with many years of experience in Puerto Rico and other tropical areas and cover the history of research in these mountains, a framework for understanding disturbance and response, the environmental setting, the disturbance regime, response to disturbance, biotic mechanisms of response, management implications, and future directions. The text provides a strong perspective on tropical ecosystem dynamics over multiple scales of time and space.
Soil organic carbon (SOC) in a humid subtropical forest in Puerto Rico is higher at ridge locations compared to valleys, and therefore opposite to what is commonly observed in other forested hillslope catenas. To better understand the spatial distribution of SOC in this system, plots previously characterized by topographic position, vegetation type and stand age were related to soil depth and SOC. Additional factors were also investigated, including topographically-related differences in litter dynamics and soil chemistry. To investigate the inﬂuence of litter dynamics, the Century soil organic model was parameterized to simulate the effect of substituting valley species for ridge species. Soil chemical controls on C concentrations were investigated with multiple linear regression models using iron, aluminum and clay variables. Deeper soils were associated with indicators of higher landscape stability (older tabonuco stands established on ridges and slopes), while shallower soils persisted in more disturbed areas (younger non-tabonuco stands in valleys and on slopes). Soil depth alone accounted for 77% of the observed difference in the mean 0 to 60 cm SOC between ridge soils (deeper) and valley soils (shallower). The remaining differences in SOC were due to additional factors that lowered C concentrations at valley locations in the 0 to 10 cm pool. Model simulations showed a slight decrease in SOC when lower litter C:N was substituted for higher litter C:N, but the effects of different woody inputs on SOC were unclear. Multiple linear regression models with ammonium oxalate extractable iron and aluminum, dithionite–citrate-extractable iron and aluminum, and clay contents explained as much as 74% of the variation in C concentrations, and indicated that organo-mineral complexation may be more limited in poorly developed valley soils. Thus, topography both directly and indirectly affects SOC pools through a variety of inter-related processes that are often not quantiﬁed or captured in terrestrial carbon models.
Relationships between litter invertebrate communities, climate, and forest net primary productivity (NPP) were investigated in two microhabitats along an elevational gradient in the Luquillo Experimental Forest. In forest floor litter communities, using palm litter as a control for forest type, although overall NPP declined with increasing elevation and rainfall, animal abundance, biomass and species richness were remarkably similar along the gradient. In non-palm litter, all community parameters declined with increasing elevation, along with NPP and litter nutrient concentra-tions. Nutrient concentrations were higher in palm than non-palm litter, and did not decline significantly along the gradient, and palm litter provided a more stable physical environment than other litter types. Thus, adaptations of invertebrates to the secondary factors of resource base and habitat (i.e. forest type), as seen in palm litter, may be more important in determining community structure than the primary effects of changing climate along the gradient. In structurally more complex bromeliad microcosms, which contain both litter and aquatic detritivore communities, abundance and species richness were independent of each other. Animal abundance declined, along with NPP, with increasing elevation, but species richness and animal biomass peaked at mid-elevation, confirming the monotonic pattern reported in other tropical elevation studies. The mid-elevation palo colorado forest is more architecturally complex, and richer in epiphytes and ground species than the other forests, and its intermediate climatic conditions may be more favourable to invertebrate survival than other forests. Bromeliad community parameters reflect forest characteristics specifically, rather than NPP and climate. The aquatic component of the bromeliad fauna was habitat-specific
The structure and function of forest ecosystems often change along altitudinal gradients in the tropics, culminating in short-stature, low-productivity cloud forests at the uppermost elevations. Field data and literature values were used to examine patterns in nitrogen mineralization on tropical mountains and to discuss the potential for nitrogen limitation to net primary productivity. Few trends in net nitrogen mineralization within and across elevation gradients in the tropics were found, and rates were generally comparable to those found in tropical forests at low elevations. Gross nitrogen mineralization rates were much higher than net rates, and in Puerto Rico upper montane forests exhibited higher gross nitrogen mineralization than lower elevation forests. Work from Puerto Rico found no effect of short-term anaerobic conditions. In Hawai'i gross nitrogen mineralization increased with substrate age. Ammonium availability was augmented by dissimilatory nitrate reduction to ammonium in montane forests; nitrogen conservation via this pathway exceeded losses via N2O production. Patterns in nitrogen circulation in upper montane forests in Puerto Rico showed that elfin and palm forests had lower nitrogen use efficiency and a higher proportion of nitrogen mineralized from decomposing litter relative to other forest types, further indicating that rates of nitrogen supply in these forests are considerable. In summary, the data reviewed in this chapter suggest that nitrogen limitation alone cannot explain patterns in the structure and function of tropical montane forest vegetation. Alternative factors are offered that warrant further investigation.
Appraising the social-ecological processes influencing the inflow, transformation, and storage of materials and energy in urban ecosystems requires scientific attention. This appraisal can provide an important tool for assessing the sustainability of cities. Socio-economic activities are mostly responsible for these fluxes, which are well manifested in the household unit. Human behavior associated with cultural traditions, belief systems, knowledge, and lifestyles are important drivers controlling the transfer of materials throughout the urban environment. Within this context, we explored three aspects of household consumption and waste disposal activities along the Río Piedras Watershed in the San Juan metropolitan area of Puerto Rico. These included: the source of food consumed by residents, recycling activities, and trends in connection to the municipality’s sewerage system. We randomly interviewed 437 households at six sites along the watershed. We also conducted analysis to estimate accessibility to commercial food services for residents in the study areas. Our surveys revealed that nearly all interviewed households (~97 percent) consumed products from supermarkets. In neighborhoods of the upper portion of the watershed, where residential density is low with large areas of vegetative cover, more than 60 percent of residents consumed food items cultivated in their yards. Less than 36 percent of residents in the in densely urbanized parts of the lower portion of the watershed consumed items from their yards. Accessibility to commercial stores for food consumption contrasted among study sites. Recycling activities were mostly carried out by residents in the lower portion of the watershed, with better access to recycling programs provided by the municipality. The surveys also revealed that only 4 to 17 percent of residences in the upper watershed are connected to the sewerage system while the large majority uses septic tanks for septic water disposal. For these residents wastewater from house maintenance is disposed of directly into the environment. In the lower portion of the watershed all residents were connected to the sewerage system. Our study suggests the need to understand human behavioral attitudes in the acquirement and processing of resources, as a tool to generate informed-based strategies promoting sustainable consumption and disposal patterns.
The freshwater fauna (crustaceans, molluscs, fish) of many tropical islands in the Caribbean and Pacific share an amphidromous life-cycle, meaning their larvae need to develop in saline conditions before returning to freshwater as juveniles. This community dominates the freshwaters of much of the tropics, but is poorly known and at risk from development, in particular dam construction. Amphidromy can theoretically lead to dispersal between different freshwater areas, even to distant oceanic islands, via the sea. The extent and scale of this presumed dispersal, however, is largely unknown in the Caribbean. Recent genetic work in Puerto Rico has shown that many fresh-water species have little or no population structure among different river catchments, implying high levels of connectivity within an island, whereas between-island structure is unknown. We used genetic techniques to infer the geographic scales of population structure of amphidromous invertebrates (a gastropod and a number of crustacean species) between distant parts of the Caribbean, in particular Puerto Rico, Panama and Trinidad. We found virtually no geographic population structure across over 2000 km of open sea for these freshwater species. This implies that they are indeed moving between islands in sea currents as larvae, meaning that continued recruit-ment requires a continuum of healthy habitat from the freshwater to marine environment. We further discuss the role of amphidromy and suggest its ecological and biogeographic role may be more important than previously presumed.
Background The Luquillo Mountains have served as a focal point for people since pre-Columbian times. The Taíno Indians in Puerto Rico believed that the spirit of Yuquiyú – god of order, after which El Yunque National Forest (or Luquillo Experimental Forest) is named – dwelled and protected the people from Juracán (the phonetic name given by the Spanish settlers to the god of chaos and disorder believed to control the weather, particularly hurricanes). The chapters in this book illustrate how the Luquillo Mountains continue to serve as a focal point for understanding climate and for using our knowledge to better plan for and ‘protect’ people from climate-induced disturbances in the future. The juxtaposition of a rise in elevation from sea level to nearly 1100 m over the course of 10 km, the tropical climate, and the location of the Luquillo Mountains on the easternmost of the Greater Antilles, exposed to moisture laden trade winds, gives rise to gradients in rainfall, temperature, and seasonality that have structured ecological systems and human use of the landscape. Assessing the patterns and mechanisms of control along gradients that are often interdependent requires thoughtful analyses from many perspectives, rigorous experimental design, and long-term study. The value in understanding these gradients arises because knowledge is the basis for independently predicting responses to changes in rainfall, temperature, seasonality, storm intensity or frequency, and human use of the landscape. Gradient analysis uses spatial measures of physical, chemical, and biological properties of the environment to understand the spatiotemporal dynamics of species, biological communities, biogeochemical fluxes, and ecosystem properties. The study of environmental gradients traces its history to the origins of the discipline of ecology (Clements 1904, Gleason 1917, Shelford 1951, Andrewartha and Birch 1954, Odum 1959). Indeed, the relation between environmental gradients and the distribution of organisms and communities is a favorite topic of ecologists and biogeographers, and papers on the subject abound in the literature. Much of this work focuses on environmental gradients that arise in concert with changes in elevation, such as temperature and rainfall (Whittaker 1967). Thus, studies of montane ecosystems are frequent in the literature on environmental gradients, and tropical mountains, because of their high biodiversity compared to those in temperate latitudes, provide an excellent laboratory for examining ecological responses to environmental change. The chapters in this book take advantage of gradients that exist in the Luquillo Mountains, one of the most intensively studied tropical landscapes in the world (Brokaw, et al. 2012). Moreover, the work described in this book furthers the studies of ecological gradients that have been undertaken in other parts of the tropics (Olson 1963, Terborgh 1977, Patton and Smith 1992, Stevens 1992, Lieberman et al. 1996, Schneider et al. 1999, Givnish 2001, 2002, Smith et al. 2001, Ogden and Thorpe 2002, McCain 2005, Presley et al. 2012 ). The extensive literature on environmental gradients provides a general conceptual framework for studies in the Luquillo Mountains. For example, formulation of an energy-based mechanism tha provided insight into how individuals and species respond to gradients (Hall et al. 1992) stimulated development of a conceptual model for the Luquillo LTER program that uses the concept of gradients to link landscape-scale patterns and processes with disturbance regimes (Willig and Walker 1999, Waide and Willig 2012). In this model, spatial gradients of environmental factors in the landscape are dynamic. Disturbance modifies environmental conditions so that any location in the landscape is subject to a range of environmental conditions over time. The distribution and abundance of individuals and species are determined by the tolerances to this range of conditions. Much of the work reported in this volume has been stimulated by this conceptual model. Yet, the way human actions interact with the environment at all scales (e.g. global warming, urban heat island effect, land use, roads and landslides, and water extraction) make future scenarios difficult to predict. The elevational and climatic gradients within the Luquillo Mountains and northeastern Puerto Rico provide a natural in situ simulation of climate change, as a difference of about 5°C in mean annual temperature and more 3000 mm in mean annual precipitation occur from the top of the Luquillo Mountains to the coast. Thus, the use or exploration of the elevation gradient as a proxy for climate change can be a practical and informative way to study climate change scenarios, or to conduct experiments based on translocation manipulations. Manipulative experiments along elevation gradients could focus on populations, communities or ecosystem processes, and can be designed to anticipate and understand the effects of warming and drying.
We analyzed responses of canopy arthropods on seven representative early and late successional overstory and understory tree species to a canopy trimming experiment designed to separate effects of canopy opening and debris pulse (resulting from hurricane disturbance) in a tropical rainforest ecosystem at the Luquillo Experimental Forest Long-Term Ecological Research (LTER) site in Puerto Rico. We expected that either canopy opening or added debris would result in increased abundances of certain scale insects and other hemipterans, and thereby affect arthropod diversity. Six of thirteen arthropod taxa tested showed significant responses to treatments as main effects or interactions. No taxon responded significantly to trim treatment alone. The red wax scale, Ceroplastes rubens (on Manilkara bidentata), was significantly less abundant in treatments with added debris than in treatments without added debris, and salticid spiders (on Sloanea berteriana) were significantly more abundant in treatments with added debris than in other treatments. Canopy trimming generally did not have a significant effect on assemblage diversity, whereas debris deposition significantly increased diversity on three late successional tree species. A number of significant treatment interactions were observed. Overall, the debris pulse had a greater effect on canopy arthropods than did canopy opening, suggesting that changes in plant condition resulting from nutrient availability associated with debris deposition have a greater effect on canopy arthropods than do the more direct and immediate changes in abiotic conditions resulting from canopy opening.
1. Abiotic variables are critical drivers of succession in most primary seres, but how their inuence on biota changes over time is rarely examined. Landslides provide good model systems for examining abiotic inuences because they are spatially and temporally heterogeneous habitats with distinct abiotic and biotic gradients and post-landslide erosion. 2. In an 18-year study on 6 Puerto Rican landslides, we used structural equation models to interpret the changing effects of abiotic inuences (landslide dimensions, slope, aspect, elevation, parent material and related soil properties) on seed plants (density and diversity), tree fern density, scrambling fern cover, canopy openness and soil development (nitrogen, soil organic matter, pH and cation exchange capacity). 3. Seven years after landslide formation, catchment size (the landslide area above the point of measurement) was the key abiotic factor inuencing plants. The larger the catchment the greater was the diversity and density of seed plants. Conversely, the smaller the catchment the greater was the density of tree ferns and the cover of scrambling ferns. 4. Eighteen years after landslide formation, landslide slope was the key abiotic inuence. The greater the slope, the lower was the density and diversity of seed plants and the greater the scrambling fern cover. 5. Aspect, particularly east-facing slopes exposed to wind disturbances, positively inuenced tree fern densities at both 7 and 18 years and negatively inuenced seed plants and scrambling ferns after 18 years. Soils were least developed, that is, had lowest soil nitrogen and organic matter concentrations, after 18 years on steep slopes (like seed plants) and were most developed near landslide edges, on hurricane-exposed slopes (like tree ferns), and where there were high soil potassium concentrations. 6. Synthesis. Abiotic variables have important inuences on plant succession on landslides and the relative inuence of different abiotic variables changes with time. Improved predictability of temporal dynamics will rely not only on understanding the effects of initial disturbances and subsequent biological responses but also on the different and changing inuences exerted by each abiotic variable.
Spacial and temporal variability in the climate of the Luquillo Mountains of eastern Puerto Rico is influenced by large-scale movements of air masses, extreme events, and regional and global climate change. Because of the long history of the ecosystem research in the Luquillo Mountains, their status as a U.S. Dept of Agriculture (USDA) Experimental Forest, and their role as a source of drinking water for many communities, climate of the Luquillo Moutaines has been a topic of interest for many different public and private entities. Long-term and spatially-diverse records of climate and simulation models suggest that climate is changing in the Luquillo Mountains. Precipitation is decreasing slowly in the lowlands of Puerto Rico and global models suggest that this trend will continue. Annual maximum and minimum temperatures are increasing slowly, and may be affected by accelerating urbanization around the Luquillo Mountains. Cyclonic storms are a major influence on community composition and ecosystem processes, and some studies have suggested trends in intensity and frequency of these storms. Cumulative effects of these changes may include a more pronounced dry season, changes in spatial distribution of species, shifts in the distribution of soil organic carbon, decreases in primary productivity, and increases in extreme rainfall events. Because predictions of the possible effects of climate change bear high levels of uncertainty, future research needs to focus on understanding the direction and magnitude of ecosystem responses to change. A coordinated effort to expand collection of meteorological data and to improve the quality of such data is a fundamental necessity if we are to understand the effect of future climate change on the Luquillo Mountains.
Seen from space, the Luquillo Mountains stand out on the eastern edge of Puerto Rico like a dark-green eye in the midst of a heterogeneous matrix of fields, forests, roads and cities in the surrounding lowlands (Fig. 1). Intercepting clouds, rain, and Saharan dust as these cross the Atlantic and make landfall in the Caribbean, the peaks of the Luquillo Mountains – El Yunque, El Toro, East Peak, and West Peak – capture resources to sustain coupled human and natural systems. The Luquillo Mountains are not tall by global standards, rising only to just over 1000 m in the short distance from the coast to the peaks, but this moderate elevational relief creates gradients that are expressed in the form and functioning of the landscape. Understanding the way in which species and ecosystems respond to gradients of climate and land use intensity is vital to the sustainability of populations, water resources, and ecosystem services. In the Luquillo Mountains these patterns are expressed in the context of the rich biodiversity of the tropics and the complex interplay of land use, hurricanes, and plant and animal responses to resources and competition. The patterns expressed by the distributions of species and the ecosystem functions that they carry out are dynamic. Understanding the nature of gradients, and responses of species to them, helps to better predict responses to future conditions and ultimately to develop and sustain the kinds of landscapes that support societal interests and human wellbeing.
Landslides represent one of the most severe disturbances in montane forests because the main consequence of theiroccurrence is loss or downslope redistribution of the majority of the above- and below-ground biomass. We examinedamong-landslide gradients (size, slope, aspect, age, elevation) on 142 landslides in the Luquillo Mountains, PuertoRico, created by three storms in 2003–2004. We also examined within-landslide gradients (top to bottom, edge tocenter, successional development) by reviewing 20 yr of landslide data in the Luquillo Mountains. Landslide abun-dance and plant successional patterns do not closely reflect the elevation gradient that is characteristic of this moun-tain range, unlike many abiotic and biotic factors that do. Numerous physical gradients resulting from landslides,including soil nutrients, slope, age, and distance to edges and the base of a landslide, strongly influence colonization,growth, and survival of vegetation in the Luquillo Mountains. However, some gradients appear more pronouncedthan others, and the influence of each gradient on landslide recovery likely depends on both biotic responses to thenet effects of multiple, overlapping interactions among gradients (e.g. soil slope and fertility) and the temporal andspatial scale at which attributes are measured. Therefore, even when the many gradients that influence plant coloniza-tion and landslide development are known, accurate predictions of species composition and time to forest recoveryremain challenging.
Soil properties, carbon and nutrient distribution were studied along an elevation gradient covering eight distinctforest types in eastern Puerto Rico. The dominant soil forming parent materials are granodiorite saprolite and vol-canic rocks which exert a strong influence on the soil properties. Soils on middle to high elevations are very stronglyacidic with low base saturation (<20%) due to a strong leaching environments. Soils formed in lowlands, dry forestand coastal wetlands are moderately to weakly acidic with higher base saturation (>40%). Generally, organic carbon(OC), nitrogen (N) and nutrients accumulated in the surface horizons and decrease with depth. Soils formed in areasinfluenced by landslides or alluvium were the exception to this pattern. Soil OC and N stores follow the elevationgradient: from 26.7 kg OC m–2and 1.4 kg N m–2in soils of the colder and wetter mountain tops to 12.3 kg OCm–2and 1.0 kg N m–2in soils of the lower elevation dry forests. The C:N ratio decreased from 20 to 11 as elevationdecreased along the gradient. Landscape movement on uplands through landslides, slumps and fluvial/alluvial proc-esses has a significant effect on variation of C stores which must be considered when estimating C stores by landscapeunits. The coastal wetlands have exceptionally high OC stores (>90 kg m–2) due to their water-saturated and reducingenvironment. Soil OC content showed an inverse relation with soil bulk density and played a controlling role oncation exchange capacity (CEC) and nutrient distribution. Clay content has no effect on CEC. Elevation through itsinfluence on precipitation and temperature exerts strong influence in the quantity and quality of terrestrial OC stores,and the depth-distribution pattern of C, N and other nutrients.
We measured the ion composition of rainfall collected in stations located at different elevations in northeastern Puerto Rico to explore relationships between rainfall volume and distribution, and influence of marine aerosols on ion composition. Rainfall collected during the last two weeks of each month between February and December 2009 at different altitudes from sea level to 1000 m a.s.l., was analyzed by ion chromatography for Cl–, SO42–, Br–, NO3–, Na+, K+, Mg2+, Ca2+, and NH4+. Electrical conductivity was inversely related to rainfall volume following a power relationship. Conductivity varied little for rainfall above 100 mm. Upper montane stations showed smaller conductivity and variability than lowland stations. Na+ and Cl– accounted for the largest ion fraction in all rainfall stations, and were highly correlated indicating strong influence of marine aerosols. Cl–/Na+ and SO42–/Na+ ratios in rain water did not vary significantly with altitude, but other ion/Na+ ratios showed significantly higher values in rainfall stations located below 600 m a.s.l., indicating contributions of different aerosol sources. All stations had several times larger K+/Na+, Ca2+/Na+, and SO42–/Na+ ratios (μeq l–1), than the marine reference value, a result requiring confirmation with continued measurements. Comparison with previous studies in the Luquillo Experimental Forest showed that volume-weighted ion concentrations measured in the lower montane stations during 2009 (200–500 m a.s.l.) were similar to those of Bisley watershed and higher than those of El Verde station. Ion concentrations measured in rainfall from seven stations at and above the average cloud formation level (600 m a.s.l.) are much lower than those reported by a previous report from Pico del Este. Results emphasize the need to continue routine analysis of rainfall chemistry as a mean to monitor changes in element supply to terrestrial ecosystems.
The maximum power principle predicts that maximum transformation of available energy into useful work occurs when a system operates at an intermediate rate and efficiency. This relation is apparent in everyday situations as we shift gears to keep near the middle of each gear range when we accelerate an automobile, or operate chain saws and other machines at a load about half their stalling rate. We tested the validity of the maximum power principle in a complex natural system by quantifying patterns of photosynthesis and respiration – an ecosystem’s energy currency – along an elevation gradient in a subtropical forest of Puerto Rico. This mountain system was a useful proxy for testing the hypothesis over broader climatic gradients elsewhere. Our results indicate that metabolic rates (defined as gross primary productivity) decrease up the gradient, efficiency (defined as the ratio of net to gross primary productivity) increases up this gradient, and power (defined as net primary productivity, or the amount of useful energy produced within a given ecosystem per unit time) is maximum near the midpoint of the gradient where rate and efficiency are intermediate. These observations are non-trivially consistent with the maximum power principle and support a scalable, energy-based definition of evolutionary fitness. Given a set of environmental forcing functions in a given location, those individuals (or populations or ecosystems) that optimize the trade-off between metabolic rate and efficiency to achieve maximum power will be most fit. As environmental conditions change over the long term, this rate vs efficiency optimum will shift and those that are able to achieve maximum power in the new environment will be favored over those that are maximizing power for the old environment. We think that this net energy-as-fitness view allows for a richer series of possibilities for testing the consequences of natural selection.
To examine whether stream nitrogen concentrations in forested reference catchments have changed over time and if patterns were consistent across the USA, we synthesized up to 44 yr of data collected from 22 catchments at seven USDA Forest Service Experimental Forests. Trends in stream nitrogen presented high spatial variability both among catchments at a site and among sites across the USA. We found both increasing and decreasing trends in monthly flow-weighted stream nitrate and ammonium concentrations. At a subset of the catchments, we found that the length and period of analysis influenced whether trends were positive, negative or non-significant. Trends also differed among neighboring catchments within several Experimental Forests, suggesting the importance of catchment-specific factors in determining nutrient exports. Over the longest time periods, trends were more consistent among catchments within sites, although there are fewer long-term records for analysis. These findings highlight the critical value of long-term, uninterrupted stream chemistry monitoring at a network of sites across the USA to elucidate patterns of change in nutrient concentrations at minimally disturbed forested sites.
A transition to a gradient-based systems approach to scientific research in the Luquillo Experimental Forest (LEF) has yielded a new ability to understand the patterns of variability in abiotic conditions and the resultant structure and function of the LEF ecosystems. The location and topography of the Luquillo Mountains result in strong elevational gradients of temperature, soil moisture, microtopography, solar radiation and other physical characterizations. These gradients offer an exceptional opportunity to understand the response of species to environmental conditions across spatial and temporal scales through the use of spatial simulation models. We know now that the four forest type classification does not capture the biotic reality of the gradual transitions of vegetation and environmental gradients with elevation. We review the evolution of the increasing use of gradients in research in the LEF, and provide evidence that ecosystem structure and function are responding to gradients in environmental conditions based on a series of empirical studies and spatial simulation models of structure and function. Based on these studies, we believe that the gradients perspective is an appropriate conceptual framework for investigating the biota of this ecosystem and provides a strong basis for understanding their relation with the physical environment of the LEF over the long term.
Woody debris is an ecologically important component of forests as well as a potentially large contributor to the carbonpool of forested terrestrial ecosystems. We characterized coarse woody debris, fine woody debris, litter, and duff bio-mass at 24 sites along an elevation gradient in northeastern Puerto Rico. These sites are representative of eight matureforest types that include Elfin woodland, Sierra palmPrestoea montana, Palo ColoradoCyrilla racemosa, TabonucoDacryodes excelsa, lowland moist, lowland dry, fresh waterPterocarpusswamps, and flooded mangrove forests. Weexpected the amount and composition of both woody debris and forest floor components to vary by forest type. Wehypothesized mid- to upper-elevation forests, exhibiting the greatest basal area and amount of aboveground biomass,would have the greatest amounts of woody debris. In addition we expected the fine woody debris (wood < 7.60 cmdiameter), litter, and duff fractions to be an important source of organic matter in some forest types, representinga significant percentage of total woody debris. We found significant differences in mean total woody debris, coarsewoody debris, and fine woody debris among forest types along the elevation gradient. The mean total woody debriswas significantly greater in the Palo Colorado forest, than the low-elevation Dry (14.79 Mg ha–1) and highest eleva-tion Elfin (17.38 Mg ha–1) forests, with the other forest types containing intermediate amounts of woody debris.In addition, the total fine woody debris fraction was an important component of total carbon storage, representing22–56% of total carbon stored in each forest.
Urbanization through the addition of impervious cover can alter catchment hydrology, often resulting in increased peak flows during floods. This phenomenon and the resulting impact on stream channel morphology is well documented in temperate climatic regions, but not well documented in the humid tropics where urbanization is rapidly occurring. This study investigates the long-term effects of urbanization on channel morphology in the humid sub-tropical region of Puerto Rico, an area characterized by frequent high-magnitude flows, and steep coarse-grained rivers. Grain size, low-flow channel roughness, and the hydraulic geometry of streams across a land-use gradient that ranges from pristine forest to high density urbanized catchments are compared. In areas that have been urbanized for several decades changes in channel features were measurable, but were smaller than those reported for comparable temperate streams. Decades of development has resulted in increased fine sediment and anthropogenic debris in urbanized catchments. Materials of anthropogenic origin comprise an average of 6% of the bed material in streams with catchments with 15% or greater impervious cover. At-a-station hydraulic geometry shows that velocity makes up a larger component of discharge for rural channels, while depth contributes a larger component of discharge in urban catchments. The average bank-full cross-sectional area of urbanized reaches was 1.5 times larger than comparable forested reaches, and less than the world average increase of 2.5. On average, stream width at bank-full height did not change with urbanization while the world average increase is 1.5 times. Overall, this study indicates that the morphologic changes that occur in response to urban runoff are less in channels that are already subject to frequent large magnitude storms. Furthermore, this study suggests that developing regions in the humid tropics shouldn't rely on temperate analogues to determine the magnitude of impact of urbanization on stream morphology. Copyright © 2012 John Wiley & Sons, Ltd.
How best to define and quantify plant communities was investigated using long-term plot data sampled from a recovering pasture in Puerto Rico and abandoned sugarcane and banana plantations in Ecuador. Significant positive associations between pairs of old field species were first computed and then clustered together into larger and larger species groups. I found that (1) no pasture or plantation had more than 5% of the possible significant positive associations, (2) clustering metrics showed groups of species participating in similar clusters among the five pasture/plantations over a gradient of decreasing association strength, and (3) there was evidence for repeatable communities-especially after banana cultivation-suggesting that past crops not only persist after abandonment but also form significant associations with invading plants. I then showed how the clustering hierarchy could be used to decide if any two pasture/plantation plots were in the same community, that is, to define old field communities. Finally, I suggested a similar procedure could be used for any plant community where the mechanisms and tolerances of species form the "cohesion" that produces clustering, making plant communities different than random assemblages of species.
The degree to which turnover in biological communities is structured by deterministic or stochastic factors and the identities of inﬂuential deterministic factors are fundamental, yet unresolved, questions in ecology. Answers to these questions are particularly important for projecting the fate of forests with diverse disturbance histories worldwide. To uncover the processes governing turnover we use species-level molecular phylogenies and functional trait data sets for two long-term tropical forest plots with contrasting disturbance histories: one forest is older-growth, and one was recently disturbed. Having both phylogenetic and functional information further allows us to parse out the deterministic inﬂuences of different ecological ﬁlters. With the use of null models we ﬁnd that compositional turnover was random with respect to phylogeny on average, but highly nonrandom with respect to measured functional traits. Furthermore, as predicted by a deterministic assembly process, the older-growth and disturbed forests were characterized by less than and greater than expected functional turnover, respectively. These results suggest that the abiotic environment, which changes due to succession in the disturbed forest, strongly governs the temporal dynamics of disturbed and undisturbed tropical forests. Predicting future changes in the composition of disturbed and undisturbed forests may therefore be tractable when using a functional-trait-based approach. Also access article in: http://www.esajournals.org/doi/pdf/10.1890/11-1180.1
Litterfall traps could preferentially represent certain kinds of leaf litter. Several factors may cause bias while sampling litterfall leading to over- or under-representation of the species present in the surrounding vegetation. For example, species standing precisely above litterfall traps, having big and wide crowns, and/or with high leaf fall rate may be over-represented in litterfall samples. Additionally, species standing upslope or in the windward side of litterfall traps may be more likely to be collected in litterfall traps (Staelens et al., 2003). Conversely, species with big and/or heavy leaves or fronds such as palms or species from the Cecropia and Heliconia genera may be under-represented in litterfall traps (Clark et al., 2001). However, the few studies dealing with patterns of litterfall dispersal and collection have found contradictory results. For example, in Australian rainforests Lowman (1988) found that collected litterfall was not necessarily biased toward leaves coming from trees located precisely above traps. Similarly, in a dry forest in Costa Rica, Burnham (1997) found a low spatial correspondence between location of source stems and litterfall samples. In contrast, for a temperate mixed forest in northeastern Japan, Hirabuki (1991) found that estimated patterns of litterfall spatial distribution corresponded to the distribution of stems in the studied plot. In this chapter we report results from a study that takes advantage of an ongoing experiment in the Luquillo Experimental Forest, Puerto Rico, to examine the correspondence between litterfall samples and standing vegetation. Such correspondence was analyzed at three different spatial scales defined by the sampling units already in place: forest stand (106 m2), sampling blocks (4x104 m2), and plots (4x102 m2). Our first objective was to examine which factors, in addition to relative abundance of species in the vegetation, could affect the relative abundance of species in litterfall samples. Specifically, we evaluated the effect of tree size (measured as height and crown area), leaf size (measured as leaf area), and distance to litter traps using a stepwise regression procedure. We hypothesized that bigger trees (i.e., having high height and crown area) would produce more leaf litter and therefore would tend to occur more abundantly in litterfall samples; while trees with relatively big leaves would be in general under-estimated in litterfall samples because traps would fail to catch those leaves. Finally, if traps were capturing leaves from trees standing precisely above, then trees being closer to litter traps would tend to present higher relative abundances in litterfall samples. Additionally, we analyzed the similarity between litterfall and particular sub-sets of the whole vegetation community. Sub-sets were defined by tree height, crown area, and distance to traps, such that if litter traps were preferentially collecting leaves from any particular sub-set of the vegetation (e.g., bias toward either canopy or understory trees, wide-crowned trees, or trees located closer to traps), those sub-sets should bear a higher compositional similarity with the litterfall samples than the whole vegetation community. The particular experimental set up used in this study (cf., Fig. 1), allowed us also to ask if litter traps located in the center of vegetation plots (i.e., surrounding plots. See Fig. 1) provided more representative samples of the surrounding vegetation than traps located adjacent to vegetation plots (i.e., adjacent plots. See Fig. 1). To address this second objective, we compared the composition and relative abundance of species collected in litter traps with the same parameters of the vegetation from the surrounding and adjacent plots (Fig. 1), using similarity indexes and parametric and non-parametric correlations. We hypothesized that if litter traps were collecting litterfall coming from all directions with the same likelihood, a higher similarity between litterfall samples and vegetation would be found for surrounding than for adjacent plots, both for the scale of the forest as for the scale of individual plots and for particular species. Finally, our third objective was to gain insights for the scaling of litterfall data from the level of sampling plots up to the level of the forest stand. We addressed this by comparing the similarity between vegetation and litterfall across the three different scales mentioned before (i.e., plots, blocks and the forest stand; cf. Fig. 1) using similarity indexes, correlations, multivariate ordinations, and Mantel tests. An important aspect when examining the correspondence between litterfall and vegetation across different spatial scales is related to whether litter traps are capturing leaves from a wide range or only from the near vicinity around traps. On one hand, considering the potential far-ranged and random patterns of leaf dispersal (Jonard et al., 2006), a high compositional similarity between litterfall and vegetation at the scale of the forest type together with a low similarity at the smaller scales of sampling units might be expected. On the other hand, if litter traps are collecting leaf litter mainly from the vegetation in the near vicinity (for example, 10 m around traps), a high similarity between litterfall and vegetation at the scale of sampling units should be encountered as well. Particularly, the following outcomes could be expected: 1) high correlation between litterfall and vegetation dissimilarity matrices calculated for the smallest sampling units (i.e., plots), namely, pairs of plots with high dissimilarity in their vegetation should be also highly dissimilar in their litterfall; 2) litterfall and vegetation samples from the same plots should cluster together in an ordination space accurately representing compositional distances among sampling units; and 3) strong correlation between similarity among pairs of litterfall samples and the physical distance separating those samples (i.e., distance among plots), namely, the more distant the plots were located, the higher the dissimilarity between them would be. Litterfall collection using litter traps has become a ubiquitous method in terrestrial ecology. Thus it is important to understand the relevant variables behind the method and the implications of its limitations. We believe our findings will prove instrumental for the improvement of methods in terrestrial and forest ecology especially in the tropics were the high species diversity and structural complexity of forests impose tough challenges to the study of forest structure and dynamics.
Key Points • The abundance and distribution of organisms and the attendant ecosystem processes vary across the landscape of the Luquillo Mountains in relation to underlying patterns of spatial heterogeneity and gradients of environmental factors. • The ecosystems of the Luquillo Mountains are affected by frequent climate- induced disturbances such as treefalls, landslides, tropical storms, and droughts, as well as by human-induced disturbances associated with land use (i.e., agriculture and forest harvest). • The term “ecological space” refers to multivariate dimensions defined by a suite of environmental characteristics. Disturbances can disrupt or create gradients by altering the mapping of ecological characteristics onto geo- graphic space. • Because the relationship between geographic space and ecological space is dynamic, the relationship between the physical template and the distribution and abundance of animal, plant, and microbial species cannot be understood without reference to the disturbance regime. • The resilience of an ecosystem to anthropogenic disturbances might be low because such disturbances often produce severe modifications to the environ- ment, creating novel combinations of environmental characteristics that are outside of the ecological space that was characteristic of the site or which are characterized by the absence of biological residuals.
Millipedes are considered to be important organisms involved in decomposition, both for their direct feeding on detritus and their indirect effects on microbial activity. Hanlon (1981a, 1981b) suggested that fragmentation of leaf litter by soil fauna increases microbial biomass by increasing leaf surface area and diminishing pore sizes. The passage of litter through the gut of macroarthropods, such as millipedes, can help in the establishment of soil bacteria (Anderson & Bignell, 1980; Hanlon, 1981a, 1981b; Tajovsky et al., 1991; Maraun & Scheu, 1996). The presence of millipedes has been shown to increase the decomposition of litter as well as increase growth of seedlings (Cárcamo et al., 2001). In a beech forest, Bonkowski et al. (1998) also found that the presence of millipedes significantly increased the decomposition of litter, much more so than endogeic earthworms. The presence of millipedes has also been found to greatly increase the release of litter nutrients into the soil, especially calcium and nitrates (Pramanik et al., 2001). Millipedes are selective about what leaves they eat (Lyford, 1943; Kheirallah, 1979; Cárcamo et al., 2000). The chemical composition of leaf litter, especially the lignin and nitrogen content, can greatly affect soil fauna populations, although this effect is not clear for millipedes (Tian et al., 1993). Van der Drift (1975) estimated that in temperate areas millipedes are responsible for ingesting 5–10 percent of the annual leaf litter fall and Cárcamo et al. (2000) estimated that a single species of millipede consumed 36 percent of the annual leaf litter in a British Columbian Cedar-Hemlock forest. Tropical studies have also found a large influence of millipedes on decomposition (Tian et al., 1995). In a Tabonuco forest in Puerto Rico, Ruan et al. (2005) found that millipede density explained 40 percent of the variance in leaf litter decomposition rates, while soil microbial biomass explained only 19 percent of the variance.Millipedes make up a large part of the arthropod community on the forest floor in the Tabonuco forests of Puerto Rico. Richardson et al. (2005, pers. com.) found that diplopods in El Verde (a Tabonuco forest) constituted about 11.4 percent (73.09 mg dry/m2) of the microarthropod biomass, second only to Isoptera. In the same forest, we found that Stemmiulidae were the most abundant millipede with a density of ca. 22 individuals/m2 (Murphy et al., 2008). In this study, we use a microcosm approach to answer the direct (leaf fragmentation) and indirect (microbial biomass) effects of millipedes on the decomposition of leaf litter and how these outcomes are influenced by the substrate (litter) quality and the density of millipedes. We expect that higher the litter quality (lower lignin content) and the higher density of millipedes would result in more leaf area lost, decreased leaf mass remaining, and higher biomass of soil microbes. We used microcosms containing one of three litter species with varying lignin to nitrogen (L/N) ratios and three different densities of millipedes. Access article in: http://www.fs.fed.us/global/iitf/pubs/bc_iitf_2012_Gonzalez001.pdf
Key Points • The Luquillo Mountains are affected by a wide array of environmental processes and disturbances. Events that concurrently alter the environmental space of several different areas of the Luquillo Mountains occur every 2 to 5 years. Events such as hurricanes that cause widespread environmental modification occur once every 20 to 60 years. • The most common disturbance-generating weather systems that affect the Luquillo Mountains are (1) cyclonic systems, (2) noncyclonic intertropical systems, (3) extratropical frontal systems, and (4) large-scale coupled ocean- atmospheric events (e.g., North Atlantic Oscillation, El Niño-Southern Oscilla- tion). Unlike some tropical forests, disturbances associated with the passage of the Inter-Tropical Convergence Zone or monsoonal rains do not occur. • Hurricanes are considered the most important natural disturbance affecting the structure of forests in the Luquillo Mountains. Compared to other humid tropical forests, Luquillo has a high rate of canopy turnover caused by hurricanes but a relatively low rate caused by tree-fall gaps. Historically, pathogenic disturbances have not been uncommon. • Human-induced disturbances have historically included tree harvesting for timber and charcoal, agriculture, and agroforestry. In the past few decades, water diversions, fishing and hunting, and road building have been important disturbances. Present and future human-induced disturbances are related to regional land use change, the disruption of migratory corridors, and forest drying related to coastal plain deforestation and regional climate change. • Hurricane-related storm discharges can cause significant geomorphic modifications to Luquillo stream channels, and stream water concentrations of sediments and nutrients can be elevated for months to years following a major hurricane. However, the largest floods are not necessarily associated with hurricanes, and the annual peak discharge can occur in any month of the year but is most common in the late summer and fall. • Over the entire island of Puerto Rico, 1.2 landslide-producing storms occur each year. In the Luquillo Mountains, landslides are typically covered with herbaceous vegetation within 2 years, have closed canopies of woody vegetation in less than 20 years, and have aboveground biomass of the adjacent forest after several decades.
Key Points - The ecosystems of the Luquillo Mountains are representative of large areas of the frost-free tropical world, particularly those with high rainfall, periodic hurricane disturbances, a maritime climate, and insularity. - The natural history of the Luquillo Mountains spans over 30 million years, whereas human presence has been an influence over the past 2,200 years. - Indigenous peoples, Spanish conquistadors, and a steady stream of 20th and 21st century scientists have observed, studied, and experimented with the ecosystems of the Luquillo Mountains, and in the process they have left a legacy of ideas and heuristic models concerning ecosystem organization and function. The Luquillo Long-Term Ecological Research (LTER) program is rooted in this legacy. - Important contributions to tropical science made by the Luquillo LTER program are a systematic investigation of disturbance and the identification of a number of mechanisms that contribute to the resistance and resilience of forested ecosystems. - The LTER program has also contributed to a basic understanding of the ecology and biogeochemistry of the Luquillo Mountains and to an under- standing of the long-term consequences of human activity on populations, communities, and ecosystem function. - This book focuses on the response of the ecosystems of the Luquillo Moun- tains to natural and anthropogenic disturbances, with a particular focus on hurricanes and land cover change. Also access article in: http://www.fs.fed.us/global/iitf/pubs/bc_iitf_2012_lugo002x.pdf
Ecosystem services science has developed at a fast rate in Latin America, a region characterized by a high biological and cultural diversity, strong emphasis in foreign investment, and high socioeconomic inequities. Here we conducted the following analyses at the regional and national scales: (1) how and when did the study of ecosystem services arise in each country?, (2) what is our present understanding of ecosystem service supply, delivery to societies, and social and economic values?, (3) what is the state of the art in integrating tradeoffs among services and in using interdisciplinary perspectives?, and (4) how has ecosystem service research been connected to policy design or management for sustainability? A large literature review (>1000 references) showed that in Latin America ES supply and links to policy have been the most frequently assessed. Overall, emphasis has been placed on a few services, namely carbon and water. Payments for ecosystem services have received considerable attention in the region, though with strong differences across nations and with important limitations in their application. The future of the ecosystem service paradigm in Latin America will largely depend on its capacity to demonstrate effectiveness in meeting both conservation and development goals.
Key Points • The Luquillo Mountains in northeastern Puerto Rico are geologically dynamic, with recurrent hurricanes, landslides, and earthquakes. • Puerto Rico has never been physically connected to continents by land bridges, which, together with the island’s long distance from North and South America, contributes to its relatively low numbers of native plant and animal species for a tropical location and its high rate of endemism. • The climate is warm, wet, and relatively aseasonal but shows strong gradi- ents with elevation. • Soils are deep and highly weathered, with carbon and nutrient concentrations and standing stocks similar to those in many other tropical forests. Soils contain much to most of the available nutrients and total carbon, but plant biomass is a particularly important pool of potassium. • Nutrient inputs in precipitation are dominated by marine aerosols; these aerosols and rapid weathering contribute to a substantial export of base cations in streams. • Nitrogen budgets are unbalanced at the watershed scale, suggesting that significant amounts of N fixation are occurring. • The Luquillo Mountains contain many types of forest, but four are common and particularly well studied: tabonuco, colorado, palm, and elfin • Aboveground net primary productivity is high, as it is in many other tropical sites, and aboveground biomass, productivity, and forest stature decrease with elevation. • Large mammalian herbivores and predators are absent; lizards, frogs, snakes, and a few birds are the top terrestrial predators. • Stream and river food webs are dominated by freshwater shrimp and fish species that migrate to the estuary; nonmigratory freshwater crabs are also important, but aquatic insects are neither diverse nor abundant. • Leaf litter decomposition is rapid in both the forest and streams, and detrital pathways provide a major energy source to higher trophic levels. Also access article in: http://www.fs.fed.us/global/iitf/pubs/bc_iitf_2012_McDowell001.pdf
Topographically complex islands present opportunities for in situ (within-island) allopatric speciation because of increased chances for isolation in separate mountain ranges, as well as greater opportunity for fragmentation by high sea levels and climate-driven changes in habitat distribution. Climatic oscillations of the Quaternary (Pleistocene – Holocene; ~2.5 million years ago to the present) may have influenced the severity of vicariant barriers among and within islands, yet how these events influenced evolution of tropical insular biota is not well understood. This dissertation explores the role of topographic complexity and climate-driven range shifts resulting from sea-level changes and habitat suitability in shaping genetic diversity of two Eleutherodactylus frogs (Anura: Eleutherodactylidae) in the Puerto Rican Bank, an archipelago in the eastern Caribbean Sea. Sea level changes significantly altered the size, area, and degree of isolation of terrestrial habitats in the Puerto Rican Bank, and habitat shifts may have occurred in the main island of Puerto Rico. Whereas the Mountain Coquí, E. portoricensis, is restricted to cool and moist understory montane forest habitat in Puerto Rico, the Red-eyed Coquí, E. antillensis, is a habitat generalist with a broad elevational distribution on most of the larger islands of the Puerto Rican Bank. Hypotheses of population history were formulated using data from paleoenvironmental records and ecological niche models, and tested using a suite of population genetic, phylogenetic, and coalescent analyses of DNA sequence data. I show how basin barriers and Quaternary climatic fluctuations shaped the distribution of genetic diversity in E. portoricensis in the Luquillo and Cayey Mountains in eastern Puerto Rico, and how varying degrees of terrestrial connectivity and isolation influenced the persistence and colonization dynamics of E. antillensis across the Puerto Rican Bank. To infer whether climate-driven, historical shifts in distributions occurred in E. portoricensis and E. antillensis, this dissertation also compares patterns of genetic isolation and demography of these species in Puerto Rico, where elevational gradients may have accommodated range shifts during climatic extremes of the Quaternary. The collective findings of this dissertation improve our understanding of topographic and historic factors that promote population divergence and that ultimately produce regional patterns of biodiversity in tropical archipelagos.
Disturbances are abrupt events that dramatically alter habitat conditions and resource distribution for populations and communities. Terrestrial landscapes are subject to various disturbance events that create a matrix of patches with different histories of disturbance and recovery. Species tolerances to extreme conditions during disturbance or to altered habitat or resource conditions following disturbances determine responses to disturbance. Intolerant populations may become locally extinct, whereas other species respond positively to the creation of new habitat or resource conditions. Local extinction represents a challenge for conservation biologists. On the other hand, outbreaks of herbivorous species often are triggered by abundant or stressed hosts and relaxation of predation following disturbances. These insect responses can cause further changes in ecosystem conditions and predispose communities to future disturbances. Improved understanding of insect responses to disturbance will improve prediction of population and community dynamics, as well as ecosystem and global changes.
In heterotrophic streams the retention and export of coarse particulate organic matter and associated elements are fundamental biogeochemical processes that influence water quality, food webs and the structural complexity of forested headwater streams. Nevertheless, few studies have documented the quantity and quality of exported organic matter over multiple years and under a range of conditions that includes both droughts and hurricanes. This study quantifies the export of coarse particulate organic matter (CPOM, > 12.7 mm), over 18 y in two headwater streams in north-east Puerto Rico. Daily exports ranged from 0 to over 170 g ha−1 d−1 and averaged 7.39 g ha−1 d−1,with similar amounts coming from leaves (3.5 g ha−1 d−1) and wood (3.2 g ha−1 d−1). Export of coarse particulate organic carbon was 3.0 g ha−1 d−1 which constitutes only 1.32% of carbon exports.Most litter falling into the streams was processed in place as only 2.3% of the leaf litter falling directly into these perennial channels was exported as CPOM. On average, 6 wk y−1 had no exports while events transporting more than 10 g ha−1 d−1 occurred every 2.8 mo. Instead of a single annual pulse as observed in deciduous systems, there were annual peaks in CPOM exports during May and September and less export during the drier period from December to February. Ratios of C:N in the exportedmaterial were highest in the driest month and lowest during rainymonths, while leaf fluxes for nitrogen, phosphorus and calcium were highest in rainy months and lowest during February. Although median daily exports and exports during low- and base-flow periods were similar before and after Hugo, after 16 y exports during moderateand high-flow periods were still less than those in the 2 y prior to the hurricane. Our observations indicate a system with high rates of internal processing that quickly returns to median daily conditions following hurricanes but requires several decades for storm-flow exports to return to pre-disturbance conditions and indicates that the long-term pattern of CPOM export is associated with the level of maturity of watershed vegetation. Also access article in: http://www.fs.fed.us/global/iitf/pubs/ja_iitf_2012_Heartsill001.pdf
Key Points • The biota responds to disturbance and, equally important, influences the frequency, magnitude, and intensity of disturbances. • Environmental gradients provide a context for contrasting the roles of particular species with respect to resilience and resistance during the interplay between disturbance and succession. • Disturbance increases the complexity of interactions (i.e., macro- and microclimatic, biogeochemical, biotic) that control the flow of energy and cycling of materials through ecosystems. • Soil microorganisms, as well as the timing, quantity, and quality of litter deposition, play a critical role in affecting the dynamics of carbon and nutrient cycling over short and long temporal scales. • Disturbance affects the life history and demographic parameters of species at fine spatial scales and creates a mosaic of patches at large spatial scales, which together influence the dispersal of individuals among patches (i.e., the degree of connectedness) in a species-specific fashion. Such a cross-scale perspective provides a spatially explicit metacommunity framework for understanding the assembly of species in disturbance-mediated environments. • Differences in biodiversity affect ecosystem processes through species complementarity, organismal traits, and trophic interactions. These effects are mediated by scale and ultimately determine the resistance and resilience of ecosystems to disturbance. • The effects of multiple disturbances on riparian and stream communities have complex spatial and temporal linkages. Life histories of species that connect freshwater and marine communities with those in headwater tributaries and riparian forests provide pathways for pulsed flows of energy and materials. • Anthropogenic disturbance facilitates invasions by introduced tree species, sometimes culminating in the emergence of new forest communities domi- nated by introduced taxa. The development of these new emerging forests does not necessarily result in the loss of native species or a reduction in species richness. • In mature forests not subject to intense anthropogenic degradation, intro- duced species can occur sporadically as rare species in hurricane-induced gaps, but these populations rapidly decrease in numbers after canopy closure. • The recognition and study of emerging new forests is important for devel- oping an ecological understanding of how organisms respond to anthropo- genic disturbances, including global climate change. • Forecasting environmental change requires the integration of biophysical and social science perspectives. We outline an approach for developing an integrated social-ecological system for the Luquillo Experimental Forest of Puerto Rico. Also access article in: http://www.fs.fed.us/global/iitf/pubs/bc_iitf_2012_Willig001.pdf
Key Points • Uses and conservation of tropical forests reflect the economic and social circumstances of their associated human populations. • Conservation efforts in the Luquillo Mountains have benefited from research activity since the 1920s. • Early research in Puerto Rico focused on descriptions of flora and fauna, tree nurseries and plantation establishment, tree growth, and forest products, whereas recent research focuses on ecosystem functioning and services, climate change, landscape scale patterns, disturbances, and land use legacies. • Ecological information from both aquatic and terrestrial ecosystems facili- tates the sustainable use of natural resources while informing methods for conserving ecosystems and their services. • Results from research also help in the interpretation of environmental change and in the design of resource conservation strategies in the face of uncertainty. • A new era of conservation based on ecological knowledge is emerging. Conservation is increasingly based on sustainable development goals and implemented in collaboration with citizens. Management in this era will be more flexible in outlook and adaptable to a continuously changing environment. • We give examples of surprise events for which we have no explanation, and which we did not have the means to anticipate. These examples collectively demonstrate that the management of complex ecosystems requires contin- uous long-term research.
Species employ diverse strategies to cope with natural disturbance, but the importance of these strategies for maintaining tree species diversity in forests has been debated. Mechanisms that have the potential to promote tree species coexistence in the context of repeated disturbance include life history trade-offs in colonization and competitive ability or in species' ability to survive at low resource conditions and exploit the temporary resource-rich conditions often generated in the wake of disturbance (successional niche). Quantifying these trade-offs requires long-term forest monitoring and modeling. We developed a hierarchical Bayes model to investigate the strategies tree species employ to withstand and recover from hurricane disturbance and the life history trade-offs that may facilitate species coexistence in forests subject to repeated hurricane disturbance. Unlike previous approaches, our model accommodates temporal variation in process error and observations from multiple sources. We parameterized the model using growth and mortality data from four censuses of a 16-ha plot taken every five years (1990-2005), together with damage data collected after two hurricanes and annual seed production data (1992-2005). Species' susceptibilities to hurricane damage as reflected by changes in diameter growth and fecundity immediately following a storm were weak, highly variable, and unpredictable using traditional life history groupings. The lower crowding conditions (e.g., high light) generated in the wake of storms, however, led to greater gains in growth and fecundity for pioneer and secondary-forest species than for shade-tolerant species, in accordance with expectation of life history. We found moderate trade-offs between survival in high crowding conditions, a metric of competitive ability, and long-distance colonization. We also uncovered a strong trade-off between mean species fecundity in low crowding conditions, a metric of recovery potential, and competitive ability. Trade-offs in competitive and colonization ability, in addition to successional niche processes, are likely to contribute to species persistence in these hurricane-impacted forests. The strategies species employ to cope with hurricane damage depend on the degree to which species rely on sprouting, repair of adult damage, changes in demographic rates in response to enhanced resource availability after storms, or long-distance dispersal as recovery mechanisms.
Gradient analysis is rarely used in studies of fungal communities. Data on macromycetes from eight sites along an elevation gradient in central Veracruz, Mexico, were used to demonstrate methods for gradient analysis that can be applied to studies of communities of fungi. Selected sites from 100 to 3,500 m altitude represent tropical dry forest, tropical montane cloud forest, conifer forest, and their ecotones. From May to October 2010, macromycetes were collected monthly within ten 10 9 10 m permanent plots per site. In total, 672 individuals of 213 species of macromycetes were recorded. Models for richness and diversity for all macromycete and ectomycorrhizal communities displayed peaks in the mid-part of the gradient, and a tendency to increase with elevation, whereas xylophagous fungi displayed a peak in the mid-lower part but tended to decrease with elevation. Cluster and Maximum Likelihood analyses distinguished four communities for both macromycetes and trees, but plant and fungal communities were only partly concordant. Canonical correspondence analysis indicated that macromycete distribution along the gradient is related to slope, relative humidity, soil temperature, soil water con- tent, canopy openness, and litter depth. Spearman’s correlation and regression trees sug- gested that air and soil temperature, relative humidity, soil water content, canopy openness, vegetation structure and tree species richness were most strongly related to macrofungal functional groups, but these environmental variables were often correlated to the forest type and may not be causal. Variation in the environment along the elevation gradient differentially affected macromycete functional groups. Results from the different methods used in this work were concordant and showed significant patterns.
In ecological studies, the use of different sampling methods for the same purpose influence data quality and thus the resulting conclusions (Coddington et al. 1996; Fisher 1999). For example, to collect arthropods from soil and litter samples a soil corer or a shovel may be used. Soil corers compact the soil (Meyer 1996) making difficult for organisms to leave the sample while shovels create a large disturbance (Longino et al. 2002) promoting mobile organisms to leave and reducing their apparent abundance in the sample. As a consequence the diversity of collected arthropods will vary between these two procedures, resulting in either an under- or overestimate of the diversity of the collected fauna (André et al. 2002 ). These different results will lead the researcher to infer different conclusions. Therefore it is essential to assess how different procedures affect the abundance, richness and species composition of the retrieved arthropods. Arthropods are usually retrieved from soil/litter samples with Berlese-Tullgren funnels (Walter et al. 1987; Rohitha 1992; MacFadyen 1961; Bremner 1990; Lakly & Crossley 2000; MacFadyen 1953; Haarlov 1947). In these funnels, a source of heat (i.e. a light bulb) is placed above the sample, and a collecting vial filled with a killing solution (e.g. 70% ethanol) is placed below the sample. Light from the bulb has a double effect because light per se forces photophobic organisms to move away from the source, and light heats the sample. As the sample dries, a temperature and humidity gradient is created between the upper and lower surfaces of the sample (Haarlov 1947; Block 1966). As this gradient moves downwards, animals are forced down into the collecting liquid (Coleman et al. 2004). By increasing the temperature within the funnel, heat speeds drying (Coleman et al. 2004) but may also burn organisms before their collection and thus decreases estimates of their abundance (Walter et al. 1987). Alternatively, in remote field conditions, extractions without light are logistically more affordable and feasible, in which case the establishment of the gradient and the drying out of the sample depends on the room temperature in which the extractions are performed (Krell et al. 2005). Both, extractions with and without light, create different conditions within the sample, as a consequence, the use, or no use, of light during extractions, can result in different groups of arthropods being extracted, and thus a different set of data (Agosti et al. 2000). The duration of arthropod extraction can also affect diversity estimates. Extraction periods reported in the literature vary from 2 d (Burgess et al. 1999), 3 d (Hasegawa 1997), to 4 d (Oliver & Beattie 1996; Bestelmeyer et al. 2000) and up to 7 d (Chen & Wise 1999; Walter et al. 1987). Long extraction periods are generally assumed to result in more complete extractions and higher abundance of the extracted fauna (Oliver & Beattie 1996) as organisms with low mobility require more time to exit the sample, but longer extraction periods may expose the samples to potential contamination with foreign organisms. On the other hand, to establish an adequate period of extraction, the environment of origin and the developmental stage should be taken into account (André et al. 2002). For example, organisms adapted to extreme environments, such as areas devoid of vegetation cover that have large temperature fluctuations, may require longer extraction periods than organisms adapted to less extreme environments. Furthermore, organisms from the same habitat but occurring in the dry or wet seasons (Oliver & Beattie 1996) or different developmental stages (Søvik & Leinaas 2002) may differ in the extraction period required to retrieve them. As a consequence, in order to collect reliable data, it is necessary to assess how an adequate duration of the extraction varies among environments of origin and developmental stages of the focal organism. The present study was carried out in the Caribbean island of Puerto Rico, specifically in tropical dry and wet forests with contrasting environmental conditions (Ewel & Whitmore 1973). The objective of this study was to assess how the diversity of extracted arthropods was affected by variations in the collection and extraction methodologies, and by variations in the duration of the extraction. We present abundance, richness and composition of the collected fauna. The information presented here will provide researchers with data to simplify the logistics of arthropod sampling and extraction, and to better choose a specific procedure for a given focal organism in a given habitat.
Key Points • Background treefall gaps (not caused by hurricanes) are filled with plant regrowth as in other tropical forests. There is limited response by animals to treefall gaps, probably because background treefall gaps are relatively less important in these forests, which are dominated by chronic, widespread hurricane effects. • Despite substantial effects on trees, the tree species composition changed little in the tabonuco forest after two recent hurricanes. • Animal species show various responses to the changes in forest architecture and food resources caused by hurricanes. Bird species tend to be plastic in habitat and dietary requirements, probably due to the large changes in forest structure caused by hurricanes and regrowth, which require birds to change their foraging locations and diets. • Although hurricane-produced debris is substantial (litterfall up to 400 times the average daily amount), decomposition, nutrient export, and trace gas emissions after hurricanes change only briefly, as rapid regrowth reasserts control over most ecosystem processes. • In general, terrestrial ecosystem functions recover faster than structure. • Hurricanes dump debris in streams, and floods redistribute inorganic and detrital material, as well as stream organisms, throughout the benthic environment along the stream continuum. • Succession in landslides is slow and primarily limited by the availability of seed and by low nutrient availability, and early plant colonists, especially ferns, have a strong influence on later dynamics. • Past land use is the most important determinant of species composition in tabonuco forest, despite repeated hurricane effects and underlying environ- mental variation such as in soil and topography. • The native organisms of the Luquillo Mountains are more resilient after natural than human disturbances.
Quaternary climatic oscillations caused changes in sea level that altered the size, number and degree of isolation of islands, particularly in land-bridge archipelagoes. Elucidating the demographic effects of these oscillations increases our understanding of the role of climate change in shaping evolutionary processes in archipelagoes. The Puerto Rican Bank (PRB) (Puerto Rico and the Eastern Islands, which comprise Vieques, Culebra, the Virgin Islands and associated islets) in the eastern Caribbean Sea periodically coalesced during glaciations and fragmented during interglacial periods of the quaternary. To explore population-level consequences of sea level changes, we studied the phylogeography of the frog Eleutherodactylus antillensis across the archipelago. We tested hypotheses encompassing vicariance and dispersal narratives by sequencing mtDNA (c. 552 bp) of 285 individuals from 58 localities, and four nuDNA introns (totalling c. 1633 bp) from 173 of these individuals. We found low support for a hypothesis of divergence of the Eastern Islands populations prior to the start of the penultimate interglacial c. 250 kya, and higher support for a hypothesis of colonization of the Eastern Islands from sources in eastern Puerto Rico during the penultimate and last glacial period, when a land bridge united the PRB. The Río Grande de Loíza Basin in eastern Puerto Rico delineates a phylogeographic break. Haplotypes shared between the PRB and St. Croix (an island c. 105 km south-east of this archipelago) likely represent human-mediated introductions. Our findings illustrate how varying degrees of connectivity and isolation influence the evolution of tropical island organisms.
Soil moisture is a key driver of biogeochemical processes in terrestrial ecosystems, strongly affecting carbon (C) and nutrient availability as well as trace gas production and consumption in soils. Models predict increasing drought frequency in tropical forest ecosystems, which could feed back on future climate change directly via effects on trace gasdynamics and indirectly through changes in nutrient availability. We used throughfall exclusion shelters to determine effects of short-term (3 month) drought on trace gas fluxes and nutrient availability in humid tropical forests in Puerto Rico. Exclusion and control plots were replicated within and across three topographic zones (ridge, slope, valley) to account for spatial heterogeneity typical of these ecosystems. Throughfall exclusion reduced soil moisture in all sites and lowered exchangeable phosphorus (P) on ridges and slopes. Drought decreased soil carbon dioxide (CO2) emissions by 30% in ridge sites and 28% in slope sites, and increased net methane (CH4) consumption by 480% in valley sites. Both valley and ridge sites became net nitrous oxide (N2O) sinks in response to soil drying. Emissions of CO2 and N2O, as well as CH4 consumption were positively related to exchangeable P and the nitrate:ammonium ratio. These findings suggest that drought has the potential to decrease net trace gas emissions from humid tropical forest soils. The differential response of trace gas emissions and nutrients from different topographic zones to drought underscores the complexity of biogeochemical cycling in these ecosystems and the importance of considering spatial heterogeneity when estimating whole system responses.
Key Points • Individual biota or taxa sometimes have a disproportionate effect on food web or ecosystem dynamics. • The differences in the architecture of tree species (e.g., Dacryodes excelsa) alter wind disturbance magnitude and effects through the dissipation of wind energy. • Ferns and earthworms can enhance the recolonization rate on bare soils following disturbances through modification of the physical microenviron- ment and nutrient availability. • Freshwater shrimp and earthworms alter nutrient availability in the streams and soils, altering processing rates through effects on detrital processing. • Small vertebrate species such as anolis lizards and tree frogs (Coquis) signifi- cantly alter food web dynamics through direct consumption of herbivorous insects and their cycling of important, limiting nutrients. Also view article in: http://www.fs.fed.us/global/iitf/pubs/bc_iitf_2012_Crowl001.pdf
Forest transitions (FT) occur when socioeconomic development leads to a shift from net deforestation to reforestation; these dynamics have been observed in multiple countries across the globe, including the island of Puerto Rico in the Caribbean. Starting in the 1950s, Puerto Rico transitioned from an agrarian to a manufacturing and service economy reliant on food imports, leading to extensive reforestation. In recent years, however, net reforestation has leveled off. Here we examine the drivers of forest transition in Puerto Rico from 1977 to 2000 at two subnational, nested spatial scales (municipality and barrio) and over two time periods (1977-1991 and 1991-2000). This study builds on previous work by considering the social and biophysical factors that influence both reforestation and deforestation at multiple spatial and temporal scales. By doing so within one analysis, this study offers a comprehensive understanding of the relative importance of various social and biophysical factors for forest transitions an the scales at which they are manifest. Biophysical factors considered in these analyses included slope, soil quality, and land-cover in the surrounding landscape. We also considered per capita income, population density, and the extent of protected areas as potential factors associated with forest change. Our results show that, in the 1977-1991 period, biophysical factors that exhibit variation at municipality scales (~100km²) were more important predictors of forest change than socioeconomic factors. In this period, forest dynamics were driven primarily by abandonment of less productive, steep agricultural land in the western,central part of the island. These factors had less predictive power at the smaller barrio scale (~10 km²) relative to the larger municipality scale during this time period. The relative importance of socioeconomic variables for deforestation, however, increased over time as development pressures on available land increased. From 1991-2000, changes in forest cover reflected influences from multiple factors, including increasing population densities, land development pressure from suburbanization, and the presence of protected areas. In contrast to the 1977-1991 period, drivers of deforestation and reforestation over this second interval were similar for the two spatial scales of analyses. Generally, our results suggest that although broader socioeconomic changes in a given region may drive the demand for land, biophysical factors ultimately mediate where development occurs. Although economic development may initially result in reforestation due to rural to urban migration and the abandonment of agricultural lands, increased economic development may lead to deforestation through increased suburbanization pressures.
Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3−) concentrations, but that <1% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3−. We suggest that increased stream NO3− loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y−1 of anthropogenic N inputs to N2O in river networks, equivalent to 10% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.
Improving our understanding of the impacts of urbanization on tropical island streams is critical as urbanization becomes a dominant feature in tropical areas. Although the “urban stream syndrome” has been successful in summarizing urban impacts on streams, the response of some island streams is different to that expected. Here we review available information on urban impacts to tropical island streams and describe unique responses to urbanization. We identified three key aspects that play particularly important or unique roles in determining tropical-island stream integrity: biotic response to water pollution, movement barriers along the stream network, and altered geomorphology that results in habitat loss. As expected, water pollution negatively impacts stream ecosystems in tropical islands and in some regions impacts can be severe, as untreated wastewaters are directly discharged into streams. While aquatic insects show the expected responses to pollution, other native fauna (e.g., shrimps and fishes) appear to be less impacted by moderate levels of pollution. Movement barriers along the stream network are especially important as much of the tropical island fauna have diadromous (either amphidromous or catadromous) life histories. Most native freshwater mollusks, shrimps, and fishes inhabiting tropical islands are diadromous and depend on unimpeded connections between freshwater and marine environments to complete their life cycles. The presence of these species in urban streams is best explained by longitudinal connectivity rather than by the degree of urban impact. Finally, in streams that remain connected to marine environments, the presence of native shrimps and fishes is strongly related to the physical habitat. Fish assemblages in channelized and severely altered stream reaches are almost completely devoid of native fauna and tend to be dominated by non-native species. In contrast, relatively diverse shrimp and fish assemblages can be found in reaches that retain their physical habitat complexity, even when they are impacted by urbanization. Our understanding of urban impacts on tropical island streams remains limited. However, the identification of key aspects can help us better understand urban impacts on streams in tropical islands, and best focus our management and research efforts to protect these unique ecosystems.
Agricultural and urban development alters nitrogen and other biogeochemical cycles in rivers worldwide. Because such biogeochemical processes cannot be measured empirically across whole river networks, simulation models are critical tools for understanding river-network biogeochemistry. However, limitations inherent in current models restrict our ability to simulate biogeochemical dynamics among diverse river networks. We illustrate these limitations using a river-network model to scale up in situ measures of nitrogen cycling in eight catchments spanning various geophysical and land-use conditions. Our model results provide evidence that catchment characteristics typically excluded from models may control river-network biogeochemistry. Based on our findings, we identify important components of a revised strategy for simulating biogeochemical dynamics in river networks, including approaches to modeling terrestrial-aquatic linkages, hydrologic exchanges between the channel, floodplain/riparian complex, and subsurface waters, and interactions between coupled biogeochemical cycles.
Lignin is often the most difficult portion of plant biomass to degrade, with fungi generally thought to dominate during late stage decomposition. Lignin in feedstock plant material represents a barrier to more efficient plant biomass conversion and can also hinder enzymatic access to cellulose, which is critical for biofuels production. Tropical rain forest soils in Puerto Rico are characterized by frequent anoxic conditions and fluctuating redox, suggesting the presence of lignin-degrading organisms and mechanisms that are different from known fungal decomposers and oxygen-dependent enzyme activities. We explored microbial lignin-degraders by burying bio-traps containing lignin-amended and unamended biosep beads in the soil for 1, 4, 13 and 30 weeks. At each time point, phenol oxidase and peroxidase enzyme activity was found to be elevated in the lignin-amended versus the unamended beads, while cellulolytic enzyme activities were significantly depressed in lignin-amended beads. Quantitative PCR of bacterial communities showed more bacterial colonization in the lignin-amended compared to the unamended beads after one and four weeks, suggesting that the lignin supported increased bacterial abundance. The microbial community was analyzed by small subunit 16S ribosomal RNA genes using microarray (PhyloChip) and by high-throughput amplicon pyrosequencing based on universal primers targeting bacterial, archaeal, and eukaryotic communities. Community trends were significantly affected by time and the presence of lignin on the beads. Lignin-amended beads have higher relative abundances of representatives from the phyla Actinobacteria, Firmicutes, Acidobacteria and Proteobacteria compared to unamended beads. This study suggests that in low and fluctuating redox soils, bacteria could play a role in anaerobic lignin decomposition.
Microbial metabolism couples elemental reactions, driving biogeochemical cycles. Assimilatory coupling of elemental cycles, such as the carbon (C), nitrogen (N), and phosphorus cycles, occurs when these elements are incorporated into biomass or released through its decomposition. In addition, many microbes are capable of dissimilatory coupling, catalyzing energy-releasing reactions linked to transformations in the oxidation state of elements, and releasing the transformed elements to the environment. Different inorganic elements provide varying amounts of energy yield, and the interaction of these processes creates a microbial energy economy. Dissimilatory reactions involving C, N, iron, and sulfur provide particularly important examples where microbially mediated oxidation–reduction (redox) transformations affect nutrient availability for net primary production, greenhouse-gas emissions, levels of contaminants and natural toxic factors, and other ecosystem dynamics. Recent discoveries of previously unrecognized microbial dissimilatory processes are leading to reevaluation of traditional perceptions of biogeochemical cycles.
Increased inputs of greenhouse gases have altered the composition of the atmosphere over the past 150 years (IPCC, 2001, 2007), resulting in shifts in temperature and precipitation around the globe. The scientific community has put an enormous effort into understanding the causes of these changes, and predicting future climate and the interactions between climate and the biosphere that may moderate or accelerate current trends. Most of the research on climate change has focused on boreal and north temperate ecosystems where temperature shifts are predicted to be the largest (IPCC, 2001, 2007). These ecosystems are often characterized by deep organic soils that present the potential for a strong positive feedback to climate change (Oechel et al., 1998; Vourlitis and Oechel, 1997; Hobbie et al., 2002).
Biogenic aerosols are relevant for the Earth system, climate, and public health on local, regional, and global scales. Up to now, however, little is known about the diversity and biogeography of airborne microorganisms. We present the first DNA-based analysis of airborne fungi on global scales, showing pronounced geographic patterns and boundaries. In particular we find that the ratio of species richness between Basidiomycota and Ascomycota is much higher in continental air than in marine air. This may be an important difference between the “blue ocean” and “green ocean” regimes in the formation of clouds and precipitation, for which fungal spores can act as nuclei. Our findings also suggest that air flow patterns and the global atmospheric circulation are important for the understanding of global changes in biodiversity.
In The Biology of Disturbed Habitats, readers will find a pithy but wellbalanced review of the relevant research on ecological theory, but disturbance ecology is the focal point throughout the book. In reading, we view the whole of ecology through the lens of disturbance—as a modulator of biodiversity, ecosystem processes, and stability. I do not know of any previously published work more comprehensive in its inclusiveness of the types of natural and human disturbances, in spatial and temporal scales, within aquatic and terrestrial ecosystems, affecting both plants and animals. The interplay between the natural and anthropogenic disturbances and how they affect human use of the environment and sustainability is the book's key component. The statement “To survive, humans have adapted to disturbances that we cannot manage and manipulated those we can” (p. 211) captures its essence. Author Lawrence R. Walker, a professor of plant ecology at the University of Nevada and a published writer of many classic papers, has addressed this book to ecologists, naturalists, and land managers. The prose is straightforward and readable by an audience with a wide array of specialties. Although the subject matter is vast, the book narrows in on the specifics with illustrative examples of disturbance within various ecosystems, showing how the small observable details fit into the much larger concepts. Tables and conceptual diagrams summarize major points, making the book useful for teachers who wish to introduce students to the variety of processes and effects of disturbance. I found the tables that show the chronological development of ecological concepts over the last century to be particularly convenient, because I had forgotten certain items since graduate school. Walker's message is clear: Disturbances are constantly affecting all life, everywhere, in an integrative fashion. This point of view is likely to be eye opening for many students and researchers who believe that it is possible to experimentally isolate one factor of interest, one disturbed element, in field study. Carcasses create nutrient pulses; sand dunes move across the landscape. Floods, tsunamis, treefalls, insect infestations, fires, volcanoes, and landslides are all considered and then put in a larger temporal and spatial context. Anthropogenic disturbances, including forestry, mining, agriculture, dams, urbanization, and military activities—which are novel compared with the evolutionary history of most species and ecosystems—are given a particularly thorough treatment. An estimate of the proportion of the Earth's surface affected by each disturbance type is given when data are available. The book is not flawless, however. For example, fire intensity is measured in kilowatts (not temperature); tornadoes and thunderstorm downbursts, which probably affect areas as large as cyclones (hurricanes in North America), are absent; and large herbivores are not discussed in chapter 2 (“Terrestrial habitats”) but curiously appear later, in chapter 8 (“Temporal dynamics”), where they are certainly relevant but lack the necessary introduction that should have occurred earlier. The usefulness and limitations of ecological theory are given reviews throughout the book. For example, Clements's theory on climax vegetation is put in the proper perspective with a discussion steering between the extreme viewpoints that have often been voiced elsewhere and the more indulgent approach, which allows it to mingle with the supposedly opposing individualistic theory. Discussions of ecological function, biodiversity, landscape ecology, patch dynamics (including interface interactions, such as along shorelines), and restoration ecology appear in various chapters; these are like miniature textbooks on each of these topics and are among the highlights of the book. Chapter 8, on temporal dynamics, has a truly grand integrated discussion of the history, mechanisms, and trajectories of succession and how they are influenced by interactions with disturbance, plantanimal interactions, and environmental factors. This is the most readable discussion of the complexities of succession that I have encountered.
Microbial communities and their associated enzyme activities affect the amount and chemical quality of carbon (C) in soils. Increasing nitrogen (N) deposition, particularly in N-rich tropical forests, is likely to change the composition and behavior of microbial communities and feed back on ecosystem structure and function. This study presents a novel assessment of mechanistic links between microbial responses to N deposition and shifts in soil organic matter (SOM) quality and quantity. We used phospholipid fatty acid (PLFA) analysis and microbial enzyme assays in soils to assess microbial community responses to long-term N additions in two distinct tropical rain forests. We used soil density fractionation and 13C nuclear magnetic resonance (NMR) spectroscopy to measure related changes in SOM pool sizes and chemical quality. Microbial biomass increased in response to N fertilization in both tropical forests and corresponded to declines in pools of low-density SOM. The chemical quality of this soil C pool reflected ecosystem-specific changes in microbial community composition. In the lower-elevation forest, there was an increase in gram-negative bacteria PLFA biomass, and there were significant losses of labile C chemical groups (O-alkyls). In contrast, the upper-elevation tropical forest had an increase in fungal PLFAs with N additions and declines in C groups associated with increased soil C storage (alkyls). The dynamics of microbial enzymatic activities with N addition provided a functional link between changes in microbial community structure and SOM chemistry. Ecosystem-specific changes in microbial community composition are likely to have far-reaching effects on soil carbon storage and cycling. This study indicates that microbial communities in N-rich tropical forests can be sensitive to added N, but we can expect significant variability in how ecosystem structure and function respond to N deposition among tropical forest types.
Collecting trips in the American tropics have yielded two unusual specimens of Scutellinia. Although assigned to existing species, these specimens exhibit subtle differences from the descriptions of both species. Normally red to brownish red, a specimen of Scutellinia balansae was collected on decaying banana leaves in Venezuela that has a yellow hymenium. In Scutellinia blumenaviensis the hymenium is usually bright red, brownish red to grayish, however, a specimen with a golden-yellow hymenium was collected on a rotting log in the northeastern lowlands of Ecuador. Descriptions and illustrations of these two species are provided.
The metacommunity framework integrates species-specific responses to environmental gradients to detect emergent patterns of mesoscale organization. Abiotic characteristics (temperature, precipitation) and associated vegetation types change with elevation in a predictable fashion, providing opportunities to decouple effects of environmental gradients per se from those of biogeographical or historical origin. Moreover, expected structure is different if a metacommunity along an elevational gradient is molded by idiosyncratic responses to abiotic variables (expectation = Gleasonian structure) than if such a metacommunity is molded by strong habitat preferences or specializations (expectation = Clementsian structure). We evaluated metacommunity structure for 13 species of gastropod from 15 sites along an elevationaltransect in the Luquillo Experimental Forest of Puerto Rico. Analyses were conducted separately for the primary axis and for the secondary axis of correspondence extracted via reciprocal averaging. The metacommunity exhibited quasi-Clementsian structure along the primary axis, which represented a gradient of gastropod species specialization that was unassociated with elevation. The secondary axis represented environmental variation associated with elevation. Along this axis, the metacommunity exhibited Clementsian structure, with specialists characterizing each of three suites of sites that corresponded to three distinct forest types. These forest types are associated with low (tabonuco forest), mid- (palo colorado forest), or high (elfin forest) elevations. Thus, variation among sites in species composition reflected two independent processes: the first decoupled from elevational variation and its environmental correlates, and the second highly associated with environmental variation correlated with elevation.Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp
Hypotheses proposed for lineage diversification of tropical montane species have rarely been tested within oceanic islands. Our goal was to understand how basin barriers and Pleistocene climatic fluctuations shaped the distribution of diversity in Eleutherodactylus portoricensis (Eleutherodactylidae), a frog endemic to the montane rain forests of Puerto Rico.
Several forest dynamics plot research projects in the East-Asia Pacific region of the International Long-Term Ecological Research network actively collect long-term data, and some of these large plots are members of the Center for Tropical Forest Science network. The wealth of forest plot data presents challenges in information management to researchers. In order to facilitate the management of these data, a Forest Dynamics Plot Database and Application Workshop was held in Taiwan 2009. This paper describes the results of the workshop that produced and tested an integrated information management framework. The goal for the framework was to demonstrate how fully documented data archives can be effectively used for data discovery, access, retrieval, analysis, and integration. Results from our work included setting up a database based on the Center for Tropical Forest Science structure on a local relational database (MySQL) server, an authentication interface, a metadata query web page, and 3 workflows to test the framework. Access article in: http://www.fs.fed.us/pnw/pubs/journals/pnw_2011_lin.pdf
Forest canopy herbivores are known to increase rates of nutrient fluxes to the forest floor in a number of temperate and boreal forests, but few studies have measured effects of herbivore-enhanced nutrient fluxes in tropical forests. We simulated herbivore-induced fluxes in a tropical rainforest in Puerto Rico by augmenting greenfall (fresh foliage fragments), frassfall (insect feces), and throughfall (precipitation enriched with foliar leachates) in replicated experimental plots on the forest floor. Background rates of greenfall and frassfall were measured monthly using litterfall collectors and augmented by adding 10× greenfall or 10× frassfall to designated plots. Throughfall fluxes of NH(4), NO(3) and PO(4) (but not water) were doubled in treatment plots, based on published rates of fluxes of these nutrients in throughfall. Control plots received only background flux rates for these compounds but the same minimum amount of distilled water. We evaluated treatment effects as changes in flux rates for NO(3), NH(4) and PO(4), measured as decomposition rate of leaf litter in litterbags and as adsorption in ion-exchange resin bags at the litter-soil interface. Frass addition significantly increased NO(3) and NH(4) fluxes, and frass and throughfall additions significantly reduced decay rate, compared to controls. Reduced decay rate suggests that nitrogen flux was sufficient to inhibit microbial decomposition activity. Our treatments represented fluxes expected from low-moderate herbivore outbreaks and demonstrated that herbivores, at these outbreak levels, increase ecosystem-level N and P fluxes by >30% in this tropical rainforest.
Urbanization is degrading stream ecosystems worldwide. Tropical island streams may respond to urbanization differently than temperate streams because of their overall climate differences, and they may respond differently than continental tropical urban streams because of their reduced biological diversity and short drainages. We characterized the physicochemistry, physical habitat, and macroinvertebrate assemblages of 16 stream tributaries in the Rio Piedras Watershed (San Juan, Puerto Rico). We also described landuse patterns upstream from each sampling site for the entire subwatershed and for riparian buffers of 5- and 100-m width. Urbanization had a negative effect on the physicochemical and biological condition of the Rio Piedras. Streams were distributed in ordination space along a strong physicochemical gradient that was related to concentrations of K+, Mg2+, dissolved O2 (DO), and PO43−. Along this gradient, DO and Mg2+ decreased and PO43− and K+ increased with higher % urban cover in the subwatershed. Macroinvertebrate assemblages also were related to urbanization, and more macroinvertebrate families and pollution-sensitive taxa were found at sites where physicochemistry reflected less urban cover. Family richness and pollution-sensitive taxa were positively associated with greater % forest cover in the 5-m riparian buffer zone, a result that supports the use of riparian buffers to ameliorate the effects of urbanization on stream biointegrity in the Rio Piedras. Our results are similar to findings in urban streams in temperate zones and in tropical continental streams. Therefore, despite island characteristics, tropical island stream physicochemistry and macroinvertebrate assemblages responded to urbanization in ways that are in general agreement with the predictions of the Urban Stream Syndrome.
Lessons learned from the study of ecological succession have much to offer contemporary environmental problem solving but these lessons are being underutilized. As anthropogenic disturbances increase, succession is more relevant than ever. In this review, we suggest that succession is particularly suitable to address concerns about biodiversity loss, climate change, invasive species, and ecological restoration. By incorporating modern experimental techniques and linking results across environmental gradients with meta-analyses, studies of succession can substantially improve our understanding of other ecological phenomena. Succession can help predict changes in biodiversity and ecosystem services impacted by invasive species and climate change and guide manipulative responses to these disruptions by informing restoration efforts. Succession is still a critical, integrative concept that is central to ecology.
Most hypotheses explaining the general gradient of higher diversity toward the equator are implicit or explicit about greater species packing in the tropics. However, global patterns of diversity within guilds, including trophic guilds (i.e., groups of organisms that use similar food resources), are poorly known. We explored global diversity patterns of a key trophic guild in stream ecosystems, the detritivore shredders. This was motivated by the fundamental ecological role of shredders as decomposers of leaf litter and by some records pointing to low shredder diversity and abundance in the tropics, which contrasts with diversity patterns of most major taxa for which broad-scale latitudinal patterns haven been examined. Given this evidence, we hypothesized that shredders are more abundant and diverse in temperate than in tropical streams, and that this pattern is related to the higher temperatures and lower availability of high-quality leaf litter in the tropics. Our comprehensive global survey (129 stream sites from 14 regions on six continents) corroborated the expected latitudinal pattern and showed that shredder distribution (abundance, diversity and assemblage composition) was explained by a combination of factors, including water temperature (some taxa were restricted to cool waters) and biogeography (some taxa were more diverse in particular biogeographic realms). In contrast to our hypothesis, shredder diversity was unrelated to leaf toughness, but it was inversely related to litter diversity. Our ﬁndings markedly contrast with global trends of diversity for most taxa, and with the general rule of higher consumer diversity at higher levels of resource diversity. Moreover, they highlight the emerging role of temperature in understanding global patterns of diversity, which is of great relevance in the face of projected global warming.
The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding inﬂuence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO2 production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a signiﬁcant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.
There is a pressing need to understand the consequences of human activities, such as land transformations,on watershed ecosystem services. This is a challenging task because different indicators of water quality and yield are expected to vary in their responsiveness to large versus local-scale heterogeneity in land use and land cover (LUC). Here we rely on water quality data collected between 1977 and 2000 from dozens of gauge stations in Puerto Rico together with precipitation data and land cover maps to (1) quantify impacts of spatial heterogeneity in LUC on several water quality indicators; (2) determine the spatial scale at which this heterogeneity influences water quality; and (3) examine how antecedent precipitation modulates these impacts. Our models explained 30–58% of observed variance in water quality metrics. Temporal variation in antecedent precipitation and changes in LUC between measurements periods rather than spatial variation in LUC accounted for the majority of variation in water quality. Urbanization and pasture development generally degraded water quality while agriculture and secondary forest regrowth had mixed impacts. The spatial scale over which LUC influenced water quality differed across indicators. Turbidity and dissolved oxygen (DO) responded to LUC in large-scale watersheds, in-stream nitrogen concentrations to LUC in riparian buffers of large watersheds, and fecal matter content and instream phosphorus concentration to LUC at the subwatershed scale. Stream discharge modulated impacts of LUC on water quality for most of the metrics. Our findings highlight the importance of considering multiple spatial scales for understanding the impacts of human activities on watershed ecosystem services.
The third edition of Insect Ecology: An Ecosystem Approach provides a modern perspective of insect ecology that integrates two approaches traditionally used to study insect ecology: evolutionary and ecosystem. This integration substantially broadens the scope of insect ecology and contributes to prediction and resolution of the effects of current environmental changes, as these affect and are affected by insects. The third edition includes an updated and expanded synthesis of feedback and interactions between insects and their environment. This updated material and a new chapter on applications of insect ecology to social and environmental issues effectively demonstrates how evolutionary and ecosystem approaches complement each other, with the intent of stimulating further integration of these approaches in experiments that address insect roles in ecosystems. Effective management of ecosystem resources depends on evaluation of the complex, often complementary, effects of insects on ecosystem conditions, as well as insect responses to changing conditions.
The global reach of human activities affects all natural ecosystems, so that the environment is best viewed as a social–ecological system. Consequently, a more integrative approach to environmental science, one that bridges the biophysical and social domains, is sorely needed. Although models and frameworks for social–ecological systems exist, few are explicitly designed to guide a long-term interdisciplinary research program. Here, we present an iterative framework, “Press–Pulse Dynamics” (PPD), that integrates the biophysical and social sciences through an understanding of how human behaviors affect “press” and “pulse” dynamics and ecosystem processes. Such dynamics and processes, in turn, influence ecosystem services –thereby altering human behaviors and initiating feedbacks that impact the original dynamics and processes. We believe that research guided by the PPD framework will lead to a more thorough understanding of social–ecological systems and generate the knowledge needed to address pervasive environmental problems. Also access article in: http://www.fs.fed.us/pnw/pubs/journals/pnw_2010_collins001.pdf
Disturbance is an integral part of every ecosystem, but humans are altering disturbance regimes in fundamental ways that can alter outcomes for ecosystem structure and function. Fortunately, advances in understanding ecosystem responses to natural disturbances can address the ecological consequences of the novel suite of disturbances now created by humans. Complex interactions among both natural and anthropogenic disturbances at many overlapping spatial and temporal scales can be examined across severity gradients. The gradient approach applies ecological tools to differential conditions of stability and fertility, degrees of biological legacy and rates of successional recovery and can help address modern concerns about socio-economic consequences of disturbance and the sustainability of ecosystem services.
Understanding the effects of disturbance and secondary succession on spatio-temporal patterns in the abundance of species is stymied by a lack of long-term demographic data, especially in response to infrequent and high intensity disturbances, such as hurricanes. Moreover, resistance and resilience to hurricane-induced disturbance may be mediated by legacies of previous land use, although such interactive effects are poorly understood, especially in tropical environments. We address these central issues in disturbance ecology by analyzing an extensive dataset, spanning the impacts of Hurricanes Hugo and Georges, on the abundance of a Neotropical walking stick, Lamponius portoricensis, in tabonuco rainforest of Puerto Rico during the wet and dry seasons from 1991 to 2007. By synthesizing data from two proximate sites in tabonuco forest, we show that resistance to Hurricane Hugo (97% reduction in abundance) was much less than resistance to Hurricane Georges (21% reduction in abundance). Based on a powerful statistical approach (generalized linear mixed-effects models with Poisson error terms), we documented that the temporal trajectories of abundance during secondary succession (i.e., patterns of resilience) differed between hurricanes and among historical land use categories, but that the effects of hurricanes and land use histories were independent of each other. These complex results likely arise because of differences in the intensities of the two hurricanes with respect to microclimatic effects (temperature and moisture) in the forest understory, as well as to time-lags in the response of L. portoricensis to changes in the abundance and distribution of preferred food plants (Piper) in post-hurricane environments.
The 326 ha Río Icacos watershed in the tropical wet forest of the Luquillo Mountains, northeastern Puerto Rico, is underlain by granodiorite bedrock with weathering rates among the highest in the world. We pooled stream chemistry and total suspended sediment (TSS) data sets from three discrete periods: 1983–1987, 1991–1997, and 2000–2008. During this period three major hurricanes crossed the site: Hugo in 1989, Hortense in 1996, and Georges in 1998. Stream chemistry reflects sea salt inputs (Na, Cl, and SO4), and high weathering rates of the granodiorite (Ca, Mg, Si, and alkalinity). During rainfall, stream composition shifts toward that of precipitation, diluting 90% or more in the largest storms, but maintains a biogeochemical watershed signal marked by elevated K and dissolved organic carbon (DOC) concentration. DOC exhibits an unusual “boomerang” pattern, initially increasing with flow but then decreasing at the highest flows as it becomes depleted and/or vigorous overland flow minimizes contact with watershed surfaces. TSS increased markedly with discharge (power function slope 1.54), reflecting the erosive power of large storms in a landslide-prone landscape. The relations of TSS and most solute concentrations with stream discharge were stable through time, suggesting minimal long-term effects from repeated hurricane disturbance. Nitrate concentration, however, increased about threefold in response to hurricanes then returned to baseline over several years following a pseudo first-order decay pattern. The combined data sets provide insight about important hydrologic pathways, a long-term perspective to assess response to hurricanes, and a framework to evaluate future climate change in tropical ecosystems.
Migratory freshwater fauna depend on longitudinal connectivity of rivers throughout their life cycles. Amphidromous shrimps spend their adult life in freshwater but their larvae develop into juveniles in salt water. River fragmentation resulting from pollution, land use change, damming and water withdrawals can impede dispersal and colonization of larval shrimps. Here we review current knowledge of river fragmentation effects on freshwater amphidromous shrimp in the Neotropics, with a focus on Puerto Rico and Costa Rica. In Puerto Rico, many studies have contributed to our knowledge of the natural history and ecological role of migratory neotropical shrimps, whereas in Costa Rica, studies of freshwater migratory shrimp have just begun. Here we examine research ﬁndings from Puerto Rico and the applicability of those ﬁndings to continental Costa Rica. Puerto Rico has a relatively large number of existing dams and water withdrawals, which have heavily fragmented rivers. The effects of fragmentation on migratory shrimps’ distribution have been documented on the landscape-scale in Puerto Rico. Over the last decade, dams for hydropower production have been constructed on rivers throughout Costa Rica. In both countries, large dams restrict shrimps from riverine habitat in central highland regions; in Puerto Rico 27% of stream kilometers are upstream of large dams while in Costa Rica 10% of stream kilometers are upstream of dams. Research about amphidromy speciﬁc to non-island shrimps is increasingly important in light of decreasing hydrologic connectivity.
The final larval stadium of four species of Cora are described and compared with known species in the genus. Cora skinneri Calvert, 1907, C. semiopaca Selys, 1878 and C. lugubris Navás, 1934 are described and illustrated for the first time using material from Costa Rica for the first two and from Colombia for the latter. A redescription of C. marina Selys, 1868 from specimens collected in Costa Rica is also included for comparison. Although all species are very similar as larvae, two major groups can be differentiated based on the shape of the caudal gills. The three species here described for the first time are very similar, but can be separated from each other using a combination of characters.
Throughout the tropics, population movements, urban growth, and industrialization are causing conditions that result in elevated temperatures within urban areas when compared with that in surrounding rural areas, a phenomenon known as the urban heat island (UHI). One such example is the city of San Juan, Puerto Rico. Our objective in this study was to quantify the UHI created by the San Juan Metropolitan Area over space and time using temperature data collected by mobile- and fixed-station measurements. We also used the fixed-station measurements to examine the relationship between average temperature at a given location and the density of remotely sensed vegetation located upwind. We then regressed temperatures against regional upwind land cover to predict future temperature with projected urbanization. Our data from the fixed stations show that the average nighttime UHI calculated between the urban reference and rural stations (ΔTCBD–rural) was 2.15 °C during the usually wet season and 1.78 °C during the usually dry season. The maximum UHI value for San Juan was calculated as 4.7 °C between the urban and forest sites and 3.9 °C between the urban and an open, rural site. Comparisons of diurnal temperature trends at urban, grassland, and forested sites indicate that canopy cover reduced daytime warming. Temperature was predicted best (r2 = 0.94) by vegetation in upwind easterly directions, especially that within 180 m of the sensor. Results from the mobile measurements show that the UHI has reached the base of the Luquillo Mountains. Predictions of future development and temperatures suggest that if the present pattern of development continues, over 140 km2 of land that showed no signs of UHI in 2000 will have an average annual UHI between + 0.4 and + 1.55 °C by 2050. Furthermore, more than 130 km2 of land area with a current UHI between + 0.4 and + 1.4 °C in 2000 will have an average UHI greater than + 1.55 °C by 2050. Copyright © 2010 Royal Meteorological Society
Tropical rain forests play a dominant role in global biosphere-atmosphere CO2 exchange. Although climate and nutrient availability regulate net primary production (NPP) and decomposition in all terrestrial ecosystems, the nature and extent of such controls in tropical forests remain poorly resolved. We conducted a meta-analysis of carbon-nutrient-climate relationships in 113 sites across the tropical forest biome. Our analyses showed that mean annual temperature was the strongest predictor of aboveground NPP (ANPP) across all tropical forests, but this relationship was driven by distinct temperature differences between upland and lowland forests. Within lowland forests (< 1000 m), a regression tree analysis revealed that foliar and soil-based measurements of phosphorus (P) were the only variables that explained a significant proportion of the variation in ANPP, although the relationships were weak. However, foliar P, foliar nitrogen (N), litter decomposition rate (k), soil N and soil respiration were all directly related with total surface (0–10 cm) soil P concentrations. Our analysis provides some evidence that P availability regulates NPP and other ecosystem processes in lowland tropical forests, but more importantly, underscores the need for a series of large-scale nutrient manipulations – especially in lowland forests – to elucidate the most important nutrient interactions and controls.
Cloud water, rainwater, and aerosol particles were collected in Puerto Rico from December 2004 to March 2007 in order to investigate their chemical composition, relation to sources, and removal processes. The species analyzed were inorganic ions, metals, total and dissolved organic carbon (TOC, DOC), total nitrogen (TN), and organic acids. For all samples, the dominant species were marine (Na+, Cl−), representing about 50%–65% of total content. Non-sea-salt fraction was dominated by SO42− (17%–25%), followed by water-soluble organic (2%–8%) and total nitrogen (2% –6%) compounds. Organic acids represented contributions to the organic fraction in cloud water of 20% and 6% for aerosol particles. Inorganic species were predominant in total nitrogen portion. The chemical composition of cloud water, rainwater, and aerosol particles were observed to be sensitive to transport patterns. Air masses from northwest Africa showed the highest concentrations of nss-Ca2+, Fe, and Al, suggesting a crustal origin. The pH values for cloud water and rainwater observed under this transport pattern were higher than background conditions, probably due to the alkalinity associated with nss-Ca2+. The highest concentrations of Cl− and SO42−, with lower pH, were measured during periods of influence from Soufriere Hills volcano eruptions, most likely due to emitted SO2 and HCl. Air masses from North America had an anthropogenic influence, where levels of nss-SO42−, TOC, and TN were higher (∼4 times) than in clean air masses. These results suggest that long-range transport could be an extra source of metals and organic/nitrogen species to the Caribbean region ecosystems.
Soil redox plays a key role in regulating biogeochemical transformations in terrestrial ecosystems, but the temporal and spatial patterns in redox and associated controls within and across ecosystems are poorly understood. Upland humid tropical forest soils may be particularly prone to fluctuating redox as abundant rainfall limits oxygen (O2) diffusion through finely textured soils and high biological activity enhances O2 consumption. We used soil equilibration chambers equipped with automated sensors to determine the temporal variability in soil oxygen concentrations in two humid tropical forests with different climate regimes. We also measured soil trace gases (CO2, N2O, and CH4) as indices of redox-sensitive biogeochemistry. On average, the upper elevation cloud forest had significantly lower O2 concentrations (3.0 ± 0.8%) compared to the lower elevation wet tropical forest (7.9 ± 1.1%). Soil O2 was dynamic, especially in the wet tropical forest, where concentrations changed as much as 10% in a single day. The periodicity in the O2 time series at this site was strongest at 16 day intervals and was associated with the hourly precipitation. Greenhouse gas concentrations differed significantly between sites, but the relationships with soil O2 were consistent: O2 was negatively related to both CO2 and CH4 and positively related to N2O. These results are among the first to quantify the temporal and spatial scale of variability in soil redox in humid tropical forests, and show that the timing of precipitation plays a strong role in biogeochemical cycling on the scale of hours to weeks.
Elevational gradients provide a natural experiment for assessing the extent to which the structure of animal metacommunities is molded by biotic and abiotic characteristics that change gradually, or is molded by aspects of plant community composition and physiognomy that change in a more discrete fashion. We used a metacommunity framework to integrate species-specific responses to environmental gradients as an approach to detect emergent patterns at the mesoscale in the Luquillo Mountains of Puerto Rico. Elements of metacommunity structure (coherence, species turnover and range boundary clumping) formed the basis for distinguishing among random, checkerboard, Gleasonian, Clementsian, evenly spaced and nested patterns. Paired elevational transects (300–1000 m a.s.l.) were sampled at 50 m intervals to decouple underlying environmental mechanisms: a mixed forest transect reflected changes in abiotic and biotic conditions, including forest type (i.e. tabonuco, palo colorado and elfin forests), whereas another transect reflected changes in environmental conditions but not forest type, as its constituent plots were located within palm forest. Based on distributional data (presence versus absence of species), the mixed forest transect exhibited Clementsian structure, whereas the palm forest transect exhibited quasi-Gleasonian structure. In contrast, the distribution of modes in species abundance was random with respect to the latent environmental gradient in the mixed forest transect and clumped with respect to the latent environmental gradient in the palm forest transect. Such contrasts suggest that the environmental factors affecting abundance differed in form or type from those affecting distributional boundaries. Variation among elevational strata with respect to the first axis of correspondence from reciprocal averaging was highly correlated with elevation along each transect, even though axis scores were not correlated between mixed forest and palm forest transects. This suggests that the identity of the environmental characteristics, or the form of response by the fauna to those characteristics, differed between the two elevational transects. Despite the proximity of the transects, the patchy configuration of palm forest, and the pervasive distribution of the dominant palm species, the relative importance of abiotic variables and habitat in structuring gastropod metacommunities differed between transects, which is remarkable and attests to the sensitivity of metacommunity structure to environmental variation.
Fungi are found in all aerobic ecosystems, colonizing a diversity of substrates and performing a wide diversity of functions, some of which are not well understood. Many spices of fungi are cosmopolitan and generalists or habitats. Unusual fungal niches are habitats where extreme conditions would be expected to prevent the development of a mycobiota. In this review we describe five unusual fungal habitats in which fungi occupy poorly understood niches: Antarctic dry valleys, high Arctic glaciers, salt flats and salterns, hypersaline microbial mats and plant trichomes. Yeasts, black yeast-like fungi, melanized filamentous species as well as representatives of Aspergillus and Penicillium seem to be dominant among the mycobiota adapted to cold and saline niches. Plant trichomes appear to be a taxa. The advent of new sequencing technologies is helping to elucidate the microbial diversity in many ecosystems, but more studies are needed to document the functional role of fungi in the microbial communities thriving in these unusual environments.
The growing need to protect stream ecosystems in Puerto Rico requires the development of monitoring procedures that help determine management priorities. Physical habitat assessments have been used to make quick evaluations that are cost efficient and easy conduct, yet they need to be studied further to understand their accuracy at predicting stream health. This study evaluated the efficiency of the Hawaii Stream Visual Assessment Protocol (HSVAP) at determining integrity of streams within the highly urbanized Rio Piedras watershed in Puerto Rico. To validate the protocol we compared results from HSVAP assessments conducted at 16 reaches with water quality and macroinvertebrate data collected at the same sites. Results from linear regressions between the water quality measures and HSVAP scores showed that there was no significant relationships (R2 = 0.48; p = 0.08). This implies that the protocol is not supported by the water quality data. However, results from regressions between macroinvertebrate diversity and the number of families per site showed a significant positive relation with HSVAP scores (R2 = 0.30; p = 0.02; R2 = 0.24; p = 0.05). In addition, a significant negative relation was observed between HSVAP scores and the Family Biotic Index (FBI) (R2 = 0.32; p = 0.02). Comparisons between ratings obtained from the FBI and HSVAP scores suggest that the HSVAP classified sites as having higher quality than the biological metric. Based on these results, it can be concluded that the HSVAP is a good tool for a general assessment of the physical characteristics of a stream, but it needs modifications to accurately assess ecological quality of streams in Puerto Rico.
Species number, functional traits, and phylogenetic history all contribute to characterizing the biological diversity in plant communities. The phylogenetic component of diversity has been particularly difficult to quantify in species-rich tropical tree assemblages. The compilation of previously published (and often incomplete) data on evolutionary relationships of species into a composite phylogeny of the taxa in a forest, through such programs as Phylomatic, has proven useful in building community phylogenies although often of limited resolution. Recently, DNA barcodes have been used to construct a robust community phylogeny for nearly 300 tree species in a forest dynamics plot in Panama using a supermatrix method. In that study sequence data from three barcode loci were used to generate a well-resolved specieslevel phylogeny. Methodology/Principal Findings: Here we expand upon this earlier investigation and present results on the use of a phylogenetic constraint tree to generate a community phylogeny for a diverse, tropical forest dynamics plot in Puerto Rico. This enhanced method of phylogenetic reconstruction insures the congruence of the barcode phylogeny with broadly accepted hypotheses on the phylogeny of flowering plants (i.e., APG III) regardless of the number and taxonomic breadth of the taxa sampled. We also compare maximum parsimony versus maximum likelihood estimates of community phylogenetic relationships as well as evaluate the effectiveness of one- versus two- versus three-gene barcodes in resolving community evolutionary history. Conclusions/Significance: As first demonstrated in the Panamanian forest dynamics plot, the results for the Puerto Rican plot illustrate that highly resolved phylogenies derived from DNA barcode sequence data combined with a constraint tree based on APG III are particularly useful in comparative analysis of phylogenetic diversity and will enhance research on the interface between community ecology and evolution.
Ecology Letters (2010) 13: 1503-1514 ABSTRACT: The phylogenetic structure and distribution of functional traits in a community can provide insights into community assembly processes. However, these insights are sensitive to the spatial scale of analysis. Here, we use spatially explicit, neighbourhood models of tree growth and survival for 19 tree species, a highly resolved molecular phylogeny and information on eight functional traits to quantify the relative efficacy of functional similarity and shared ancestry in describing the effects of spatial interactions between tree species on demographic rates. We also assess the congruence of these results with observed phylogenetic and functional structure in the neighbourhoods of live and dead trees. We found strong support for models in which the effects of spatial neighbourhood interactions on tree growth and survival were scaled to species-specific mean functional trait values (e.g., wood specific gravity, leaf succulence and maximum height) but not to phylogenetic distance. The weak phylogenetic signal in functional trait data allowed us to independently interpret the static neighbourhood functional and phylogenetic patterns. We observed greater functional trait similarity in the neighbourhoods of live trees relative to those of dead trees suggesting that environmental filtering is the major force structuring this tree community at this scale while competitive interactions play a lesser role.
Headwater streams have a demonstrated ability to denitrify a portion of their nitrate (NO3-)load but there has not been an extensive consideration of where in a stream this process is occurring and how various habitats contribute to total denitriﬁcation capability. As part of the Lotic Intersite Nitrogen Experiment II (LINX II) we measured denitriﬁcation potential in 65 streams spanning eight regions of the US and draining three land-use types. In each stream, potential denitriﬁcation rates were measured in common substrate types found across many streams as well as locations unique to particular streams. Overall, habitats from streams draining urban and agricultural land-uses showed higher potential rates of denitriﬁcation than reference streams draining native vegetation. This difference among streams was probably driven by higher ambient nitrate concentrations found in urban or agricultural streams. Within streams, sandy habitats and accumulations of ﬁne benthic organic matter contributed more than half of the total denitriﬁcation capacity (mg N removed m-2h-1). A particular rate of potential denitri-ﬁcation per unit area could be achieved either by high activity per unit organic matter or lower activities associated with larger standing stocks of organic matter. We found that both small patches with high rates (hot spots) or more widespread but less active areas (cool matrix) contributed signiﬁcantly to whole stream denitriﬁcation capacity. Denitriﬁcation estimated from scaled-up denitriﬁcation enzyme assay (DEA) potentials were not always dramatically higher than in situ rates of denitriﬁcation measured as 15N gas generation following 24-h 15N–NO3 tracer additions. In general, headwater streams draining varying land-use types have signiﬁcant potential to remove nitrate via denitriﬁcation and some appear to be functioning near their maximal capacity.
Ecosystems have long been categorized by their function and structure (Odum 1953) and dominance and diversity are among the most common structural parameters measured in plant communities. The dominance of a plant species over another has been defined by having (1) more individual plants in a given sampling plot, (2) more collective biomass, or (3) greater cover (i.e. leaf area), but implicit in all definitions is a better ability to capture resources, grow and compete. Alternatively, diversity is a community-wide parameter encompassing both the raw number of species and the distribution of individual plants among them. Combining dominance and diversity together gives curves that show the distribution of abundances within a community (Whittaker 1965, Wilson 1991) and explore how these two aspects of structure relate to each other, change over time, and compare between communities separated in time or space. These curves are also effective in displaying contrasting patterns of species richness, highlighting differences in evenness among assemblages, and comparing species abundance patterns across communities (Magurran 2004). Indeed combining the dominance of particular species and the relative importance of that species within a community can suggest a ranking in competitive success and niche differentiation.
An extensive survey and topographic analysis of five watersheds draining the Luquillo Mountains in north-eastern Puerto Rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS-based topographic analysis was used to examine channel profiles, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial-alluvial transitions. Non-fluvial processes, such as landslides, deliver coarse boulder-sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fining in the downstream reaches; a pattern associated with a mid-basin transition between colluvial and fluvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid-basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self-forming ‘threshold’ alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach-scale channel morphology, strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin-scale patterns. Copyright © 2010 John Wiley and Sons, Ltd.
Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem-level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole-system nutrient fluxes. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem-level N and P excretion and uptake. Finally, we used a 3-year record of discharge and NH4-N concentration in the high-biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH4-N pool. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. Excretion rates ranged from 0.16–3.80 μmol NH4-N shrimp−1 h−1, 0.23–5.76 μmol total dissolved nitrogen (TDN) shrimp−1 h−1 and 0.002–0.186 μmol total dissolved phosphorus (TDP) shrimp−1 h−1. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH4-N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. Large between-stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 μmol NH4-N m−2 h−1, 37.5 versus 0.26 μmol TDN m−2 h−1 and 1.1 versus 0.015 μmol TDP m−2 h−1), equivalent to 21% and 0.5% of NH4-N uptake and 5% and <0.1% of P uptake measured in the high- and low-biomass stream, respectively. Distances required for excretion to turn over the ambient NH4-N pool varied more than a hundredfold over the 3-year record in the high-shrimp stream, driven by variability in discharge and NH4-N concentration. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community-level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems.
Hurricanes cause canopy removal and deposition of pulses of litter to the forest floor. A Canopy Trimming Experiment (CTE) was designed to decouple these two factors, and to investigate the separate abiotic and biotic consequences of hurricane-type damage and monitor recovery processes. As part of this experiment, effects on forest floor invertebrate communities were studied using litterbags. Canopy opening resulted in increased throughfall, soil moisture and light levels, but decreased litter moisture. Of these, only throughfall and soil moisture had returned to control levels 9 months after trimming. Canopy opening was the major determinant of adverse changes in forest floor invertebrate litter communities, by reducing diversity andbiomass, irrespective of debris deposition, which played a secondary role. Plots subjected to the most disturbance, with canopy removed and debris added, had the lowest diversity and biomass. These two parameters were higher than control levels when debris was added to plots with an intact canopy, demonstrating that increased nutrient potential or habitat complexity can have a beneficial effect, but only if the abiotic conditions are suitable. Animal abundance remained similar over all treatments, because individual taxa responded differently to canopy trimming. Mites, Collembola, and Psocoptera, all microbiovores feeding mainly on fungal hyphae and spores, responded positively,with higher abundance in trimmed plots, whereas all other taxa, particularly predators and larger detritivores, declined in relative abundance. Litterbag mesh size and litter type had only minor effects on communities, and canopy trimming and debris deposition explained most variation between sites. Effects of trimming on diversity, biomass, and abundance of some invertebrate taxa were still seen when observations finished and canopy closure was complete at 19 months. This suggests that disturbance has a long-lasting effect on litter communities and may, therefore, delay detrital processing, depending on the severity of canopy damage and rate of regrowth.
The thirty-year-old United States Long Term Ecological Research Network has developed extensive metadata to document their scientific data. Standard and interoperable metadata is a core component of the data-driven analytical solutions developed by this research network Content management systems offer an affordable solution for rapid deployment of metadata centered information management systems. We developed a customized integrative metadata management system based on the Drupal content management system technology. Building on knowledge and experience with the Sevilleta and Luquillo Long Term Ecological Research sites, we successfully deployed the first two medium-scale customized prototypes. In this paper, we describe the vision behind our Drupal based information management instances, and list the features offered through these Drupal based systems. We also outline the plans to expand the information services offered through these metadata centered management systems. We will conclude with the growing list of participants deploying similar instances.
The trajectory of hurricane-induced succession was evaluated in a network of forest plots measured immediately before and 3 mo, 5, 10, and 15 yr after the direct impact of a Category 4 hurricane. Comparisons of forest structure, composition, and aboveground nutrients pools were made through time, and between species, life-history groups and geomorphic settings. The hurricane reduced aboveground biomass by 50 percent, causing an immediate decrease in stem density and diversity indices among all geomorphic settings. After 15 yr, basal area and aboveground biomass returned to pre-hurricane levels, while species richness, diversity indices, and stem densities exceeded pre-hurricane levels. Differences in species composition among geomorphic settings had not returned after 15 yr but differences in stem densities and structure were beginning to emerge. Significant differences were observed in the nutrient concentration of the three species that comprised the most aboveground biomass, and between species categorized as secondary high-light species and primary, low-light species. Species whose abundance was negatively correlated with the mature forest dominant also had distinct nutrient concentrations. When total aboveground nutrient pools were compared over time, differences in leaf nutrients among species were hidden by similarities in wood nutrient concentrations and the biomass dominance of a few species. The observed successional trajectory indicates that changes in species composition contributed to fast recovery of aboveground biomass and nutrient pools, while the influence of geomorphic setting on species composition occurs at time scales >15 yr of succession.
Emerging new ecosystems are products of human activity. They occur everywhere but particularly in degraded sites and abandoned managed lands. These ecosystems have new species combinations and dominance by invasive species and appear to be increasing in land cover. As new ecosystems emerge on landscapes, issues of social values and attitudes toward alien species and naturalness increase in relevance. Despite their ecological and socioeconomic importance, however, very little empirical information exists about the basic ecology and social relevance of these ecosystems. We propose regional and transcontinental ecological and socioecological research to address questions about the structure, functioning, and ecological services of new ecosystems.
Integrating research and education is a fundamental goal of institutions and agencies supporting science because of the benefits to society of a more informed and scientifically literate population.The value of engaging public interest in ecological research is to maintain support for and integrate science in solutions to environmental problems (Hudson, 2001; Avila, 2003). The National Science Foundation lists criteria for assessing broader impacts of research projects which include (1) the integration ofresearch and educationadvancing discovery and understanding while promoting teaching, training, and learning; and (2)developing opportunities to broaden the participation of groups underrepresented in science (NSF, 2006). Educational researchers are developing models and assessing outcomes of integrating research and education at diverse grade levels (Trautmann and Krasny, 2006; Bowen and Roth, 2007); the value of integratingtraditional ecological knowledge (TEK) in research has been demonstrated in several ecosystems (Huntington, 2000; Kimmerer, 2002; Ford et al., 2007; GarciŽa-Quijano, 2007) but specific approaches and achievements of efforts integrating research and education are not widely disseminated in environmental researchjournals. Thus, while there is a call for environmental scientists to broaden their activities to engage in outreach (i.e., have broader impact) there is a lag in the assessment of the effectiveness of these activities and of their value in mainstream scientific culture. Environmental scientists seldom evaluate these impacts, and there are few venues or incentives to report on these activities in ways that would enhance their research careers. For an individual scientist, efforts expended in integrating research and education often occur at the expense of research productivity and this results in a lack of reward for a researchers efforts to broaden research impacts (Andrews et al., 2005; Uriarte et al., 2007). One way to address the imbalance between efforts devoted to broader impacts vs. avenues for reporting on these efforts is through the publication of case studies and assessments of integration efforts in journals that reach a research audience as opposed to an education audience. This venue exists in a very few, high-profile, broad-interest research journals (e.g., Science, Bioscience) but could be more widespread in journals addressing a range of environmental research. Examples of successful integration help researchers and institutions evolve better mechanisms to achieve goals beneficial to society, including improved public understanding of science, greater diversity of research and stakeholders, and better application of current scientifically based information to managing environmental issues. In that spirit, we present as an example an effort integrating an interdisciplinary research project investigating the interactions of climate, vegetation, and permafrost in the study Biocomplexity of Arctic Tundra Ecosystems with a university field course, ArcticField Ecology, and with indigenous Inuit students and elders. The integration allowed university students and native community members to participate with the research team, drawn by the opportunity to gain education and experience. This participation has had synergistic benefits with the research agenda and diversified the pool of stakeholders involved in the research (see Box 1).
We analyzed results from 10-year long field incubations of foliar and fine root litter from the Long-term Intersite Decomposition Experiment Team (LIDET) study. We tested whether a variety of climate and litter quality variables could be used to develop regression models of decomposition parameters across wide ranges in litter quality and climate and whether these models changed over short to long time periods. Six genera of foliar and three genera of root litters were studied with a 10-fold range in the ratio of acid unhydrolyzable fraction (AUF, or ‘lignin’) to N. Litter was incubated at 27 field sites across numerous terrestrial biomes including arctic and alpine tundra, temperate and tropical forests, grasslands and warm deserts. We used three separate mathematical models of first-order (exponential) decomposition, emphasizing either the first year or the entire decade. One model included the proportion of relatively stable material as an asymptote. For short-term (first-year) decomposition, nonlinear regressions of exponential or power function form were obtained with r2 values of 0.82 and 0.64 for foliar and fine-root litter, respectively, across all biomes included. AUF and AUF : N ratio were the most explanative litter quality variables, while the combined temperature-moisture terms AET (actual evapotranspiration) and CDI (climatic decomposition index) were best for climatic effects. Regressions contained some systematic bias for grasslands and arctic and boreal sites, but not for humid tropical forests or temperate deciduous and coniferous forests. The ability of the regression approach to fit climate-driven decomposition models of the 10-year field results was dramatically reduced from the ability to capture drivers of short-term decomposition. Future work will require conceptual and methodological improvements to investigate processes controlling decadal-scale litter decomposition, including the formation of a relatively stable fraction and its subsequent decomposition.
Reducing atmospheric carbon emissions from tropical deforestation is at present considered a cost-effective option for mitigating climate change. However, the forces associated with tropical forest loss are uncertain1. Here we use satellite-based estimates of forest loss for 2000 to 2005 (ref. 2) to assess economic, agricultural and demographic correlates across 41 countries in the humid tropics. Two methods of analysis—linear regression and regression tree—show that forest loss is positively correlated with urban population growth and exports of agricultural products for this time period. Rural population growth is not associated with forest loss, indicating the importance of urban-based and international demands for agricultural products as drivers of deforestation. The strong trend in movement of people to cities in the tropics is, counter-intuitively, likely to be associated with greater pressures for clearing tropical forests. We therefore suggest that policies to reduce deforestation among local, rural populations will not address the main cause of deforestation in the future. Rather, efforts need to focus on reducing deforestation for industrial-scale, export-oriented agricultural production, concomitant with efforts to increase yields in non-forested lands to satisfy demands for agricultural products.
Rapid increases in nitrogen (N) loading are occurring in many tropical watersheds, but the fate of N in tropical streams is not well documented. Rates of nitrate uptake and denitrification were measured in nine tropical low-order streams with contrasting land use as part of the Lotic Intersite Nitrogen eXperiment II (LINX II) in Puerto Rico using short term (24-hour) additions of K15NO3 and NaBr. Background nitrate concentrations ranged from 105 to 997 μg N/L, and stream nitrate uptake lengths were long, varying from 315 to 8480 m (median of 1200 m). Other indices of nitrate uptake (mass transfer coefficient, Vf [cm/s], and whole-stream nitrate uptake rate, U [μg N·m−2·s−1]) were low in comparison to other regions and were related to chemical, biological, and physical parameters. Denitrification rates were highly variable (0–133 μg N·m−2·min−1; median = 15 μg N·m−2·min−1), were dominated by the end product N2 (rather than N2O), and were best predicted by whole-stream respiration rates and stream NO3 concentration. Denitrification accounted for 1–97% of nitrate uptake with five of nine streams having 35% or more of nitrate uptake via denitrification, showing that denitrification is a substantial sink for nitrate in tropical streams. Whole-stream nitrate uptake and denitrification in our study streams closely followed first-order uptake kinetics, indicating that NO3 uptake is limited by delivery of substrate (NO3) to the organisms involved in uptake or denitrification. In the context of whole-catchment nitrogen budgets, our finding that in-stream denitrification results in lower proportional production of N2O than terrestrial denitrification suggests that small streams can be viewed as the preferred site of denitrification in a watershed in order to minimize greenhouse gas N2O emissions. Conservation of small streams is thus critical in tropical ecosystem management.Read More: http://www.esajournals.org/doi/full/10.1890/09-1110.1
Anthropogenic nitrogen (N) deposition is increasing rapidly in tropical regions, adding N to ecosystems that often have high background N availability. Tropical forests play an important role in the global carbon (C) cycle, yet the effects of N deposition on C cycling in these ecosystems are poorly understood. We used a field N-fertilization experiment in lower and upper elevation tropical rain forests in Puerto Rico to explore the responses of above- and belowground C pools to N addition. As expected, tree stem growth and litterfall productivity did not respond to N fertilization in either of these N-rich forests, indicating a lack of N limitation to net primary productivity (NPP). In contrast, soil C concentrations increased significantly with N fertilization in both forests, leading to larger C stocks in fertilized plots. However, different soil C pools responded to N fertilization differently. Labile (low density) soil C fractions and live fine roots declined with fertilization, while mineral-associated soil C increased in both forests. Decreased soil CO2 fluxes in fertilized plots were correlated with smaller labile soil C pools in the lower elevation forest (R2 = 0.65, p < 0.05), and with lower live fine root biomass in the upper elevation forest (R2 = 0.90, p < 0.05). Our results indicate that soil C storage is sensitive to N deposition in tropical forests, even where plant productivity is not N-limited. The mineral-associated soil C pool has the potential to respond relatively quickly to N additions, and can drive increases in bulk soil C stocks in tropical forests.
Soil turnover by tree uprooting in primary and secondary forests on the island of Puerto Rico was measured in 42 study plots in the months immediately after the passage of a Category 3 hurricane. Trunk basal area explained 61% of the variability of mound volume and 53% of the variability of mound area. The proportion of uprooted trees, the number of uprooted trees, or the proportion of uprooted basal area explained 84–85% of the variation in hurricane-created mound area. These same variables explain 79–85% of the variation in mound volume. The study indicates that the soil turnover period from tree uprooting by Puerto Rican hurricanes is between 1600 and 4800 years. These rates are faster than soil turnover by landslides and background treefall in the same area and provide a useful age constraint on soil profile development and soil carbon sequestration in these dynamic landscapes.
This comprehensive book of more than 400 pages is divided into ten chapters. The first chapter sets the scene, describing the increasing number of publications on fern ecology and showing a departure from an earlier tendency to concentrate more on seed plants. There is a brief explanation of the current phylogeny of vascular plants showing the lineage of lycophytes, seed plants and ferns. The position of what used to be called ‘fern allies’ is indicated, as they are now included in both ferns and lycophytes. More detail is provided in an appendix giving key characters for classes, orders and families. A range of life cycles is described for ferns and lycophytes and comparisons made with seed plants.The second chapter on biogeography discusses dispersal, range size, the incidence of endemism and factors determining species richness. It stresses the need for more fieldwork to support laboratory studies. The third chapter on fern population dynamics deals with spore viability, gametophyte and sporophyte growth. Mature sporophytes have adaptations for colonising different habitats and there is emphasis on the importance of long-term monitoring both of populations and individual fronds on individual plants to further understanding in this area.In the fourth chapter the essential role of ferns in nutrient cycling is explored. In less-fertile areas ferns provide a greater proportion of the biomass compared with seed plants, although they generally have lower concentrations of nitrogen, phosphorus and calcium in their fronds (referred to throughout the book as leaves). The importance of mycorrhizal associates is stressed, together with the symbiosis between species of Azolla and the nitrogen-fixing bacteria Anabaena azollae. The fifth chapter deals with the surprising variety of adaptations that ferns exhibit within xeric habitats, with a greater proportion of desiccation-tolerant ferns when compared with seed-plants. The robust nature of many ferns is continued into the next chapter on disturbance and succession. In the late-Cretaceous extinction ferns were reported to have formed a major proportion of the vegetation that survived. Ferns still play a significant role as first colonisers after natural disasters and anthropogenic activity has provided built structures with new habitats for ferns. However, too-frequent disturbance of natural habitats, all too often associated with human activity, can lead to a decline in species richness. Chapter 7 is a discussion of how fungi and animals interact with ferns. This is another under-researched area with the popular perception that few invertebrates eat ferns, when further examination reveals many more herbivore activity, with areas yet unexplored. Chapter 8 deals in great detail with invasive opportunist ferns that can create problems, with a long section on Pteridium aquilinum, which is not an alien in the UK and has some benefits, although it also causes severe problems. Species that have been introduced outside their natural ranges tend to cause the greatest problems, as with floating mats of Salvinia up to 1 m deep, and native vegetation in Florida being smothered by a species of climbing Lygodium. Chapter 9 is on fern conservation and summarises the work that is currently taking place and measures used to protect vulnerable species. As in so many areas, more work is needed as conservation initiatives only cover a few species. The final chapter brings together the themes that have been explored in the earlier chapters and suggests potential areas of research.The experimental biology of ferns edited by A. F. Dyer (1979) is a precursor to some of the material in Fern ecology. Much of The experimental biology of ferns is laboratory-based, using valid traditional methods but written before molecular methods were developed. Ferns: their habitats in the British and Irish landscape by C. N. Page (1988) mentions many of the themes that have been expanded in Fern ecology, where much recent work is reported. A natural history of ferns by R. C. Moran (2004) is a more contemporary book, written using a more popular, descriptive style utilising similar language and fern classification complementing Fern ecology. The latter is illustrated with a mixture of colour and black and white photographs, frequent, clear graphs and occasional line drawings. This book will be of general interest to those who want to know more about ferns around the world. Despite so much information being presented, it is written in an accessible style with terms usually explained in the text or in the glossary. It will be of especial interest for students as a review of current knowledge and a source of inspiration for those looking for further projects.
The Amazon is the most productive and diverse terrestrial ecosystem in the world and among its importantstructuring forces are flooding gradients and treefall gap dynamics. Consequently, in order to more fully understandAmazonian forests, I sampled Peruvian vegetation plots for five years both in forest and in fresh gaps within each of threeforest types (terra firme, varzea, tahuampa) that differed in flooding duration and maximum water depth. I found a signi-ficant effect of degree of openness (i.e. treefall gap formation) on canopy average height, canopy maximum height, basalarea, density, above-ground biomass, turnover, and alpha diversity, and a significant effect of forest type (i.e. the floodinggradient) on species richness, genera richness, density, turnover, and alpha diversity. In general, there were fewer trees, butthey were larger, and more productive in the forest plots compared with the gap plots; and the most flooded plots hadfewer trees, species, and genera compared with both the less flooded forest and unflooded forest. Also the greatest amountof turnover was found in the most flooded forests, and the intermediately flooded forest had the greatest richness and alphadiversity. These results taken together support a “mass effects” hypothesis where species from both the unflooded and mostflooded forests and their gaps have overlapping ranges in the less flooded forest and gaps, causing continuous immigrationwhich boosts diversity. In summary, (1) canopy structure was determined by traditional gap dynamics, but much of cano-py diversity depended on the type of forest, (2) tree density decreased as flooding increased, especially among the smalleststems, and (3) there was evidence to suggest that the high biodiversity of the Amazon may be maintained in part by theexistence of moderately flooded forest and gaps.
Fungi have gone unnoticed in many prokaryote-dominated ecosystems such as hydrothermal vents and microbial mats. The objectives of this study were to document the diversity of fungi in a subtropical hypersaline microbial mat located at Cabo Rojo, Puerto Rico, and to determine the influence of different seasons on fungal community composition. Terminal restriction fragment length polymorphism (TRFLP) results suggest that when compared to the dry season, the total abundance and richness of phylotypes are higher during the wet season. Moreover, TRFLP data demonstrate that the diversity of fungal phylotypes decreased from the first layer (at a depth of 0–1 mm) to the third layer (at a depth of 2–20 mm). A total of 131 clones from the mat were sequenced. Cloned sequences were identified as species of Acremonium, Aspergillus, Cladosporium and Penicillium. The accumulated data suggest that fungi thrive within this hypersaline environment.
Tropical species with narrow elevational ranges may be thermally specialized and vulnerable to global warming. Local studies of distributions along elevational gradients reveal small-scale patterns but do not allow generalizations among geographic regions or taxa. We critically assessed data from 249 studies of species elevational distributions in the American, African, and Asia-Pacific tropics. Of these, 150 had sufficient data quality, sampling intensity, elevational range, and freedom from serious habitat disturbance to permit robust across-study comparisons. We found four main patterns: (1) species classified as elevational specialists (upper- or lower-zone specialists) are relatively more frequent in the American than Asia-Pacific tropics, with African tropics being intermediate; (2) elevational specialists are rare on islands, especially oceanic and smaller continental islands, largely due to a paucity of upper-zone specialists; (3) a relatively high proportion of plants and ectothermic vertebrates (amphibians and reptiles) are upper-zone specialists; and (4) relatively few endothermic vertebrates (birds and mammals) are upper-zone specialists. Understanding these broad-scale trends will help identify taxa and geographic regions vulnerable to global warming and highlight future research priorities.
Aim To use molecular data to test for dispersal structuring in the immigration history of an amphidromous community on an island.Location The Caribbean island of Puerto Rico.Methods Mitochondrial DNA sequences were obtained from 11 amphidromous species, including shrimps, fish and a gastropod, sampled from throughout the island. The timing of population expansion (TE) in each species was calculated using nucleotide variation and molecular clock dating methods. The order of species accumulation was then reconstructed (oldest to most recent estimate for TE), and groups of species with non-overlapping estimates for TE were identified. The temporal span and average immigration rate for each group were calculated and compared with expectations of two previously published models of island immigration [the ‘dispersal-structured model of island recolonization’ (Whittaker & Jones, Oikos, 1994, 69, 524–529), which predicts short phases of rapid immigration followed by extended phases with relatively slow immigration rates; and the ‘colonization window hypothesis’ (Carine, Taxon, 2005, 54, 895–903), which suggests that opportunities for island colonization are temporally constrained to discrete waves of colonization].Results The molecular data indicated the immigration history of Puerto Rican amphidromous fauna from the late Pleistocene through the Holocene and identified two groups of species with non-overlapping estimates for TE and one group that overlapped with the other two groups. The temporal span, average immigration rate and lack of discreteness between all three groups indicated a continuum of immigration rather than distinct phases of species arrivals.Main conclusions This study did not support the expectations of the immigration models and suggested that amphidromous species from Puerto Rico comprise a single class of marine-based dispersers. The immigration sequence we report probably reflects a recolonization chronology in this community, in keeping with the notion of species turnover through time. Four areas of future research into the immigration history of amphidromous species on islands are identified, and indicated the possibility that equilibrium processes govern long-term community change in amphidromous biota on islands.
Rates of whole-system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first-order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban-influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land-use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land-use categories in the catchment as regulators of ER. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land-use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. Overall, consideration of the data separated by land-use categories showed reduced inter-regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.
Human-impacted forests are increasing in extent due to widespread regrowth of secondary forests on abandoned lands. The degree and speed of recovery from human disturbance in these forests will determine their value in terms of biodiversity conservation and ecosystem function. In areas subject to periodic, severe natural disturbances, such as hurricanes, it has been hypothesized that human and natural disturbance may interact to either erase or preserve land use legacies. To increase understanding of how interactions between human and natural disturbance influence forest regeneration and recovery, we monitored seedlings in a human- and hurricane-impacted forest in northeastern Puerto Rico over a 10-yr period and compared seedling composition and dynamics in areas that had experienced high- and low-intensity human disturbance during the first half of the 20th century. We found that land use history significantly affected the composition and diversity of the seedling layer and altered patterns of canopy openness and seedling dynamics following hurricane disturbance. The area that had been subject to high-intensity land use supported a higher density, but lower diversity, of species. In both land use history categories, the seedling layer was dominated by the same two species, Prestoea acuminata var. montana and Guarea guidonia. However, seedlings of secondary-successional species tended to be more abundant in the high-intensity land use area, while late-successional species were more abundant in the low-intensity area, consistent with patterns of adult tree distributions. Seedlings of secondary-forest species showed greater increases in growth and survival following hurricane disturbance compared to late-successional species, providing support for the hypothesis that hurricanes help preserve the signature of land use history. However, the increased performance of secondary-forest species occurred predominantly in the low-intensity land use area, suggesting that hurricanes act to homogenize differences in species composition between areas with differing land use histories by increasing secondary-forest species regeneration in areas that experienced little direct human disturbance. Our results suggest that, through effects on seedling dynamics, hurricanes may extend the signature of land use history beyond the average recovery time of forests not subject to intense natural disturbance events. Also access article in: http://www.columbia.edu/~mu2126/publications_files/Comita%20et%20al%202010%20EcolApplic.pdf
Estimates of the percent of Earth’s land surface that has either been transformed or degraded by human activity range between 39 and 50 percent, with agriculture accounting for the vast majority of these changes. Although much of the focus of research on land use and cover change in the tropics has been on deforestation, ongoing socioeconomic changes both locally and globally have made land transitions in the tropics extremely fluid. In addition, feedbacks between land cover change and human behavior constrain the extent and trajectories of land transitions. The sustainability of land use systems in the tropics depends on an understanding of coupled human–natural systems that can lead to general frameworks for management and prediction. The unprecedented availability of land use/cover data together with ecological data collected at large spatial scales offer exciting opportunities for advancing our understanding of socioecological systems. We rely on six studies of land transitions in the tropics to illustrate some promising approaches and pose critical questions to guide this body of research.
Introduction. Understanding the geographic and environmental characteristics of islands that affect aspects of biodiversity is a major theme in ecology (Begon et al. 2006; Krebs 2001) and biogeography (Cox and Moore 2000; Drakare et al. 2006; Lomolino et al. 2006). Such understanding has become particularly relevant over the past century because human activities on continents have fragmented natural landscapes, often creating islands of isolated habitat dispersed within a sea of land uses that include agriculture, forestry, and various degrees of urban and suburban development. The increasingly fragmented or islandlike structure of mainland habitats has critical ramifications to conservation biology, as it provides insights regarding the mechanisms leading to species persistence and loss. Consequently, the study of patterns and mechanisms associated with island biodiversity is of interest in its own right Whittaker 1998; Williamson 1981), and may provide critical insights into mainland phenomena that otherwise could not be studied because of ethical, financial, or logistical considerations involved with the execution of large-scale manipulative experiments.
Rapidly fluctuating environmental conditions can significantly stress organisms, particularly when fluctuations cross thresholds of normal physiological tolerance. Redox potential fluctuations are common in humid tropical soils, and microbial community acclimation or avoidance strategies for survival will in turn shape microbial community diversity and biogeochemistry. To assess the extent to which indigenous bacterial and archaeal communities are adapted to changing in redox potential, soils were incubated under static anoxic, static oxic or fluctuating redox potential conditions, and the standing (DNA-based) and active (RNA-based) communities and biogeochemistry were determined. Fluctuating redox potential conditions permitted simultaneous CO₂ respiration, methanogenesis, N₂O production and iron reduction. Exposure to static anaerobic conditions significantly changed community composition, while 4-day redox potential fluctuations did not. Using RNA:DNA ratios as a measure of activity, 285 taxa were more active under fluctuating than static conditions, compared with three taxa that were more active under static compared with fluctuating conditions. These data suggest an indigenous microbial community adapted to fluctuating redox potential.
A new species of Neotropical Psychodidae, Alepia apexalba sp. nov., is described from dry-forest tank bromeliads in Saba, Netherlands Antilles. Larvae, pupae and adults are described and figured. It is possible to relate larvae and adults because the latter were reared from pupae collected from the bromeliads (Tillandsia utriculata) that also contained larvae. Bromeliads are adapted to intercept canopy litter and throughfall water, and decaying litter is washed into and retained by the leaf bases. It is from this aquatic habitat that the larvae and pupae were collected.
In this review I focus on the socially and ecologically important western Amazon basin and its associated plant communities. I delineate this vast area as between the Andes to the west and the confluence of the Amazon and its first major black-water river, the Rio Negro, to the east. Although scientists have explored here, and local people have lived here for years, we still have—unfortunately—only a most basic understanding of its plant communities. This review is motivated by that lack of knowledge, and attempts both to add a level of organization to what we do know and to suggest future avenues of research. I do this by first realizing that plant communities here must be fundamentally differentiated by the degree of flooding they experience, the kind of water involved, and how regularly that flooding occurs. Within that context, plant communities can then be further defined by such characteristics as soil type, micro-topological relief, and human-induced disturbance regime. After completing the review, I suggest that the diversity of plant communities, not just the plants themselves, is large and likely to grow substantially as we sample more and more, that is to say the peak of the plant community-sampling area curve has not yet been reached. I close with basic questions to help guide future efforts, ideas on how plant communities could be defined quantitatively, and a call for more research funding of the Western Amazon.
Nitrogen (N) deposition is projected to increase significantly in tropical regions in the coming decades, where changes in climate are also expected. Additional N and warming each have the potential to alter soil carbon (C) storage via changes in microbial activity and decomposition, but little is known about the combined effects of these global change factors in tropical ecosystems. In this study, we used controlled laboratory incubations of soils from a long-term N fertilization experiment to explore the sensitivity of soil C to increased N in two N-rich tropical forests. We found that fertilization corresponded to significant increases in bulk soil C concentrations, and decreases in C loss via heterotrophic respiration (P< 0.05). The increase in soil C was not uniform among C pools, however. The active soil C pool decomposed faster with fertilization, while slowly cycling C pools had longer turnover times. These changes in soil C cycling with N additions corresponded to the responses of two groups of microbial extracellular enzymes. Smaller active C pools corresponded to increased hydrolytic enzyme activities; longer turnover times of the slowly cycling C pool corresponded to reduced activity of oxidative enzymes, which degrade more complex C compounds, in fertilized soils. Warming increased soil respiration overall, and N fertilization significantly increased the temperature sensitivity of slowly cycling C pools in both forests. In the lower elevation forest, respired CO2 from fertilized cores had significantly higher Δ14C values than control soils, indicating losses of relatively older soil C. These results indicate that soil C storage is sensitive to both N deposition and warming in N-rich tropical soils, with interacting effects of these two global change factors. N deposition has the potential to increase total soil C stocks in tropical forests, but the long-term stability of this added C will likely depend on future changes in temperature.
Many field indicators have been used to approximate the magnitude and frequency of flows in a variety of streams and rivers, yet due to a scarcity of long-term flow records in tropical mountain streams, little to no work has been done to establish such relationships between field features and the flow regime in these environments. Furthermore, the transition between the active channel of a river and the adjacent flood zone (i.e. bankfull) is an important geomorphologic and ecological boundary, but is rarely identifiable in steep mountain channels that lack alluvial flood plains. This study (a) quantifies relationships between field indicators and flow frequency in alluvial and steepland channels in a tropical mountain stream network and (b) identifies a reference active channel boundary in these channels, based on statistically defined combinations of riparian features, that corresponds to the same flow frequency of the bankfull stage and the effective discharge in adjacent alluvial channels. The relative elevation of transitions in riparian vegetation, soil, and substrate characteristics were first surveyed at nine stream gages in and around the Luquillo Experimental Forest in Northeastern Puerto Rico. The corresponding discharge, flow frequency, and recurrence intervals associated with these features was then determined from long-term 15-min discharge records and a partial duration series analysis. Survey data indicate that mosses and short grasses dominate at a stage often inundated by sub-effective flows. Herbaceous vegetation is associated with intermediate discharges that correspond to the threshold for sediment mobilization. Near-channel woody shrubs and trees establish at elevations along the channel margin inundated by a less frequent discharge that is coincident with the effective discharge of bed load sediment transport. Our data demonstrate that in alluvial channels in the study, both the bankfull stage (as marked by a flood plain) and the channel-forming (effective) discharge are associated with the presence of fine-grained substrate and soil, and tall, mature woody vegetation. In montane reaches that lack a flood plain, a boundary that is characterized by the incipient presence of soil, woody shrubs, and trees corresponds to the same flow frequency as the bankfull discharge of nearby alluvial channels. The reference discharge based on these riparian features in steepland sites has an average exceedance probability between 0.09% and 0.30%, and a recurrence interval between 40 and 90 days. We conclude that flows with similar frequencies influence the establishment of riparian vegetation, soil development, and substrate characteristics along channel margins in similar ways. Thus, these riparian features can be used as an indicator of hydrogeomorphic site conditions to identify active channel boundaries that occur at a constant flow frequency throughout the study stream network.
Producing cellulosic biofuels from plant material has recently emerged as a key US Department of Energy goal. For this technology to be commercially viable on a large scale, it is critical to make production cost efficient by streamlining both the deconstruction of lignocellulosic biomass and fuel production. Many natural ecosystems efficiently degrade lignocellulosic biomass and harbor enzymes that, when identified, could be used to increase the efficiency of commercial biomass deconstruction. However, ecosystems most likely to yield relevant enzymes, such as tropical rain forest soil in Puerto Rico, are often too complex for enzyme discovery using current metagenomic sequencing technologies. One potential strategy to overcome this problem is to selectively cultivate the microbial communities from these complex ecosystems on biomass under defined conditions, generating less complex biomass-degrading microbial populations. To test this premise, we cultivated microbes from Puerto Rican soil or green waste compost under precisely defined conditions in the presence dried ground switchgrass (Panicum virgatum L.) or lignin, respectively, as the sole carbon source. Phylogenetic profiling of the two feedstock-adapted communities using SSU rRNA gene amplicon pyrosequencing or phylogenetic microarray analysis revealed that the adapted communities were significantly simplified compared to the natural communities from which they were derived. Several members of the lignin-adapted and switchgrass-adapted consortia are related to organisms previously characterized as biomass degraders, while others were from less well-characterized phyla. The decrease in complexity of these communities make them good candidates for metagenomic sequencing and will likely enable the reconstruction of a greater number of full-length genes, leading to the discovery of novel lignocellulose-degrading enzymes adapted to feedstocks and conditions of interest.
Land cover transformations in the tropics are not limited to deforestation; they include other complex transitions such as agricultural and urban expansion, pasture development, and secondary vegetation regrowth. Understanding the causes and extent of these highly variable and complex transitions requires close collaboration between biological, physical, and social scientists. Here we address three critical issues in the study of land transitions: (1) What methodological and socioecological criteria should be used for characterizing land cover categories and transformations? Results from case studies presented here call for the creation of continuous land cover classes that allow for detection of disturbance and human use dynamics and consideration of socioeconomic and biophysical criteria in characterizing and monitoring land transitions. (2) What are the most promising theoretical frameworks? Successful theoretical frameworks must bridge disciplinary boundaries, and encompass multiple spatial, temporal, and political scales. (3) Are regime shifts, constraints, and resilience of land transformations in the tropics predictable? Resilience of land use systems requires a feedback loop between ecological constraints and management decisions. This loop may be broken by policies, migration, and flow of capital from global commodity markets. In addition, land transformations may lead to novel interactions between land-use and natural disturbance leading to unpredictable regime shifts in ecosystems. Planning for sustainable patterns of land use requires some understanding of these regime transformations.
We report that iron-reducing bacteria are primary mediators of anaerobic carbon oxidation in upland tropical soils spanning a rainfall gradient (3500–5000 mm/yr) in northeast Puerto Rico. The abundant rainfall and high net primary productivity of these tropical forests provide optimal soil habitat for iron-reducing and iron-oxidizing bacteria. Spatially and temporally dynamic redox conditions make iron-transforming microbial communities central to the belowground carbon cycle in these wet tropical forests. The exceedingly high abundance of iron-reducing bacteria (up to 1.2 × 109 cells per gram soil) indicated that they possess extensive metabolic capacity to catalyze the reduction of iron minerals. In soils from the higher rainfall sites, measured rates of ferric iron reduction could account for up to 44% of organic carbon oxidation. Iron reducers appeared to compete with methanogens when labile carbon availability was limited. We found large numbers of bacteria that oxidize reduced iron at sites with high rates of iron reduction and large numbers of iron reducers. The coexistence of large populations of iron-reducing and iron-oxidizing bacteria is evidence for rapid iron cycling between its reduced and oxidized states and suggests that mutualistic interactions among these bacteria ultimately fuel organic carbon oxidation and inhibit CH4 production in these upland tropical forests.
One of the most significant challenges in developing a predictive understanding of the long-term effects of hurricanes on tropical forests is the development of quantitative models of the relationships between variation in storm intensity and the resulting severity of tree damage and mortality. There have been many comparative studies of interspecific variation in resistance of trees to wind damage based on aggregate responses to individual storms. We use a new approach, based on ordinal logistic regression, to fit quantitative models of the susceptibility of a tree species to different levels of damage across an explicit range of hurricane intensity. Our approach simultaneously estimates both the local intensity of the storm within a plot and the susceptibility to storm damage of different tree species within plots. Using the spatial variation of storm intensity embedded in two hurricanes (Hugo in 1989 and Georges in 1998) that struck the 16 ha Luquillo Forest Dynamics Plot in eastern Puerto Rico, we show that variation in susceptibility to storm damage is an important aspect of life history differentiation. Pioneers such as Cecropia schreberiana are highly susceptible to stem damage, while the late successional species Dacryodes excelsa suffered very little stem damage but significant crown damage. There was a surprisingly weak relationship between tree diameter and the susceptibility to damage for most of the 12 species examined. This may be due to the effects of repeated storms and trade winds on the architecture of trees and forest stands in this Puerto Rican subtropical wet forest.
In an energy-scarcefuture,ecosystem services will become moreimportant in supporting the human economy. The primary role of ecology will be the sustainable management of ecosystems. Energy scarcity will afect ecology in a number of ways. Ecology will become more expensive,which will be justified by its help in solving societal problems,especially in maintaining ecosystem services. Applied research on highly productive ecosystems,including agroecosystems,will dominate ecology. Ecology may become less collegial and morecompetitive. Biodiversity preservation will beclosely tied to preservation of productive ecosystems and provision of high ecosystem services. Restoration and management of rich natural ecosystems will be as important as protection of existing wild areas. Energy-intensivemicromanagement of ecosystems will become less feasible. Ecotechnology and,more specifically,ecological engineering and self-design are appropriate bases for sustainable ecosystem management. We usethe Mississippi River basin as a casestudy forecology in times of scarcity.
Aim: Many ecological surveys record only the presence or absence of species in the cells of a rectangular grid. Ecologists have investigated methods for using these data to predict the total abundance of a species from the number of grid cells in which the species is present. Our aim is to improve such predictions by taking account of the spatial pattern of occupied cells, in addition to the number of occupied cells. Innovation: We extend existing prediction models to include a spatial clustering variable. The extended models can be viewed as combining two macroecological regularities, the abundance–occupancy regularity and a spatial clustering regularity. The models are estimated using data from ﬁve tropical forest censuses, including three Panamanian censuses (4, 6 and 50 ha), one Costa Rican census (16 ha) and one Puerto Rican census (16 ha). A serpentine grassland census (8 × 8 m) from northern California is also studied. Main conclusions: Taking account of the spatial clustering of occupied cells improves abundance prediction from presence–absence data, reducing the mean square error of log-predictions by roughly 54% relative to a benchmark Poisson predictor and by roughly 34% relative to current prediction methods. The results have high statistical signiﬁcance.
— Riparian zones are dynamic areas adjacent to flowing freshwater that connect aquatic and terrestrial environments. We measured individual tree distances relative to two permanent streams in the Luquillo Forest Dynamics Plot, Puerto Rico, to determine if tree species exhibit distinct affinities for riparian zones in a mid-montane tropical forest. In addition, we also calculated stem density, species richness, and diversity indices in 20 x 20 m randomly selected quadrats at different distances from the streams. We found that no single species in the study site was predominantly associated with riparian zones; however, almost all species were represented by a few stems in close proximity to streams. Multivariate (Fuzzy Set Ordination) ordination of plot species composition in relation to distance to streams indicated that there is greater variation in species composition closer to streams, and less variation in species composition farther from streams. This study suggests that in mid-montane wet forest, riparian zones include all tree species found throughout the forest landscape.
Landslides are excellent illustrations of the dynamic interplay of disturbance and succession. Restoration is difficult on landslide surfaces because of the high degree of spatial and temporal heterogeneity in soil stability and fertility. Principles derived from more than a century of study of ecological succession can guide efforts to reduce chronic surface soil erosion and restore both biodiversity and ecosystem function. Promotion of the recovery of self-sustaining communities on landslides is feasible by stabilization with native ground cover, applications of nutrient amendments, facilitation of dispersal to overcome establishment bottlenecks, emphasis on functionally redundant species and promotion of connectivity with the adjacent landscape. Arrested succession through resource dominance by a single species can be beneficial if that species also reduces persistent erosion, yet the tradeoff is often reduced biodiversity. Restoration efforts can be streamlined by using techniques that promote successional processes.
Chemical characterization was performed on cloud and rainwater samples collected as part of the Rain In Cumulus over the Ocean Experiment (RICO). This experiment took place at a mountaintop site (East Peak) in Puerto Rico from December 2004 to March 2007 in order to determine water-soluble organic and nitrogen fractions in a marine background environment. For cloud water, similar average concentrations of 1.0 (±0.3) mg/L were found for total organic carbon (TOC) and total nitrogen (TN) and an average concentration of 0.8 (±0.2) mg/L was found for dissolved organic carbon (DOC). In rainwater, these concentrations were lower, ranging from 0.3 to 0.5 (±0.1) mg/L. Changes in the concentrations of these species were observed in periods under the influence of anthropogenic, African dust, and volcanic ash air masses. In these periods the concentrations of TOC, DOC, and TN were 2 to 4 times higher than in periods under the influence of trade winds. The insoluble organic material arriving during African dust events showed total carbon (TC) concentrations on averaging 1.5 mg/L for cloud water. The TC was composed mainly of organic carbon with polar compounds from low to high molecular weight (MW). The polar compounds with high MW were probably associated with pollution (e.g., fossil fuel combustion) from other regions. Crustal species (A1 and Fe) dominated particles associated with dust episodes, confirming the soil origin. Our results suggested that a fraction (40-80%) of TOC and (<100%) of TN in Puerto Rican cloud/rainwater could be originated from long-range transport of dust, ash and/or pollution.
Aim: It is increasingly accepted that the mean wood density of trees within a forest is tightly coupled to above-ground forest biomass. It is unknown, however, if a positive relationship between forest biomass and mean community wood density is a general phenomenon across forests. Understanding spatial variation in biomass as a function of wood density both within and among forests is important for predicting changes in stored carbon in response to global change, and here we evaluated the generality of a positive biomass–wood density relationship within and among six tropical forests. Location Costa Rica, Panama, Puerto Rico and Ecuador. Methods: Individual stem data, including diameter at breast height and spatial position, for six forest dynamics plots were merged with an extensive wood density database. Individual stem biomass values were calculated from these data using published statistical models. Total above ground biomass, total basal area and mean community wood density were also quantified across a range of subcommunity plot sizes within each forest. Results: Among forests, biomass did not vary with mean community wood density. The relationship between subcommunity biomass and mean wood density within a forest varied from negative to null to positive depending on the size of subcommunities and forest identity. The direction of correlation was determined by the associated total basal area–mean wood density correlation, the slope of which increased strongly with whole forest mean wood density. Main conclusions: There is no general relationship between forest biomass and wood density, and in some forests, stored carbon is highest where wood density is lowest. Our results suggest that declining wood density, due to global change, will result in decreased or increased stored carbon in forests with high or low mean wood density, respectively.
As part of the Rain In Cumulus over the Ocean Experiment (RICO) and the Puerto Rico Aerosol and Cloud Study (PRACS), cloud water was collected at East Peak (EP) in Puerto Rico. The main objective of this study was to determine the concentrations of water-soluble species (Cl−, NO3 −, SO4 2−, NH4 +, Ca2+, H+, Mg2+, K+, and Na+) in water samples taken from clouds influenced by tropical trade winds. The most abundant inorganic species were Na+ (average 465 µeq l−1) and Cl− (434 µeq l−1), followed by Mg2+ (105 µeq l−1), SO4 2− (61 µeq l−1), and NO3 − (25 µeq l−1). High concentrations of nss–SO4 2 (28 µeq l−1), NO3 − (86 µeq l−1), and H+ (14.5 µeq l−1) were measured with a shift in air masses origin from the North Atlantic to North American continent, which reflected a strong anthropogenic influence on cloud chemistry at EP. Long-range transport of particles and acid gases seems to be the factor responsible for fluctuations in concentrations and pH of cloud water at East Peak. When under trade wind influences the liquid phase concentrations of all inorganic substances were similar to those found in clouds in other clean maritime environments.
Decomposition is a critical process in global carbon cycling. During decomposition, leaf and fine root litter may undergo a later, relatively slow phase; past long-term experiments indicate this phase occurs, but whether it is a general phenomenon has not been examined. Data from Long-term Intersite Decomposition Experiment Team, representing 27 sites and nine litter types (for a total of 234 cases) was used to test the frequency of this later, slow phase of decomposition. Litter mass remaining after up to 10 years of decomposition was fit to models that included (dual exponential and asymptotic) or excluded (single exponential) a slow phase. The resultant regression equations were evaluated for goodness of fit as well as biological realism. Regression analysis indicated that while the dual exponential and asymptotic models statistically and biologically fit more of the litter type–site combinations than the single exponential model, the latter was biologically reasonable for 27–65% of the cases depending on the test used. This implies that a slow phase is common, but not universal. Moreover, estimates of the decomposition rate of the slowly decomposing component averaged 0.139–0.221 year−1 (depending on method), higher than generally observed for mineral soil organic matter, but one-third of the faster phase of litter decomposition. Thus, this material may be slower than the earlier phases of litter decomposition, but not as slow as mineral soil organic matter. Comparison of the long-term integrated decomposition rate (which included all phases of decomposition) to that for the first year of decomposition indicated the former was on average 75% that of the latter, consistent with the presence of a slow phase of decomposition. These results indicate that the global store of litter estimated using short-term decomposition rates would be underestimated by at least one-third.
The input of phosphorus (P) through mineral aerosol dust deposition may be an important component of nutrient dynamics in tropical forest ecosystems. A new dust deposition calculation is used to construct a broad analysis of the importance of dust-derived P to the P budget of a montane wet tropical forest in the Luquillo Mountains of Puerto Rico. The dust deposition calculation used here takes advantage of an internal geochemical signal (Sr isotope mass balance) to provide a spatially integrated longer-term average dust deposition flux. Dust inputs of P (0.23 ± 0.08 kg ha-1 year-1) are compared with watershed-average inputs of P to the soil through the conversion of underlying saprolite into soil (between 0.07 and 0.19 kg ha-1 year-1), and with watershed-average losses of soil P through leaching (between 0.02 and 0.14 kg ha-1 year-1) and erosion (between 0.04 and 1.38 kg ha-1 year-1). The similar magnitude of dust-derived P inputs to that of other fluxes indicates that dust is an important component of the soil and biomass P budget in this ecosystem. Dust-derived inputs of P alone are capable of completely replacing the total soil and biomass P pool on a timescale of between 2.8 ka and 7.0 ka, less than both the average soil residence time (*15 ka) and the average landslide recurrence interval (*10 ka).
1. Many forests experience periodic, large-scale disturbances, such as hurricanes and cyclones, which open the forest canopy, causing dramatic changes in understorey light conditions and seedling densities. Thus, in hurricane-impacted forests, large variations in abiotic and biotic conditions likely shape seedling dynamics, which in turn will contribute to patterns of forest recovery. 2. We monitored 13 836 seedlings of 82 tree and shrub species over 10 years following Hurricane Georges in 1998 in a subtropical, montane forest in Puerto Rico. Wequantified changes in the biotic and abiotic environment of the understorey and linked seedling dynamics to changes in canopy openness and seedling density, and to spatial variation in soil type, topography and tree density. 3. Canopy openness was highest when first measured after Hurricane Georges and dropped significantly within c. 3 years, while seedling densities remained high for c. 5 years post-hurricane. When all species and census intervals were analysed together, generalized linear mixed effects models revealed that canopy openness, seedling and adult tree densities were significant drivers of seedling survival. 4. The relative importance of abiotic and biotic factors changed over time. Separate analyses for each census interval revealed that canopy openness was a significant predictor of survival only for the first census interval, with lower survival at the highest levels of canopy openness. The effect of conspecific seedling density was significant in all intervals except the first, and soil type only in the final census interval. 5. When grouping species into life-history guilds based on adult tree susceptibility to hurricane damage, we found clear differences among guilds in the effects of biotic and abiotic factors on seedling survival. Seedlings of hurricane-susceptible and intermediate guilds were more strongly influenced by canopy openness, while seedlings of the hurricane-resistant group were less affected by conspecific seedling density. Individual species-level analyses for 12 common species, however, showed considerable variation among species within guilds. 6. Synthesis. Our results suggest that hurricanes shape species composition by altering understorey conditions that differentially influence the success of seedlings. Thus, predicted increases in the intensity and frequency of hurricanes in the Caribbean will likely alter seedling dynamics and ultimately the species composition in hurricane-impacted forests.
In the preface of a recent collection of review articles on tropical stream ecology, Dudgeon (2008) stated that there is no such thing as a “typical” tropical stream. The tropics make up the area of the globe between lat 23°N and 23°S, and include a great variety of climatic, geologic, and geomorphologic conditions (Boulton et al. 2008). Thus, tropical streams can flow through landscapes as varied as evergreen rain forests, deciduous seasonal forests, high-altitude grasslands, or even deserts. This diversity suggests that generalizations about tropical streams might be difficult to come by, but it also indicates that much is to be learned about stream ecology in tropical regions. Several major obstacles hinder the study of tropical streams. An obvious gap is our limited knowledge of their benthic faunas. European, North American, and, to a lesser extent, Australian and New Zealand stream invertebrates have been studied extensively and are well known, but this is not the case for most tropical stream invertebrates. Many insect larval stages have not been related to adults, and identification to species is not possible. Their life histories are unknown, but are often assumed (without good reason) to be similar to those of related temperate taxa. For example, certain traits, such as feeding habits, can differ among close relatives at different latitudes. Baetids and leptophlebiids (Ephemeroptera) are generally scrapers or collector-gatherers in temperate streams, but the baetids, Acanthiops from Kenya and Andesiops from Bolivia, and the leptophlebiids, Atalophlebia from the Australian Wet Tropics and Barba from Papua New Guinea, are shredders (Yule 1996, Dobson et al. 2002, Molina 2004, Cheshire et al. 2005). Studies of tropical streams have been restricted to intense activity by a small number of research groups in a few geographic regions, particularly in Costa Rica, Hong Kong, Kenya, Puerto Rico, Queensland, and Venezuela, although some important work has been done elsewhere. This geographic limitation constrains our ability to understand tropical regions in general. Moreover, it highlights the need for effective communication among dispersed groups of tropical researchers and between workers in tropical and temperate latitudes. Publication of compendia on tropical stream ecology, an activity that the Journal of the North American Benthological Society (J-NABS) has pioneered, is a powerful tool for enhancing communication and stimulating research in the tropics. The 1stJ-NABS special issue on tropical streams was published 20 y ago. It focused on unifying approaches to the study of streams in different biomes (Stanford and Covich 1988) and included topics such as spatial and temporal scales of patchiness and disturbance. The papers in the series mainly reviewed available data from the tropics and emphasized the need for a global perspective when constructing theories for the organization of stream ecosystems (Minshall 1988). The 2ndJ-NABS special issue on tropical streams was published in 1995 (Jackson and Sweeney 1995). It focused on descriptive research and included papers on invertebrate taxonomy and life histories, nutrient dynamics, pesticides, and gene flow in invertebrate populations. Only 2 papers, one on leaf-litter processing rates (Campbell and Fuchshuber 1995) and one on disturbance and recolonization of stony substrata (Rosser and Pearson 1995), focused on ecological processes. The range of topics reflected efforts to arrive at a broader understanding of tropical streams, but highlighted the limited amount of information that was applicable at the ecosystem level. The 3rdJ-NABS special issue on tropical streams was “New vistas in Neotropical stream ecology” and was published in 2006 (Wantzen et al. 2006). It included studies undertaken at sites in Central and South America and the Caribbean, and embraced an array of topics, including caddisfly biology, organic-matter processing, algal biomass, invertebrate distribution, fish biogeography, and ecological assessment. This issue was concerned solely with the Neotropics, but it demonstrated that understanding of tropical stream ecology had progressed substantially. This 4th compendium of tropical stream studies arose from a special session, “Are tropical streams ecologically different?,” during the 54th annual meeting of the North American Benthological Society (2006; Anchorage, Alaska, USA). The goals of the session were to present novel patterns and notions on the functioning of tropical streams with a special emphasis on energy sources and pathways fueling the ecosystem and the consumers they support and to provide a broad geographical context that would allow comparisons among different tropical areas. This strategy, it was hoped, would yield some generalizations about tropical stream ecosystem processes. This special issue includes most of the research presented at that session and some additions. Studies represent a wide array of tropical streams, including those in Central America (Costa Rica), South America (Venezuela), Asia (Hong Kong and Peninsular Malaysia), Africa (Madagascar), and the Pacific islands (Micronesia), and some subtropical streams (northern New South Wales, Australia). It includes an analysis of global-scale latitudinal patterns in freshwater biodiversity. Given the small number of studies making up this issue, the extent to which they reveal novel and unexpected patterns is surprising. They confirm the variability of conditions and environments in streams within the tropics and underscore the need for further studies of streams from all tropical regions. Such investigations should include basic taxonomic work, autecological studies, and elucidation of ecological processes and interactions, including foodweb structure and dynamics.
We investigated Ca and Sr cycling in a humid tropical forest by analyzing Ca/Sr ratios and 87Sr/86Sr ratios in soil minerals, soil exchangeable cations, soil porewater, and plant roots, wood and leaves, and calculating the relative contributions of Sr from atmospheric inputs and weathering of local bedrock. An unexpectedly large contribution of bedrock-derived Sr and presumably Ca is cycled through the vegetation, reflecting the important role of geological processes in controlling the cycling of base cation nutrients even in a system with intensely weathered soil. This is surprising because over 99% of the Ca and Sr that was originally in the bedrock is leached out of the soil and saprolite during early stages of weathering at this site, and because there are large atmospheric inputs to the site of both sea salt and Saharan dust. Substantial differences in Ca and Sr cycling are seen on small spatial scales between a ridgetop and an adjacent steep hillslope site. Measured Ca/Sr ratios reflect fractionation between these elements during biogeochemical cycling. Fractionation was particularly evident between wood and foliar tissue, but fractionation during soil exchange processes is also likely. In comparing the Ca/Sr ratios of plants, exchangeable cations, and bulk soils, we found that foliar Ca/Sr ratios were greater than exchangeable cation Ca/Sr ratios, which in turn were greater than soil Ca/Sr ratios, similar to patterns observed at other highly weathered tropical sites.