The foundations of the LUQ program stem from eight decades of research on forest and stream composition, growth, and management by Forest Service scientists as well as 30 years of work on biogeochemical cycles supported by the Department of Energy and its predecessor the Atomic Energy Commission. Results from these studies stimulated the formation of the LUQ and its focus on long-term measurements of forest and stream response to natural and anthropogenic disturbance. These core measurements are complemented by hypothesis-driven experiments focused on improving our understanding of the cumulative effects of multiple disturbances across a steep gradient in climate. Products from the first 32 years of research in the Luquillo LTER include 997 peer-reviewed publications, 21 books and special features, 208 book chapters, and 100 dissertations and theses.
Below you will find a short summary of the research approaches LUQ uses to address ecosystem change in the Luquillo Experimental Forest (LEF).
Long-term monitoring in LUQ takes place in focused field studies that promote collaboration among the diverse disciplines represented by our research group and enable efficient use of available resources. In light of the recent disturbance by Hurricane Maria, our goals for LUQ VI are to focus our early efforts on deciphering the short-term (current funding and Year 1 of the new funding cycle) impacts of the storm and responses of forest and stream ecosystems (Question I) and how these compare with previous severe storms (Walker et al 1996, Brokaw et al. 2012). In Year 2 of funding, we will return to the emphasis established in LUQ V to determine the long-term impacts of increased drying and drought (Question 2), but now in the context of increased hurricane frequency. Meanwhile, we continue to refine our mechanistic understanding of how global and regional changes in climate (Question III) drive local changes in climate and ecosystems.
3.1 Long-term Measurements and Experiments in Tabonuco Forest
We are conducting a set of complementary, long-term measurements of environmental, biotic, and system properties designed to reveal the relationships between disturbance and response in the Luquillo Mountains. Two key study areas are the Luquillo Forest Dynamics Plot (LFDP; Fig. 3.1) and the Bisley Experimental Watersheds (BEW) located in tabonuco forest (200 – 600 m asl) on leeward and windward sides of the LEF, respectively. In the 16-ha LFDP, we have followed the dynamics of >110,000 woody plant stems ≥1.0 cm dbh over three decades (Thompson et al. 2002, Hogan et al. 2016a,b). The study of species-rich communities is facilitated by large contiguous plots like the LFDP, which allow study of the mechanisms that promote species diversity and coexistence (Zimmerman et al. 2008, Anderson-Teixeira et al. 2015). We perform spatially explicit censuses of trees, shrubs, seedlings (since 1999), and phenology/seed rain (since 1992) at time intervals relevant to their dynamics (Uriarte et al. 2012). Measurements of diameter increments of living trees using dendrometers and annual measurements of coarse woody debris (CWD) allow us to detect and project long-term climate effects (Feng et al. 2018; see Hypotheses 1, 2, 4, and 5) and to estimate above and belowground C storage in the plot (Lodge et al. 2016). Data on the spatial distribution of leaf litter can also inform expected litter inputs to streams (Uriarte et al. 2015; see Hypotheses 3 and 6). The abundance of key heterotrophs, including gastropods, phasmids, lizards, frogs, and birds are measured annually at forty locations in the LFDP (e.g. Bloch et al. 2007, Willig et al. 2007, Prates et al. 2015). The next full census of the LFDP is in Year 3 of LUQ VI (2021). We are assessing tree damage ( >10 cm dbh) from Hurricane Maria and monitoring seedling recruitment via RAPID funding.
Stream Monitoring is conducted in three streams of the BEW and in three streams in the El Verde Research Area: Quebradas Sonadora, Toronja, and Prieta. Chemistry is measured weekly in these streams (since 1983), as well as in additional streams gauged by the USGS. In the Q. Prieta we monitor decapod shrimps weekly (since 1988). In two BEW streams and the Q. Prieta and Toronja, we also monitor decapod shrimp biannually in pools (since 1988). Aquatic insects, algal standing crop (chlorophyll a), organic (AFDM) and inorganic deposition are measured biannually in pools and riffles of the Q. Prieta and one BEW stream (since 2009 for insects and 2003 for algae). The upper section of Prieta is where the Stream Flow Reduction Experiment is located (see below).
The Canopy Trimming Experiment (CTE) was initiated in 2002 in the El Verde Research Area. The first trimming treatments were designed to separate canopy opening and debris deposition, the two principal effects of hurricane disturbance to our forests (Zimmerman et al. 1994, Shiels & González 2014). This long-term experiment has allowed us to separate the role of microclimate, detrital inputs, and different functional groups of decomposers on detrital processing and ecosystem resilience after canopy disturbance (Shiels et al. 2015). Through once-a-decade repeated canopy manipulations, the CTE will also allow us to assess the effects of a projected increased frequency of intense hurricanes (Knutson et al. 2010) on forest composition, soil C storage, nutrient dynamics (Sanford et al. 1991, Gutiérrez del Arroyo & Silver 2018), population dynamics, and trophic structure (see Hypotheses 1 and 2). Each treatment covers a 30 x 30 m area with an inner 20 x 20 m measurement plot. The first experiment consisted of four treatments in each of three blocks. and included: 1) canopy trimmed, with debris addition, changing microclimate, forest floor mass, and nutrient content, 2) canopy trimmed, without debris addition, 3) canopy not trimmed, but debris added, changing forest floor mass and nutrient content, 4) untreated control. To study an increased frequency of intense hurricanes, treatments 1 and 4 (hurricane and control) will be repeated every 10 years for at least 50 years duration. The first of the repeated disturbances, the second canopy trimming treatment, took place in late 2014; measurements are continuing (Table 1). In LUQ VI we plan to repeat these treatments in 2024.
The Throughfall Exclusion Experiment (TEE) is taking place in two stages. Prior to Hurricanes Irma and Maria, we deployed small (3 x 5 m) throughfall exclosures to determine the impact of multiple short-term droughts on soil biogeochemistry, as well as on microbes and litter organisms. These small clear plastic roofs significantly reduced soil moisture without significantly affecting other environmental conditions (i.e., light and temperature; Wood & Silver 2012, Bouskill et al. 2013). Replicate shelters were placed on ridges thus eliminating any upslope inputs and paired with untreated controls. Litterfall accumulating on roofs was removed weekly and placed on the soil surface underneath. Throughfall amount and chemistry were measured according to Wood & Silver (2012) allowing us to determine both water and nutrient removal as a result of simulated drought. We used volumetric moisture and temperature sensors, a Hydrosense CS620 for gravimetric moisture (Campbell Scientific, Logan, UT), and soil psychrometers (Wescor PST-55, Logan, UT) to convert soil moisture measurements to soil water potential. Soil O2 sensors (Apogee Instruments, Logan UT) were installed at 2 depths. Results from this phase of the study will provide us with a baseline measure of drought under closed canopy conditions. After Hurricane Maria, shelters were re-deployed within 3 weeks to study the effect of drought in the context of the recent hurricane disturbance (see Hypothesis 5).
The Stream Flow Reduction Experiment (StreamFRE) is situated in two 150-m long stretches of adjacent tributaries of the Q. Prieta. We took advantage of a steep natural rock barrier to divert 50% of stream flow (excepting high flow events) to below the de-watered reach between June-Aug (predicted by downscaled climate models to show the largest increase in drought). An un-manipulated arm of the stream with similar riparian vegetation, slope, width (1-3 m), biota, and chemistry serves as a reference reach. We use Randomized Intervention Analysis (RIA, Carpenter et al. 1989), a design well suited to the analysis of un-replicated whole-ecosystem manipulations. As required for RIA, all measurements are conducted concurrently in both streams to create a time-series for each characteristic. Similar to other whole-stream manipulations using RIA (Wallace et al. 1997), we collected pre-manipulation data (2016-19) before the beginning of the reduced-flow manipulation. Mapped and identified trees in the 1 ha area around the streams allowa estimation of species-specific litter inputs during experimental studies. Trimming of the canopy in part of the reference reach is planned for 2024 to coincide with the next planned trim of the CTE to study the separate and combined effects of droughts and hurricanes.
3.2. Long-term Measurements of the Elevation Gradient
Rising to 1075 m in elevation, the Luquillo Mountains present a gradient of climate and vegetation change that extends through five life zones from subtropical moist forest to lower montane rain forest (Ewel & Whitmore 1973). Forest communities extend along the gradient from mid-elevation (200-600 m asl) tabonuco forest through palo colorado forest (600-900 m) to elfin woodland (900-1075 m; Fig. 2.2). Palm forest, an edaphic formation, occurs at all elevations. We will collect long-term (80 yr) data on changes in the distribution of organisms and process rates along this gradient to examine the integrated effects of warming, drying, and more frequent disturbance over the range of biotic and abiotic conditions (Hypotheses 1 and 4). We measure changes in vegetation structure and composition every six years in three series of Long-Term Elevation Plots (LTEP) placed at 50 m intervals of elevation in three watersheds (Mameyes, Icacos, and Sonadora). In the Sonadora watershed, we compare upland forest types to adjacent palm forest to separate the effects of forest community composition and the abiotic environment (Willig et al. 2013, Table 3.2). We also monitor climate, rainfall chemistry, diameter increment, and litterfall in the Sonadora watershed.
Soil properties and plant species composition have been measured in these plots (Barone et al. 2008); we have also conducted biogeochemical and decomposition experiments along the elevation gradient as part of previous LTER research (Silver et al. 1999, McGroddy & Silver 2000, Dubinsky et al. 2010, Hall & Silver 2015). We began long-term studies of changes in plant and gastropod communities in LUQ III (Willig et al. 2013, Willig & Presley 2016), and established baseline measurements for most of the variables in Table 3.2 last year (Fig. 3.2) before Hurricanes Irma and Maria struck in September. Funds from a RAPID award are being used to conduct post-hurricane tree damage assessments in the LTEP, and other variables will be resampled during the next year and into Year 1 of LUQ VI to capture hurricane effects, and again in Year 5, the regularly scheduled timing for the LTEP census. We will continue to measure the variables in Table 3.2 at six-year intervals until the end of the century to examine the effects of changes in environmental drivers (see Hypotheses 1 and 4) on populations, communities, and key ecosystem characteristics.
3.3. Modeling – We employ a suite of models to integrate our understanding of the impacts of a changing climate and disturbance regime on ecosystems in the LEF and forge predictions of future change. We have adopted the model-experimental (ModEx) integration, an approach that recognizes that improved collaboration between modeling and process scientists is needed to develop, test, and implement process representations in models at all scales.
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Barone, J. A., J. Thomlinson, P. A. Cordero, and J. K. Zimmerman. 2008. Metacommunity structure of tropical forest along an elevation gradient in Puerto Rico. Journal of Tropical Ecology 24:525-534.
Bloch, C. P., C. L. Higgins, and M. R. Willig. 2007. Effects of large‐scale disturbance on metacommunity structure of terrestrial gastropods: temporal trends in nestedness. Oikos 116:395-406.
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