General description of Experiment and Projects: Hurricanes are important drivers of periodic disturbances on tropical forests of the Luquillo Mountauns. The immediate impact of this disturbance is on the canopy biomass which is redistributed from the canopy compartments to the detrital pool of the forest floor hence creating a wide opened canopy.  The Canopy Trimming Experiment (CTE) is a long-term experiment designed for two purposes: 1) to decouple the effect of canopy disturbance (e.g., increasing light levels, temperature, moisture, etc.) from those of increased detrital inputs on rates of germination, growth, survival, detritus processing, nutrient cycling, soil conditions, and trophic structure, and 2) to increase the frequency of simulated hurricane effects above background levels to once every six to ten years. Climate change models predict increased frequency and intensity of Caribbean hurricanes (Emmanuel 1987, Goldenberg et al. 2001), and the goal is to evaluate predictions regarding the effects of an increased rate of hurricane disturbance on tabonuco forest (Sanford et al. 1991).  The interaction of biotic and abiotic processes, all modified by the disturbance, are key in determining ecosystem responses because they regulate critical ecosystem fluxes and storage associated with detritus decomposition.  These processes define detrital dynamics and play a central role in the recovery of forest structure and function after disturbance.  Therefore, a third component of this experiment was to implement a series of short-term biotic manipulations nested within the large-scale CTE design, consisting on faunal manipulations to measure the strength of interactions between autotrophic and detrital food webs in the context of hurricane-associated disturbance, which allowed to asses the important components of the foodwebs. The overall hypothesis is as follow:  Short-term dynamics of key response variables after disturbance will be a function of the interaction between microclimate and detrital inputs, whereas long-term dynamics (particularly of SOM and NPP) will be a function of detrital inputs.

CTE Litter Basket Decomposition Experiment: The objective was to determine how green litter deposition and canopy opening associated with a hurricane independently and together affect rates of decomposition of pre-existing litter and green hurricane litter. In addition, changes in litter quality and quantity and changes in microenvironment were hypothesized to bring about changes in abundance of white-rot litter basidiomycete fungi and their activity in translocating nutrients between litter cohorts. Fungal connectivity was determined between litter cohorts that were demarcated with screens. Analysis of nutrient concentrations and content will determine which litter cohorts became nutrient sources and which ones became sinks following canopy opening and/or deposition of green litter. Litter moisture, fungal connectivity to the pre-weighed senesced leaf cohort and leaf mass loss were all slowest in the treatment with canopy trimming and debris removal and fastest in the closed canopy with added debris. Addition of green leaf litter ameliorated the environment in the senesced litter layer below and buffered it from effects of canopy opening, resulting in similar fungal connectivity and mass loss in control and trimmed canopy plus debris plots.

CTE Effect of Canopy Trimming and Debris Deposition on Litterfall Mass: Periodicity in seasonal forests has been related to rainfall regime and dry season length. While El Verde has distinct seasonal peaks in litterfall (Zou et al. 1995), it has only weakly seasonal rainfall, suggesting other possible drivers.  Zalamea and González (2008) showed using pre-treatment litter baskets placed in the same plots from which these data were collected that litterfall patterns were correlated mainly with total solar radiation, PPFD, day length, and temperature, and only secondarily with rainfall. The two main peaks of litterfall, from April–June and August–September, coincided with the periods of major solar radiation (Zalamea and González 2008). Zalamea and González (2008) also found strong variation among years that suggested litterfall was controlled by a complex interaction between environmental and physiological factors.

Manipulations and measurements of detrital processing, SOM, and soil properties associated with SOM will continue for at least three more funding periods (until 2024). In the short-term, we use faunal manipulations nested within the canopy trimming experiment to measure the strength of interactions between autotrophic and detrital food webs in the context of hurricane-like disturbance. These results are being used to parameterizing the Trophic Interaction Model. This experiment also provides a physical and intellectual focal point for the project participants. The Canopy Trimming Experiment has two parts: 1) a forest canopy manipulation with measurements of coupled changes in microclimate, structure, and biota and their associated impacts on ecosystem processes, and 2) specific manipulations of the biota that assess the importance of components of the food web.

CTE Ferns: The species composition (diversity) and abundance of all of the ferns in all subplots of the CTE plots were assessed annually beginning in January, 2003 by the researcher. Ferns in all plant measurement subplots are also monitored and measured (by staff) at regular intervals and the experiment proceeds to detect changes in growth, diversity and abundance.

CTE Snail data: The objective of the CTE snail data is to determine how green litter deposition and canopy opening associated with a hurricane independently and jointly affect population dynamics and community composition of terrestrial gastropods. Because canopy openness can be expected to increase abiotic stress on gastropods, whereas litter deposition should provide increased resources and refugia, tradeoffs can be expected.

CTE Walking Stick Data: The objective of these data is to determine how green litter deposition and canopy opening associated with a hurricane independently and jointly affect population dynamics of walking sticks (Lamponius portoricensis).

CTE Canopy Arthropods: This project addresses canopy invertebrate responses to CTE treatments, for comparison with invertebrate responses to canopy-opening hurricane disturbances dating from 1991. One year of pre-treatment and three years of post-treatment data are available. Statistical analyses are in progress.

CTE Litterbag Invertebrate Counts and Weights: A long-term experiment to increase the frequency of simulated hurricane effects above background levels to once every six years. The experiment will determine effects of repeated disturbance of the forest canopy and increased detrital inputs to the forest floor on germination, growth, survival, nutrient cycling, soil conditions, and trophic structure. Using leaf litter in litterbags, the aim of this part of the experiment was to examine the response of litter invertebrates to canopy disturbance [e.g., hurricanes], and to distinguish between the effects of canopy removal and detrital inputs. The effects of different litterbag mesh size and litter type [fresh or senescent] was also studied.

CTE Litter Basket Microbial diversity DNA data: Leaf Litter Microbial Communities at different stages of decomposition with and without canopy opening and debris deposition. Hurricanes are common disturbances affecting forest ecosystems in the Caribbean. Our objective was to determine the relative abundance and diversity of microorganisms in leaf litter at different stages of decomposition, and the influence of canopy opening and debris addition or removal. The study was conducted in the tabonuco forest (subtropical moist) at El Yunque Rain Forest, Puerto Rico. Three blocks with four treatment plots were established. TRFLP profiles of the 16S rDNA digested with MnlI and fungal ITS digested with HaeIII showed that the microbial communities at 17, 31 and 55 weeks were highly divergent among treatments. Ratio of fungal to bacterial phylotypes increased for closed canopy with debris addition. Leaf mass loss was slowest in the treatment with canopy trimming and debris removal. Microbial community changes through time can be related to microclimate and the availability of labile compounds. Fungi appeared to control the succession of microorganisms in decomposing leaves.

CTE soil and Leaf litter microbial community analysis: Microorganisms play an important role in the forest's restoration through their detritus dynamics. Our objective was to determine how the canopy openings and debris pulses affect the soil and leaf litter microbial community structure and composition in the forest before and after the canopy manipulation. We monitored the soil and leaf litter communities through the analysis of their cellular fatty acids also called ester linked fatty acid methyl ester (EL-FAME).

CTE trace gases: This research explores the trace gas dynamics from the Canopy Trimming Experiment

CTE soil solution chemistry: Soil solution was collected monthly from zero tension lysimeters installed in the CTE plots. Sampling began prior to cutting the canopy and continue to the present.

Note: For a general description of methodologies and references download file from http://luq.lternet.edu/downloads/list-methods-used-measurements-cte

Project Director: 

Project Manager

Funding Source: 

National Science Foundation - LTER; USDA Forest Service (FPL, IITF); T.D. Schowalter (shared invertebrate data voluntarily after 2001); Universidad Del Turabo (for samples storage and processing) and Environmental Protection Agency - EPA (in Athens Georgia for Gas Chromatography Analysis and scientist's time) for Microbial EL-FAME Data; University of Puerto Rico.