ABSTRACT

The Luquillo Experimental Forest LTER program (LUQ) focuses on the long-term dynamics of tropical forest ecosystems characterized by large-scale, infrequent disturbance, rapid processing of organic material, and high habitat and species diversity. Research by LUQ has stimulated a new appreciation of the significance of large-scale disturbances in tropical forested ecosystems and the key role of the biota in shaping the response to these events. Hurricanes occurring one and 10 years after the LTER program began permitted scientists to capitalize upon landscape-scale natural experiments which are followed closely. Among the most important findings from these natural experiments is that detrital dynamics plays a central role in forest recovery by influencing carbon and nutrient storage and flow.
The central theme proposed is that disturbance, through its effects on detrital dynamics, is a dominant ecosystem driver at LUQ. Pulses of detritus shift the flow of energy within the food web, modify the availability and distribution of nutrients, and feed back on the composition and productivity of plant and animal communities. Rapid processing of detritus distinguishes LUQ from other forested LTER sites, where decomposition takes 2-20 times as long.
A combination of long-term measurements, field experiments, simulation modeling, and cross-site comparisons are proposed to address five questions: (1) How do climatic factors, litter quality, and detritivore diversity regulate decomposition of detrital pulses? (2) How do terrestrial and aquatic food webs differ in response to detrital pulses? (3) What is the effect of disturbance frequency on nutrient cycling, plant community composition, and the accumulation of soil organic matter? (4) To what degree is the export of carbon and nutrients from watersheds a result of soil characteristics that are affected by detrital dynamics? (5) How do elevationally related changes in climate impact plant and detritivore communities, and how do these feed back on the quantity and quality of litter inputs and decomposition?
The research will be conducted in two spatial contexts. In mid-elevation tabonuco forest, LUQ will continue long-term measurements of ecosystem response to hurricanes, landslides, and anthropogenic disturbance. An experiment will be initiated to mimic increased frequency of hurricanes to investigate the effect of increased detrital inputs on nutrient cycling, community composition, and organic matter accumulation, along with. Manipulations of key groups of invertebrates to gain a better understanding of similarities in detrital processing between aquatic and terrestrial food webs. New plots will be established to examine the effect of elevationally related changes in climate, plant communities, and decomposers on detrital processing. Manipulative experiments will compare the relative importance of abiotic and biotic controls on decomposition in terrestrial and aquatic habitats.
Long-term measurements of hydrological and nutrient fluxes in watersheds will relate soil characteristics to stream nutrient and organic matter losses and provide information to gauge the effects of future disturbances. Simulation models of key population, community, biogeochemical, and landscape processes provide null-model predictions to inform these new observations and experiments.

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