IBL Tropical Forest Research Program Tropical forests are globally dispersed, highly productive, and exchange more carbon (C) and energy than any other ecosystem type. However, many of the factors that regulate ecosystem processes in these ecosystems remain poorly understood, confounding our understanding of how they are likely to respond to global change. Our group is using several different experimental manipulations to investigate carbon (C) and nutrient cycling in an extremely wet (>5000 mm rainfall / year), primary Pacific lowland tropical rain forest on the Osa Peninsula in southwestern Costa Rica (see map at right). The sites are located on a private reserve near Drake Bay in the town of Progresso. At this site, we have established a long-term fertilization experiment (+N, +P, +NP and control), and are using the plots (fertilized since 2000) to investigate how changes in soil nutrient availability affect soil C, N and P cycling in these nutrient-poor tropical (Ultisol) soils. Our recent data suggest that many ecosystem processes, including C decomposition and N fixation, are strongly P limited, and that increases in soil P availability drive more rapid decomposition of organic matter in these sites. We continue to investigate the long-term effects of fertilization and natural variations in soil fertility on soil processes and foliar nutrient chemistry, and are beginning to study how changes in nutrient availability alter soil microbial community function and composition. Additional work focuses on the major sources of rock-derived nutrients to forests across the Osa Peninsula, and on the biogeochemical and microbiological effects of forest-to-pasture conversion. Previous work by our group also shows the importance of precipitation in regulating decomposition and nutrient cycling in tropical forests, and that variations in precipitation may have profound effects on biogeochemical cycling in these ecosystems. In contrast to some research showing that high rates of precipitation in tropical rain forest may lead to decreases in decomposition rates, rates of decomposition in our sites are among the highest reported in the literature. Thus, to test the effects of precipitation on soil C and nutrient cycling, we have begun a precipitation manipulation experiment by establishing a set of plots where annual rainfall is reduced by 50%. Our data will expand an important dataset on how climate change in tropical forests regulates organic matter decomposition and soil biogeochemical cycling.

Aerial Picture of Drake Bay sites. A field lab that includes both basic facilities and lodging, courtesy of the Drake Bay Wilderness Resort, is in the small clearing, and pictured below.