Many tropical trees are long-lived, thus long term data are critical to document and understand the dynamics of tropical forests. Understanding tropical forest dynamics is becoming even more important because of the large role tropical forests play in global carbon cycling and because of the potential impact of climate change on these ecosystems. The Barro Colorado Island Forest Dynamics Project has conducted 6 censuses of a 50 ha plot over a 25 year period, generating data that have provided unique insights into forest dynamics and community ecology. This project will conduct a seventh 5-year census of the 50 ha plot, adding to the longest running large scale study of tropical forest dynamics. The intellectual merit of the project comes from the value of the long term data set, the insights it will provide into tropical forest dynamics, and its use in testing an array of hypotheses related to tropical forest diversity. The data from these censuses constitute an important broader impact because they are publicly available to scientists for non-commercial use. An ongoing international collaboration and student training are additional broader impacts.
Intellectual Merit This research addressed the continuing question of the long-term dynamics of the tropical forest and the mechanisms that control tropical tree species abundance. This grant funded the seventh complete census of all trees >1cm DBH a 50-hectare tropical forest research plot on Barro Colorado Island (BCI), Panama. The overall project began in 1980 and the seventh census of the plot revealed continuing rapid population dynamics of virtually all tree species, including those that are common and rare. Of particular interest on this proposal was whether the spatial dynamics seen during the first six censuses continued, namely the apparent lack of correlation of population dynamics across relatively small spatial scales. The seventh census revealed that these patterns are continuing and that there are negative spatial autocorrelations in population growth rate at large lag-distances (> 0.5 Km), supporting the Enemy Susceptibility Hypothesis (ESH) that we are testing on our current NSF grant (NSF DEB 1136626). The total number of individuals in the plot fell to 207,246, continuing a decline in total abundances of all species. We attribute these declines to two possible causes: (1) a closing of the canopy after a severe El Nino event during the 1980s and (2) possible influences of human activity in the plot during the last 30 years. We are currently evaluating whether human impact is modifying forest dynamics. Despite the decline in number of stems, the number of species inhabiting the plot has increased to 302 from the 300 species recorded during the previous census in 2005. We recently published a paper in Ecology (Feeley et al. 2011) suggesting that species that are increase are those with high wood density. These are also generally shade tolerant species, an observation supporting the hypothesis that increased canopy closure is responsible for the stem decline. An alternate hypothesis for these data is that long-term climate change in central Panama is responsible for the decline in stem number and increasing wood density of species living within the plot as a result of increasingly variable rainfall regimes and prolonged droughts. Broader Impacts This project continues a long tradition of training American and Panamanian students in tropical forest biology on Barro Colorado Island. During the term of this grant, we employed 12 Panamanians, 9 of whom were botany majors from the University of Panama, and we supported two undergraduate students from the United States (NSF REU). This grant also supported two short courses in field dendrology, led by Rolando Perez. This research project has support the publication of eight manuscripts in journals including Science, Nature, Ecology, Ecology Letters, Trends in Ecology and Evolution, and Oikos. We have two additional publications in press (Ecograpgy and Journal of Ecology). The data from the 2010 census will shortly be fully uploaded to http://ctfs.arnarb.harvard.edu/webatlas/datasets/bci/ under a license that requires users acknowledge the support of NSF and donors and the work of the PIs and their field assistants.
