9509026 HUBBELL A long-term study of a 50 ha permanent plot of neotropical forest on Barro Colorado Island (BCI) Panama, has revealed that individual pioneer species are absent from many potentially suitable gap sites. Although pioneer species are not expected to be common in a forest where suitable sites are rare, they appear to be less common than would be expected by chance. This research will test the hypothesis that distribution and abundance of pioneer species in the BCI forest are primarily controlled by dispersal limitation rather than by germination and establishment limitation. All canopy gaps greater than 25 m2 will be mapped on the 50 ha plot. A series of field experiments in gaps will be conducted to determine if the environmental conditions in gaps and closed forest affect germination and establishment. Seed fall will also be monitored as a measure of dispersal limitation. The potential importance of dispersal limitation goes far beyond finding explanations for distribution and abundance. Recent theoretical work has shown that strong dispersal limitation can promote tree species richness by greatly slowing competitive exclusion. In addition, this work will provide important insights into mechanisms that maintain species diversity in neotropical forest communities that are rapidly disappearing due to deforestation.
