The Effects of Local-scale Resource Heterogeneity on Tropical Tree Communities By

Both habitat partitioning and dispersal limitation influence the compositional structure of forest communities, but little is known about the relative contributions of these two processes, or the roles of different types of environmental variation. I used variation partitioning based on canonical redundancy analysis to decompose community compositional variation within eight large (24-50 ha) tropical forest plots into fractions explained by spatial, soil resource, and topographic variables. All environmental variables together explained 13-39% of compositional variation within a plot, with both soil resources and topography accounting for significant and approximately equal variation (9-34% and 5-29%, respectively). Additionally, a large fraction of variation (19-37%) was spatially structured yet unexplained by the environment, suggesting an important role for dispersal processes. My results, illustrated using a new graphical depiction of community structure within these plots, demonstrate the importance of small-scale environmental variation in shaping local community structure in diverse tropical forests around the globe. INTRODUCTION A major challenge for community ecology is to understand the importance of nicheassembly processes in shaping community structure. This is of particular interest in species rich communities such as tropical forests, because niche partitioning is thought to facilitate species coexistence and may therefore play an important role in biodiversity maintenance (Chesson 2000, Chave et al. 2002). Evidence for the role of habitat partitioning among tropical forest tree species has been found from local to landscape scales, and comes from observed non-random