Diversity of woody species in forest, treefall gaps, and edge in Kibale National Park, Uganda

If specialization influences species presence, then high tropical tree and shrub diversity should correspond with high environmental heterogeneity. Such heterogeneity may be found among different successional communities (i.e., canopy types). We explore species associations in three forest-dominated canopy types, forest, gap, and edge, in Kibale National Park, Uganda and determine environmental, soil and light, differences among canopy types. To determine the strength of differences among forested canopy types, they are also compared to grasslands. Tree and shrub density and species richness using rarefaction analysis were determined based on data from 24 small plots (5 x 5 m) in all four canopy types and 16 large plots (10 x 50 m) in forest and grassland canopy types. Environmental variables were determined along 10 (20 m) transects in the four canopy types. Using analysis of variance and principal components analysis, we demonstrate that forest and gap environments had similar soils, but forest had lower light levels than gap. We also found that grassland and edge were more similar to one another than to forest and gap, but differed in a number of important biotic and abiotic factors controlling soil water availability (e.g., edge had higher root length density of small roots < 2 mm diameter in the top 20 cm than grassland). Using principal components analysis to assess similarities in community composition, we demonstrate that gap and forest had indistinguishable communities and that edge was similar to but distinct from both communities. Complete species turnover only occurred between grassland and the three forested canopy types. Even though overall community composition was similar in the three forested canopy types, in analyses of individual species using randomization tests, many common species were most frequently found in only one canopy type; these patterns held across size classes. These results suggest that despite differences among environments, community composition was similar among forested canopy types, which are likely intergrading into one another. Interestingly, individual species are more frequently found in a single canopy type, indicating species specialization.