Dispersal modes and spatial patterns of tree species in a tropical forest in Arunachal Pradesh, northeast India

We characterized the dispersal modes and spatial patterns of 128 tree species in a tropical semi-evergreen forest of Arunachal Pradesh, northeastern India. A high percentage of species are animal-dispersed (78%), while about 22% (28) are dispersed abiotically (wind or gravity-dispersed). Of the animal-dispersed species, 54 species were primarily bird-dispersed, 25 were mammal-dispersed, and 21 were dispersed by both groups. We hypothesized that adult tree distribution patterns were related to dispersal mode and fruit size. We predicted that tree species with mechanisms for long-distance seed dispersal are likely to show more uniform or random spatial patterns than those with limited seed dispersal. Tree species with large fruits were also predicted to have greater levels of clumping than those that have small fruits. However, all tree species had a clumped distribution pattern. At the community level, we found no differences in spatial patterns based on dispersal mode for a subset of 50 tree species. Fruit size was, however, positively correlated with higher levels of spatial aggregation suggesting that tree species distributions are to an extent limited by dispersal. The importance of dispersal mode in determining adult tree distribution patterns at the community level may be obscured by interacting effects of other factors such as patchy habitat conditions and density-dependent mortality factors at different life-history stages that ultimately determine adult tree distributions.