Predicting which tropical tree species are vulnerable to forest disturbances

Tropical forest management often focuses on a few high-value timber species because they are thought to be the most vulnerable in logged forests. However, other tree species may be vulnerable to secondary effects of logging, like loss of vertebrate dispersers. We examined vulnerability of tree species to loss of vertebrate dispersers in Mabira, a heavily disturbed tropical rainforest in Uganda. Fruit characteristics and shade tolerance regimes of 269 tree species were compiled. Stem densities of tree species producing fruits of various sizes and having different shade tolerance regimes were computed for Mabira and compared with densities of conspecifics in Budongo, a less disturbed forest with similar floral composition. Seventy per cent of tree species in Mabira are animal-dispersed, of which 10% are large-fruited light demanders. These species are the most vulnerable because they rarely recruit beneath adult conspecifics and are exclusively dispersed by large vertebrates, also vulnerable in heavily disturbed forests. Comparison of densities between Mabira and Budongo showed that large-fruited light demanders had a lower density in Mabira. Other categories of tree species had similar densities in both forests. It is plausible that the low density of large-fruited light demanders is due to limited recruitment caused by dispersal limitations.