Analogous Environments Across the Tropics Have Similar Levels of Tree Species Αlpha Diversity

Different regions of the tropics vary in overall tree species diversity, with the tropical Americas exhibiting strikingly higher regional tree species richness than Africa and Southeast Asia. We investigated whether these differences also occur at the local scale, and whether the environmental conditions associated with tree species richness are consistent across tropical regions despite highly dissimilar species pools. A spatial random forest (RF) model was trained using a network of 429 one-hectare plots across the tropics, together with 24 environmental variables, to predict plot-level tree α diversity. A combination of climatic, soil and topographical variables explained around 86% of variation in richness. Despite differences in regional species pools and potentially disruptive effects of different geological, climatic and evolutionary histories, the relationship between environmental variables and local scale tree species richness is closely similar across different continents. Our findings imply a pervasive role of niche-based mechanisms in structuring local tree species richness, regardless of regional species assemblages. This pantropical convergence in the richness-environment relationship poses a challenge for ecology to explain.