Tropical tree diversity increases with rainfall1,2. Direct physiological effects of moisture availability and indirect effects mediated by biotic interactions are hypothesized to contribute to this pantropical increase in diversity with rainfall2-6. Previous studies have demonstrated direct physiological effects of variation in moisture availability on tree survival and diversity5,7-10, but the indirect effects of variation in moisture availability on diversity mediated by biotic interactions have not been shown11. Here we evaluate the relationships between interannual variation in moisture availability, the strength of density-dependent interactions, and seedling diversity in central Panama. Diversity increased with soil moisture over the first year of life across 20 annual cohorts. These first-year changes in diversity persisted for at least 15 years. Differential survival of moisture-sensitive species did not contribute to the observed changes in diversity. Rather, negative density-dependent interactions among conspecifics were stronger and increased diversity in wetter years. This suggests that moisture availability enhances diversity indirectly through moisture-sensitive, density-dependent conspecific interactions. Pathogens and phytophagous insects mediate interactions among seedlings in tropical forests12-18, and many of these plant enemies are themselves moisture-sensitive19-27. Changes in moisture availability caused by climate change and habitat degradation may alter these interactions and tropical tree diversity.
© 2023. The Author(s), under exclusive licence to Springer Nature Limited.