Effects of moisture and density-dependent interactions on tropical tree diversity.

Tropical tree diversity increases with rainfall 1,2 . Direct physiological effects of moisture availability and indirect effects mediated by biotic interactions are hypothesized to contribute to this pantropical increase in diversity with rainfall 2-6 . Previous studies have demonstrated direct physiological effects of variation in moisture availability on tree survival and diversity 5,7-10 , but the indirect effects of variation in moisture availability on diversity mediated by biotic interactions have not been shown 11 . 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 forests 12-18 , and many of these plant enemies are themselves moisture-sensitive 19-27 . Changes in moisture availability caused by climate change and habitat degradation may alter these interactions and tropical tree diversity.