Abstract: Understanding how biotic interactions vary across environmental gradients is critical for explaining the patterns of species coexistence in forest communities. Recent studies indicate that mycorrhizal association and environmental factors, particularly rainfall, can shape variations in conspecific negative density dependence (CNDD), although long-term community level evidence from natural forests remains limited. Using 20-year seedling censuses from a species-rich tropical forest, we investigated how interannual rainfall variability and mycorrhizal type modulated CNDD and seedling survival. Our results showed that seedling survival was greater in the years with higher rainfall, which was largely regulated by the interactions between the conspecific and heterospecific neighbors. Notably, rainfall and conspecific density were the predominant drivers of seedling survival, showing that seedlings exhibited higher survival at low conspecific densities and positive or nonlinear responses to rainfall. Heterospecific neighbors also had positive effects on seedling survival, which may be influenced by habitat conditions. Moreover, CNDD was stronger in arbuscular mycorrhizal (AM) species than in ectomycorrhizal (ECM) species, particularly in wetter years. Our findings revealed that interannual rainfall interacted with species’ mycorrhizal strategies to shape demographic outcomes, underscoring the pivotal role of mycorrhizal types in determining seedling dynamics. Thus, our study highlights the importance of incorporating mycorrhizal types into the predictions of tropical forest biodiversity.

Key words: conspecific negative density dependence, mycorrhizal types, rainfall, seedling survival, species coexistence