Seasonal differentiation in density-dependent seedling survival in a tropical rain forest

1. Density-dependent survival is prevalent in tropical forests and is recognized as a potentially important mechanism for maintaining tree species diversity. However, there is little knowledge of how density dependence changes in fluctuating environments. 2. Across the 20-ha Xishuangbanna tropical seasonal rain forest dynamics plot in southwest China, which has distinct dry and wet seasons, we monitored seedling survival in 453 1-m2 quadrats over 2 years. Density dependence was assessed using generalized linear mixed models with crossed random effects. 3. When pooling all species at the community level, there were strong negative effects of conspecific tree neighbours on seedling survival over the dry-season, wet-season and 2-year intervals. The proportion of conspecific seedling neighbours had a significant negative effect in the dry season, but not in the wet season. 4. At the species level, the effects of conspecific tree and seedling neighbours varied widely among species in the community and were significantly positively related to population basal area in the community over the dry-season interval. In contrast, over the wet-season interval, the effects of conspecific tree and seedling neighbours did not significantly vary among species in the community. Overall community- and species-level results suggest that local-scale negative density dependence (NDD) tends to be stronger in the dry than wet season in the Xishuangbanna tropical forest. 5. At the scale of the 20-ha plot, we found a community compensatory trend (CCT), in which rare species had relatively higher seedling survival than common species in both the wet and dry seasons. A positive association between potential NDD and population basal area suggests that the CCT results from local-scale NDD, specifically because of negative effects of conspecific tree neighbours. 6. Synthesis. Our results demonstrate that the strength of density-dependent seedling survival can vary between seasons and among species in tropical forests. Future research is needed to assess the underlying mechanisms of this temporal and interspecific variation in NDD and its consequences for species coexistence and community composition.