Habitat filtering masks the detection of phylogenetic and functional density dependence on seedling survival across seasons in a subtropical forest, Southwestern China

Negative density dependence and habitat filtering are crucial mechanisms driving species coexistence and community assembly. However, empirical evidence regarding how habitat filtering interacts with density dependence remains limited, particularly in terms of its influence on phylogenetic and functional density dependence in seedling dynamics. Seasonal variation in abiotic conditions (e.g., drought vs. high humidity) provides an opportunity to test whether habitat filtering masks or modifies density-dependent effects, as the strength of these mechanisms likely fluctuates with environmental stress. To address this gap, we assessed seasonal variation in seedling survival in response to phylogenetic distance and functional dissimilarity in a subtropical evergreen broad-leaved forest in Southwestern China, comparing models with and without habitat variables. We found significant positive effects of phylogenetic distance and functional dissimilarity on seedling survival in the dry season, indicating that seedlings survived better when they were more phylogenetically or functionally distant from their neighbors. This provides evidence for both phylogenetic and functional negative density dependence (PNDD and FNDD). Habitat variables (elevation, convexity, and soil organic matter content) significantly predicted seedling survival only in the dry season. Furthermore, habitat filtering masked the effect of PNDD and FNDD at both community and species levels and influenced the detection of their interactive effects on seedling survival. Overall, we observed marked seasonal variation in the effects of both density dependence and habitat variables. This study demonstrates that habitat filtering obscures the detection of phylogenetic and functional density dependence in seedling survival and reveals a seasonally modulated link between neighborhood phylogenetic and functional structure and seedling survival. These findings highlight the importance of considering environmental context when assessing density-dependent mechanisms, advancing our understanding of forest community dynamics by showing that the strength and detectability of density dependence are contingent on both habitat conditions and seasonal abiotic stress.