Conspecific and phylogenetic density-dependent survival differs across life stages in two temperate old-growth forests in Northeast China
Factors that control individual survival dynamics are pivotal determinants of forest diversity. Numerous studies have examined the relative importance of habitat variables and neighborhood effects on individual survival, while few studies have examined this importance in different forest types that largely vary topography. We examined the role of conspecific negative density dependence (CNDD), phylogenetic negative density dependence (PNDD) and habitat variables across life stages in two temperate old-growth forests in Northeast China. Using generalized linear mixed models (GLMMs), we tested whether individual survival is related to neighborhood effects and habitat variables. Our results showed that the relative importance of neighborhood effects and habitat variables to focal individual survival varied among life stages and sites. However, the best-fit models for three life stages (with the exception of seedling stage in the BKPF) at both sites all included habitat variables, indicating that these variables contribute to the patterns of focal individual survival. We found evidence of a CNDD effect, and the strength of this effect decreased as the life stages progressed and varied from a negative effect at the seedling stage to a positive one at the adult stage. This result confirms that the importance of CNDD-based survival at the seedling stage plays a particularly significant role in promoting the coexistence of tree species and maintaining forest diversity. We found evidence of PNDD in our forest, and the impact of heterospecific phylogenetic relatedness on focal individual survival was significantly negative; the strength of PNDD increased as the life stages progressed. The CNDD and PNDD widely varied among species, indicating the importance of including the relative abundance, life history strategies and functional traits of species when determining the factors that affect species sensitivity to neighborhood effects. The results of our study demonstrated that CNDD, PNDD and habitat variables all influence the individual survival of these temperate old-growth forests, but the relative importance of these factors vary among life stages and species. Our results highlight the importance of combining multiple species, life stages, functional traits and large-scale studies for investigating elements that affect species coexistence in tree communities.