Seasonal Variation of the Effects of Phylogenetic Relatedness and Functional Similarity Among Heterospecific Neighbors and Habitat on Seedling Survival in a Subtropical Forest in Gaoligong Mountains, Southwest China

Understanding the mechanisms of species coexistence is a long-standing goal in community ecology. The performance of woody seedlings across different ecological niches has been proposed as a key mechanism in community assembly. Factors influencing seedling survival can be revealed by analyzing neighboring plants in conjunction with their phylogenetic relatedness, functional traits, and the surrounding environment. In the present study, we conducted a seasonal analysis of 936 seedlings for 56 species from a 4-ha subtropical forest over 3 years in the Gaoligong Mountains, Southwest China. Our aims were to examine the relative effects of neighbor density, habitat conditions, and seasonal climate variability on seedling survival during dry and rainy seasons, and to determine whether the sensitivity of seedling survival responding to neighbor densities and abiotic factors differs between seasons. The findings indicated that the relative importance of neighbor densities, habitat factors as well as seasonal rainfall on seedling survival varied with seasonality. During the rainy season, seedling survival was comparatively less affected by conspecific neighbor density and was mainly negatively influenced by rainfall, whereas habitat factors (including topography, soil properties, and canopy openness) were detrimental for survival to a lesser extent. There is evidence suggesting that phylogenetic negative density dependence (PNDD) and functional negative density dependence (FNDD) are more pronounced in the rainy season. In contrast, the positive effect of canopy openness was more important during the dry season. Our findings further revealed that the effects of PNDD, FNDD, canopy openness, and seasonal rainfall varied widely among species across seasons. Moreover, species differed in their ability to respond to the trade-offs between canopy openness and rainfall in relation to phylogenetic relatedness and functional dissimilarity during the dry and rainy seasons. Overall, our results demonstrate that seasonality modulates the strength and importance of phylogenetic and functional density dependence, habitat preference, and climate for seedling survival in subtropical forests. The seasonal variations in these effects allow species to maintain coexistence across dry and rainy seasons. Consequently, seasonal variability should be accounted for in future studies in understanding the diversity of forest communities.