Stand, plot characteristics, and tree species diversity jointly dominate the recruitment biomass of subtropical forests

Forest dynamics is largely affected by the recruitment of trees. Despite several investigations on forest recruitment biomass (FRB), it is still not well explored how tree species diversity, stand characteristics, climate, and topography regulate FRB. Moreover, there have been limited studies and discussions on FRB in typical subtropical forests in karst areas. We examined the relationship between influencing factors and forest biomass from the perspective of tree recruitment using data from 2334 Chinese National Forest Inventory (CNFI) plots in Guizhou Province. The best predictors within each group were chosen using a stepwise regression model, and the effects of five categories, including tree species diversity, stand factors (e.g., stand density), climatic factors (e.g., mean annual temperature and annual precipitation), topographic factors (e.g., elevation and slope), and plot factors (e.g., canopy density and forest composition) on FRB were determined using structural equation models (SEM). Besides this, the relative importance of each variable among five categories in determining FRB was also quantified by hierarchical partitioning analysis (HPA). Our results indicated that diversity of tree species had the highest relative importance in determining FRB (28.85 %), followed by average DBH (21.06 %), canopy density (10.29 %) based on HPA. While for five categories, stand factors contributed the most to FRB (37.04 %), followed by plot factors (29.01 %), tree species diversity (28.85 %), climatic factors (3.81 %), and topographic factors (1.29 %). Moreover, SEM showed FRB was strongly directly and significantly controlled by tree species diversity (beta = 0.63, P < 0.05), while stand factors had a strong direct effect (beta = 0.45, P < 0.05) and indirect effect (beta = -0.78, P < 0.05). We conclude that niche complementarity is the primary mechanism driving this forest's FRB. Our findings highlight the significance of tree recruitment in maintaining subtropical forests' carbon storage capacity. Furthermore, increasing tree species diversity, particularly by combining conifers and broad-leaved trees, will encourage the recruitment of young trees.