Species asynchrony, not richness, drives fine-scale temporal stability of aboveground biomass in a subtropical karst forest

The temporal stability of biomass is fundamental to sustaining forest ecosystem functioning and services. Accumulating evidence demonstrates that biodiversity helps to maintain this stability throughout forest succession. However, the underlying mechanisms governing the biodiversity–stability relationship, particularly in pristine karst forests, remain poorly understood. Therefore, we monitored a 1-ha permanent plot in a subtropical karst evergreen–deciduous broadleaved mixed forest over 15 years, and quantified the fine-scale temporal stability of community aboveground biomass (AGB) among subplots within this single stand. We assessed how species richness, asynchrony, functional diversity, community-weighted mean (CWM) traits, structural heterogeneity and soil conditions relate to this stability. Pearson correlation and simple regression described bivariate relationships, multiple regression identified the leading predictors, and structural equation modelling (SEM) separated direct from indirect pathways. Our results showed that community AGB increased by 13.7% over the 15-year period. Species asynchrony was significantly and positively correlated with stability, whereas species richness showed a significant negative correlation; the remaining variables were non-significant. In multiple regression, only asynchrony and richness retained significant independent effects. SEM further demonstrated that asynchrony was the primary direct driver of stability, and species richness had a significant negative direct effect. CWM trait composition indirectly enhanced stability by promoting asynchrony, whereas soil fertility was strongly associated with species richness but its net indirect effect on stability was not statistically significant; functional diversity and structural heterogeneity had no significant effect on stability. Our findings suggest that maintaining asynchronous species responses—not merely raising species counts—may warrant attention in karst restoration, and confirming this at among-stand and landscape scales is a priority.