Biodiversity–ecosystem functioning relationships (BEFs) have been extensively explored across ecosystems. However, these relationships may change as the forest matures, and the underlying mechanisms remain underexplored. Using large temperate forest datasets from 2,392 permanent plots in northeastern China, we examined the relationships between biodiversity and aboveground biomass (AGB) across different developmental stages from young to over-mature stands. We found the positive BEFs using both species richness and functional diversity, but these positive effects decreased with forest development. However, the effects of community-weighted mean on AGB showed two peaks in young and mature stands. Notably, the effects of community-weighted mean on AGB became larger than the effects of functional diversity after the forests developed to near-mature/mature stands, indicating that BEFs are driven by mass-ratio effects (i.e., dominant species) rather than niche complementarity in old stands. Our findings on how the developmental stage influences the effects of biodiversity on ecosystem functioning in natural forests will help identify effective strategies for maintaining or enhancing ecosystem services at different forest successional stages.

Aim: Forests play a key role in regulating the global carbon cycle, a substantial portion of which is stored in aboveground biomass (AGB). It is well-understood that biodiversity can increase the biomass through complementarity and mass-ratio effects, and the contribution of environmental factors and stand structure attributes to AGB was also observed. However, the relative influence of these factors in determining the AGB of Quercus forests remains poorly understood. Location: Temperate Quercus forests in northeastern China. Methods: Using a large dataset retrieved from 523 permanent forest inventory plots across Northeast China, we examined the effects of integrated multiple tree species diversity components (i.e., species richness, functional and phylogenetic diversity), functional traits composition, environmental factors (climate and soil), stand age, and structure attributes (stand density, tree size diversity) on AGB based on structural equation models. Results: We found that species richness and phylogenetic diversity both were not correlated with AGB. However, functional diversity positively affected AGB via an indirect effect in line with the complementarity effect. Moreover, the community-weighted mean of specific leaf area and height increased AGB directly and indirectly, respectively; demonstrating the mass-ratio effect. Furthermore, stand age, density, and tree size diversity were more important modulators of AGB than biodiversity. Main conclusions: Our study highlights that biodiversity-AGB interaction is dependent on the regulation of stand structure that can be even more important for maintaining high biomass than biodiversity in temperate Quercus forests.