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. 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. 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. 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.

The upcoming climatic variability is expected to influence the population dynamics of tree species, but few empirical studies have been performed over broad geographical scales, which may be more convincing to address tree range shift under climate change. In the present study, the population structure and regeneration dynamics ofQuercus variabiliswere investigated along latitudinal and longitudinal gradients over its full range in China. The size structure, density, mortality rate (b), conversion rate (CR), initial recruitment (y₀), and survival from birth to the highest class of regeneration (survival rate [SR]) were analyzed. The results showed the following: (1) The population structure ofQ. variabilisexhibited an inverse‐J shape throughout the latitudinal gradient, and the relative seedling density was significantly higher in the middle populations than in the north and the south. In addition, they₀andSRexhibited the highest values in the middle, but no significant differences in the density and total basal area of adults were found across latitudes. (2) Along the longitudinal gradient, an inverse‐J‐type population structure was present in the west populations, but was not observed in the east due to the scarcity of individuals in regenerated populations. The total basal area and density of adults were similar across longitudes, but both the seedling density and the relative seedling density, along with they₀andSR, were higher in the middle than in the east and the west. Thebof adults was lower in the middle‐longitude populations than in the west and the east. (3) Climate (annual average temperature) was identified as the primary factor affecting regeneration. In addition, local factors, including the soil and stand conditions, were also found to play a significant role in determining the regeneration dynamics. Based on these results, we speculate that populations ofQ. variabilisat the range edges would decline in the context of climate change, particularly in the south and the east.

Increasing evidence shows that biodiversity–ecosystem functioning relationships (BEFs) become stronger as forests develop, but much of the evidence is drawn from experiments (less than 30 years). How the biodiversity effects vary with stand development stages remains largely unexplored. Using a large temperate forest dataset with 2392 permanent plots in northeastern China, we examined the relationships between biodiversity (i.e. tree species richness, functional diversity, and functional composition) and aboveground biomass (AGB) across different development stages of temperate forests (covering all stages from young to overmature forests). Specifically, the complementarity and mass‐ratio effects across different forest development stages were evaluated to elucidate emerging patterns that explain ecosystem functioning. We observed positive BEFs using both tree species richness and functional diversity, but these positive effects decreased with forest development. However, the effects of community‐weighted mean (CWM) on AGB showed two peaks in young and mature stands. Interestingly, the effects of CWM 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 tree species) rather than niche complementarity in old stands. The high AGB in young stands was characterized by tree species with high resource acquisition ability, however, in old stands, it was associated with tree species with both high resource acquisition ability and conservative traits. Our findings indicate how the developmental stage influences the effects of biodiversity on ecosystem functioning in natural forests. The findings tentatively advocate for a mechanistic framework of BEFs covering all developmental stages of temperate forests, which could facilitate the formulation of effective strategies for enhancing ecosystem functioning at different development stages.