Abstract:

Objective: This study aims to investigate the relationship between tree species diversity and aboveground biomass in temperate forests of northeast China with the changes of forest types and developmental stages, and to reveal the underlying mechanisms of niche complementarity and mass ratio effects, providing a scientific basis for improving the quality of secondary forests in northeast China through optimizing tree species composition. Method: Based on data from the National Forest Continuous Inventory plots in Jilin Province, linear mixed-effects models were employed to analyze the effects of multidimensional diversity (species richness, functional diversity, phylogenetic diversity) and the community-weighted mean (CWM) of functional traits on aboveground biomass across different forest types (broad-leaved forests, broad-leaved mixed forests, coniferous broad-leaved mixed forests) and developmental stages, while controlling for climatic, soil, and stand factors. Generalized additive models were used to reveal the dynamic changes in diversity effects with forest development stages, and structural equation modeling was applied to analyze the direct and indirect effects of environmental factors, forest types, developmental stages, and stand density on the relationship between tree species diversity and aboveground biomass. Result: 1) Across the three temperate forest types, species richness, functional diversity, and phylogenetic diversity all had significant positive effects on aboveground biomass. Simultaneously, the CWM, representing tree functional traits, also exhibited significant positive or negative effects. This indicates that both niche complementarity effect and the mass ratio effects operate in tandem during aboveground biomass accumulation in these temperate forests. 2) The influence of tree species diversity on aboveground biomass exhibited significant forest type dependency across different developmental stages. In the three types of forests, diversity effects (including species richness, functional diversity, and phylogenetic diversity) generally showed a decreasing trend from the young forest stage to the mature forest stage. Specifically, the community-weighted mean trait values effect (i.e., the mass ratio effect) of pure broad-leaved forests and coniferous broad-leaved mixed forests was higher in both young forests and mature/over-mature forests. In contrast, there was no significant trend in the community weighted average trait value effect of coniferous broad-leaved mixed forests across different forest developmental stages. Notably, in mature and over-mature forests, the community-weighted mean trait values effect exceeded the functional diversity effect in both broad-leaved forests and coniferous broad-leaved mixed forests. 3) Structural equation modeling indicated that environmental factors (climate, soil), stand characteristics, and forest types/developmental stages jointly regulated forest biodiversity and aboveground biomass. Specifically, forest type and developmental stage were able to indirectly influence aboveground biomass by regulating stand density and biodiversity. Conclusion: In the temperate forests of northeast China, species richness, functional diversity, phylogenetic diversity, and the community-weighted mean of functional traits all significantly influence aboveground biomass, but the strength of their effects varies with forest type and developmental stage. Therefore, forest management and ecological restoration practices should implement precise management strategies tailored to the specific forest type and its developmental stage. In young forests, in addition to increasing tree species diversity, emphasis should be placed on cultivating species with strong resource acquisition abilities. In mature forests, priority should be given to developing and retaining dominant species with long-lived and conservative traits, and attentions should be paid to the target-tree cultivation practices.

Key words: tree species diversity, functional diversity, community-weighted mean traits values, temperate forest, aboveground biomass, developmental stages