Functional traits influence biomass and productivity through multiple mechanisms in a temperate secondary forest

Niche complementarity, mass-ratio, and vegetation quantity effects have been identified as major drivers of the biodiversity-ecosystem functioning (BEF) relationships. However, their relative contribution to biomass and productivity is not yet clear in temperate secondary forests. Based on the observations from a 21.12-ha temperate secondary forest plot in northeastern China, we assessed how these mechanisms regulate forest biomass and productivity. The niche complementarity effect was quantified using a functional diversity metric that was calculated from six locally collected functional traits. The mass-ratio effect was described as functional trait composition using community-weighted mean trait values. Vegetation quantity effect was evaluated using vegetation biomass. We performed structural equation modeling to test the alternative mechanisms. Our results provide evidence for all three mechanisms. Functional diversity increased forest productivity, in line with the niche complementarity hypothesis. Acquisitive traits (e.g., greater specific leaf area and leaf nitrogen concentration) enhance productivity, while conservative traits (e.g., greater wood density) enhance the long-term accumulation of biomass, demonstrating the mass-ratio hypothesis. Furthermore, we observed a significant positive relationship between biomass and productivity, confirming the vegetation quantity hypothesis. We conclude that functional traits drive biomass and productivity through multiple mechanisms. Both niche complementarity and the mass-ratio effects play roles in this temperate secondary forest. In addition, we emphasize the importance of preserving sufficient biomass stock to ensure maximum productivity in secondary forests. Our study contributes to the identification of the mechanisms underlying BEF relationships and has practical significance for guiding temperate secondary forest management and conservation.