The mediation roles of intraspecific and interspecific functional trait diversity for linking the response of aboveground biomass to species richness across forest strata in a subtropical forest

Intraspecific functional trait diversity (FTD) has improved our understandings about the key mechanisms of species coexistence in plant communities. Yet, little is known about whether and how intraspecific and interspecific FTD mediate the response of aboveground biomass to species richness across forest strata (i.e. overstorey and understorey) and at whole-community in forests. To address this question, we tested the direct and indirect responses of aboveground biomass to species richness via intraspecific and interspecific FTD based on specific leaf area (FTD_SLA) and leaf dry matter content (FTD_LDMC) using structural equation modeling, in addition to the effects of soil nutrients, across 125 plots in a 5-ha subtropical forest in Eastern China. Results showed that intraspecific FTD mediates the response of aboveground biomass to species richness at overstorey and understorey strata, and whole-community level, while interspecific FTD did so at understorey strata only. At overstorey strata, 14% of the variation in aboveground biomass was accounted by the strong direct positive effect of species richness only. At understorey strata, soil nutrients had a strong negative direct effect followed by positive effects of species richness and FTD_LDMC on aboveground biomass with 44–45% of the variation in both intraspecific and interspecific FTD models. At whole community level, 14% of the variation in aboveground biomass was explained by the strong positive direct effect of species richness followed by negative direct effect of intraspecific FTD_SLA. Intraspecific and interspecific FTD_LDMC had positively mediated the response of aboveground biomass to species richness at understorey strata through niche differentiation. Intraspecific FTD had negligible mediation role, whereas interspecific FTD had no role, for linking the response of aboveground biomass to species richness at overstorey strata, indicating that only dominant species with a specific functional strategy may largely determine community trait space. Intraspecific FTD_SLA had negative relationship with aboveground biomass at the whole-community, probably due to the presence of a few large trees occupying larger niche space in a community. We conclude that intraspecific versus interspecific FTD plays a central role for linking the direct and indirect responses of aboveground biomass to species richness, but these relationships depend on the forest strata of a community.