Forest strata-dependent functional evenness explains whole-community aboveground biomass through opposing mechanisms

A positive biodiversity - aboveground biomass relationship is often attributed to the niche complementarity hypothesis. This hypothesis has received much less support when based on functional trait diversity and when tested in complex natural forests. Here, we hypothesized that niche complementarity (high trait diversity) in the understorey and niche overlap (low trait diversity) in the overstorey would drive whole-community aboveground biomass. To test this hypothesis, we used multiple linear regression models to evaluate how functional trait diversity (functional richness, evenness, divergence and dispersion) of overstorey, understorey, and whole community determine aboveground biomass across 125 plots in a 5-ha subtropical forest of Eastern China. A structural equation model was used to evaluate whether the functional trait diversity of overstorey affects the relationships between understorey functional trait diversity and whole-community aboveground biomass while considering for the effect of environmental factors. We additionally considered the effects of mass ratio (i.e. community-weighted mean of trait values) and stand structural complexity. The strong negative effect of overstorey functional evenness on aboveground biomass provides support to the functional dominance and/or niche overlap rather than niche complementarity effect. The negative effect of overstorey functional evenness on aboveground biomass was also partially mediated by its direct negative effect on understorey functional evenness. The weak positive effect of understorey functional evenness on aboveground biomass provides support to the niche complementarity effect. We conclude that functional evenness of overstorey and understorey strata affect whole-community aboveground biomass differently through opposing mechanisms, regardless of the effects of mass ratio and stand structural complexity.