Biodiversity-ecosystem functioning experiments found that productivity generally increases with species richness, but less is known about effects of within-species genetic richness and potential interactions between the two. While functional differences between species can explain species richness effects, empirical evidence regarding functional differences between genotypes within species and potential consequences for productivity is largely lacking. We therefore measured within-and among-species variation in functional traits and growth and determined stand-level tree biomass in a large forest experiment factorially manipulating species and genetic richness in subtropical China. Within-species variation across genetic seed families, in addition to variation across species, explained a substantial amount of trait variation. Furthermore, trait responses to species and genetic richness varied significantly within and between species. Multivariate trait variation was larger among individuals from species mixtures than those from species monocultures, but similar among individuals from genetically diverse vs genetically uniform monocultures. Correspondingly, species but not genetic richness had a positive effect on stand-level tree biomass. We argue that identifying functional diversity within and among species in forest communities is necessary to separate effects of species and genetic diversity on tree growth and community productivity.

It is increasingly being recognized that tree species diversity has positive effects on forest ecosystem carbon (C) stock. However, at broad spatial scales, this relationship may depend on climate conditions and species mycorrhizal associations. Here, observations from 667 forest plots in subtropical China were used to investigate the effects of species diversity, mean annual precipitation (MAP), mean annual temperature (MAT) and mycorrhizal type (arbuscular or ectomycorrhizal) on the forest C stock and its components (tree C stock, shrub layer C stock, herb layer C stock, litter layer C stock, root C stock and soil C stock). We found positive effect of tree species diversity on total forest C stock. MAP had positive effects on total forest C stock and its components, while MAT had consistently negative effects on total forest C stock and most of its components. Different levels of MAP and MAT did modulate the strength of effect of species diversity on forest C stock and its components. In addition, species diversity, MAT and MAP showed a significant positive relationship with arbuscular mycorrhiza‐associated tree C stock but had no or negative relationship with ectomycorrhiza‐associated tree C stock. Synthesis. Our results indicate that maintaining high level of species diversity may support the buffering of negative effects resulting from climate warming. Furthermore, under climate warming the specific C stock of AM trees can increase, which can potentially promote forest C stock. Taken together, our study suggests that afforestation policies should consider not only tree species diversity to increase forest C stock but also the effects of different tree mycorrhizal types.