Species richness and traits predict overyielding in stem growth in an early-successional tree diversity experiment

Over the last two decades, empirical work has established that higher biodiversity can lead to greater primary productivity; however, the importance of different aspects of biodiversity in contributing to such relationships is rarely elucidated. We assessed the relative importance of species richness, phylogenetic diversity, functional diversity, and identity of neighbors for stem growth 3 yr after seedling establishment in a tree diversity experiment in eastern Minnesota. Generally, we found that community-weighted means of key functional traits (including mycorrhizal association, leaf nitrogen and calcium, and waterlogging tolerance) as well as species richness were strong, independent predictors of stem biomass growth. More phylogenetically diverse communities did not consistently produce more biomass than expected, and the trait values or diversity of individual functional traits better predicted biomass production than did a multidimensional functional diversity metric. Furthermore, functional traits and species richness best predicted growth at the whole-plot level (12 m² ), whereas neighborhood composition best predicted growth at the focal tree level (0.25 m² ). The observed effects of biodiversity on growth appear strongly driven by positive complementary effects rather than by species-specific selection effects, suggesting that synergistic species' interactions rather than the influence of a few important species may drive overyielding.