The mechanisms underpinning forest biodiversity-ecosystem function relationships remain unresolved. Yet, in heterogeneous forests, ecosystem function of different strata could be associated with traits or evolutionary relationships differently. Here, we integrate phylogenies and traits to evaluate the effects of elevational diversity on above-ground biomass across forest strata and spatial scales. Community-weighted means of height and leaf phosphorous concentration, and functional diversity in specific leaf area exhibited positive correlations with tree biomass, suggesting that both positive selection effects and complementarity occur. However, high shrub biomass is associated with greater dissimilarity in seed mass and multidimensional trait space, while species richness or phylogenetic diversity is the most important predictor for herbaceous biomass, indicating that species complementarity is especially important for understory function. The strength of diversity-biomass relationships increases at larger spatial scales. We conclude that strata- and scale- dependent assessments of community structure and function are needed to fully understand how biodiversity influences ecosystem function.