Environmental controls on the causes and functional consequences of tree species diversity

Knowledge of the functional consequences of biodiversity is increasing through studies of both experimental systems and natural systems. Community assembly theory has also helped to reveal the causes of biodiversity organization. However, the causes and consequences of biodiversity have been discussed in parallel and simultaneous consideration of both has been limited, even though they are both influenced by regional environmental conditions. To understand the relationship between biodiversity and productivity, I focused on the linkages between the number of tree species and biomass productivity across forest biomes under a range of bioclimatic conditions. I found that high tree diversity generally increased productivity. This was primarily due to a selection process that results from a high probability of having high-performance species and their dominance at high diversity, regardless of the biome. In less-productive biomes, the residual importance of diversity, which likely reflects other forms of biological interactions (including species complementarity), increased productivity. These findings of differential diversity effects under different environmental conditions are consistent with the existing theory of community assembly, which predicts a shift in the assembly process from stochasticity to determinism with increasing environmental harshness. Analyses based on functional trait diversity also supported this theory: stochastic assembly (resulting in the selection effect) and deterministic assembly (possibly resulting from interspecific niche differences) became more important in productive and less-productive biomes, respectively.Synthesis. Increasing our understanding of the causalities between diversity and other characteristics, such as productivity, is crucial, particularly for forest ecosystems, because of the increasing interest in productivity-related ecosystem services supported by diverse assemblages of trees.