Global evidence for a positive relationship between tree species richness and ecosystem photosynthesis.

Forest biodiversity plays a critical role in sustaining ecosystem functioning and buffering the effects of increased extreme weather events on forests. A global assessment of the relationship between biodiversity and photosynthesis in natural forest ecosystems, however, remains elusive. We used a large dataset of the richness of tree species from a large number of globally distributed forest plots combined with satellite retrievals of sun-induced chlorophyll fluorescence, a novel proxy for photosynthesis, to evaluate the relationship between forest biodiversity and photosynthesis and its biological mechanisms at the global scale. We found that species richness and photosynthesis were often positively correlated at the global scale, with stronger relationships in tropical forests but weaker associations in high-latitude regions. This positive relationship was mainly driven by a larger role of species richness in increasing maximal photosynthesis than in prolonging the growing season. We also found that higher light capture by increasing the complexity of community structure was the basis of this increase in forest photosynthesis. Forests with high species richness also showed higher foliar nitrogen concentrations and the maximum rate of ribulose 1,5-bisphosphate carboxylase/oxygenase carboxylation, which are two crucial traits determining photosynthetic capacity. Our observation-based findings of ecosystem carbon uptake responses to changes in biodiversity suggest that the loss of biodiversity may jeopardize ecosystem carbon uptake and the terrestrial carbon sink, and will provide important constraints to Earth-system models.