ContentsSummary50I.Introduction50II.Drivers of the diversity–productivity relationship51III.Patterns of the diversity–productivity relationship55IV.Responses of mixed stands to climate change57V.Conclusions60Acknowledgements61References61SummaryAlthough the relationship between species diversity and biomass productivity has been extensively studied in grasslands, the impact of tree species diversity on forest productivity, as well as the main drivers of this relationship, are still under discussion. It is widely accepted that the magnitude of the relationship between tree diversity and forest stand productivity is context specific and depends on environmental conditions, but the underlying mechanisms of this relationship are still not fully understood. Competition reduction and facilitation have been identified as key mechanisms driving the diversity–productivity relationship. However, contrasting results have been reported with respect to the extent to which competition reduction and facilitation determine the diversity–productivity relationship. They appear to depend on regional climate, soil fertility, functional diversity of the tree species involved, and developmental stage of the forest. The purpose of this review is to summarize current knowledge and to suggest a conceptual framework to explain the various processes leading to higher productivity of species‐rich forests compared with average yields of their respective monocultures. This framework provides three pathways for possible development of the diversity–productivity relationship under a changing climate.