While it is widely acknowledged that forest biodiversity contributes to climate change mitigation through improved carbon sequestration, conversely how climate affects tree species diversity-forest productivity relationships is still poorly understood. We combined the results of long-term experiments where forest mixtures and corresponding monocultures were compared on the same site to estimate the yield of mixed-species stands at a global scale, and its response to climatic factors. We found positive mixture effects on productivity using a meta-analysis of 126 case studies established at 60 sites spread across five continents. Overall, the productivity of mixed-species forests was 15% greater than the average of their component monocultures, and not statistically lower than the productivity of the best component monoculture. Productivity gains in mixed-species stands were not affected by tree age or stand species composition but significantly increased with local precipitation. The results should guide better use of tree species combinations in managed forests and suggest that increased drought severity under climate change might reduce the atmospheric carbon sequestration capacity of natural forests.

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Northern red oak (Quercus rubra L. syn. Q. borealis F. Michx.) is a valuable broadleaved tree species originating from the eastern half of the USA and Canada. It was introduced to Europe in 1691 and currently covers over 350 000 ha, being found all over the continent, except the coldest part of Scandinavia. It is a fast-growing and valuable broadleaved tree due to its ecological characteristics, good wood properties and high economic value. Northern red oak prefers deep, loose, moderately humid and acid soils, without compact horizons and of at least moderate fertility. It does not grow well on dry, calcareous soils as well as waterlogged or poorly drained soils. It is either naturally regenerated using a group shelterwood system or planted using seedlings of European provenance, collected in certified seed stands. As northern red oak is light-demanding, its management should be ‘dynamic’ and includes heavy interventions (cleaning–respacing and thinning from above), in order to minimize crown competition between the final crop trees. These should produce large diameter trees for valuable end uses (e.g. veneer, solid furniture, lumber, etc.) within a rotation period generally of 80–100 years. The necessity for pruning (both formative and high) depends on the stand stocking at establishment, the subsequent silvicultural interventions as well as the occurrence of forking. The adaptation potential of northern red oak to predicted climate change, especially drought, seems to be higher than for European native oaks, the importance of the species is expected to increase in the future.