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For decades, ecologists have investigated the effects of tree species diversity on tree productivity at different scales and with different approaches ranging from observational to experimental study designs. Using data from five European national forest inventories (16,773 plots), six tree species diversity experiments (584 plots), and six networks of comparative plots (169 plots), we tested whether tree species growth responses to species mixing are consistent and therefore transferrable between those different research approaches. Our results confirm the general positive effect of tree species mixing on species growth (16% on average) but we found no consistency in species‐specific responses to mixing between any of the three approaches, even after restricting comparisons to only those plots that shared similar mixtures compositions and forest types. These findings highlight the necessity to consider results from different research approaches when selecting species mixtures that should maximize positive forest biodiversity and functioning relationships.

Understanding the processes that underlie drought-related tree vitality loss is essential for anticipating future forest dynamics, and for developing management plans aiming at increasing the resilience of forests to climate change. Forest vitality has been continuously monitored in Europe since the acid rain alert in the 1980s, and the intensive monitoring plots of ICP Forests offer the opportunity to investigate the effects of air pollution and climate change on forest condition. By making use of over 100 long-term monitoring plots, where crown defoliation has been assessed extensively since 1990, we discovered a progressive shift from a negative to a positive effect of species richness on forest health. The observed tipping point in the balance of net interactions, from competition to facilitation, has never been reported from real ecosystems outside experimental conditions; and the strong temporal consistency of our observations with increasing drought stress emphasizes its climate change relevance. Furthermore, we show that higher species diversity has reduced the severity of defoliation in the long term. Our results confirm the greater resilience of diverse forests to future climate change-induced stress. More generally, they add to an accumulating body of evidence on the large potential of tree species mixtures to face manifold disturbances in a changing world.

Mixing different tree species in forest plantations might increase stand productivity and resilience compared to monocultures, but mixing effects in the early stage of mixed forest plantations are still poorly understood. In general, sapling growth is affected by environmental factors, sapling species identity, direct and indirect interactions with neighbouring saplings, and competition with the ground layer vegetation. We assessed the diameter increment, height increment, and height to diameter ratio of ca. 5500 permanently marked saplings growing in local neighbourhoods that differ in number and identity of the sapling species present. We found that sapling growth was related to the sapling’s pecies identity and the characteristics of its local neighbourhood (notably phylogenetic diversity, relative size asymmetry, and ground vegetation cover). The identity of the neighbours only mattered when target and neighbour tree species differed in, for instance, light requirements. The growth of the young saplings was positively affected by tall neighbours and ground vegetation cover. This suggests that careful species selection when designing mixed forest plantations is important and that ground vegetation can be favourable for sapling growth in the first years of a new plantation.