Unprecedented tree dieback across Central Europe caused by recent global change‐type drought events highlights the need for a better mechanistic understanding of drought‐induced tree mortality. Although numerous physiological risk factors have been identified, the importance of two principal mechanisms, hydraulic failure and carbon starvation, is still debated. It further remains largely unresolved how the local neighborhood composition affects individual mortality risk. We studied 9435 young trees of 12 temperate species planted in a diversity experiment in 2013 to assess how hydraulic traits, carbon dynamics, pest infestation, tree height and neighborhood competition influence individual mortality risk. Following the most extreme global change‐type drought since record in 2018, one third of these trees died. Across species, hydraulic safety margins (HSMs) were negatively and a shift towards a higher sugar fraction in the non‐structural carbohydrate (NSC) pool positively associated with mortality risk. Moreover, trees infested by bark beetles had a higher mortality risk, and taller trees a lower mortality risk. Most neighborhood interactions were beneficial, although neighborhood effects were highly species‐specific. Species that suffered more from drought, especially Larix spp. and Betula spp., tended to increase the survival probability of their neighbors and vice versa. While severe tissue dehydration marks the final stage of drought‐induced tree mortality, we show that hydraulic failure is interrelated with a series of other, mutually inclusive processes. These include shifts in NSC pools driven by osmotic adjustment and/or starch depletion as well as pest infestation and are modulated by the size and species identity of a tree and its neighbors. A more holistic view that accounts for multiple causes of drought‐induced tree mortality is required to improve predictions of trends in global forest dynamics and to identify mutually beneficial species combinations.

Plant diversity effects on community productivity often increase over time. Whether the strengthening of diversity effects is caused by temporal shifts in species-level overyielding (i.e., higher species-level productivity in diverse communities compared with monocultures) remains unclear. Here, using data from 65 grassland and forest biodiversity experiments, we show that the temporal strength of diversity effects at the community scale is underpinned by temporal changes in the species that yield. These temporal trends of species-level overyielding are shaped by plant ecological strategies, which can be quantitatively delimited by functional traits. In grasslands, the temporal strengthening of biodiversity effects on community productivity was associated with increasing biomass overyielding of resource-conservative species increasing over time, and with overyielding of species characterized by fast resource acquisition either decreasing or increasing. In forests, temporal trends in species overyielding differ when considering above- versus belowground resource acquisition strategies. Overyielding in stem growth decreased for species with high light capture capacity but increased for those with high soil resource acquisition capacity. Our results imply that a diversity of species with different, and potentially complementary, ecological strategies is beneficial for maintaining community productivity over time in both grassland and forest ecosystems.

In recent decades, European temperate forests have repeatedly suffered from severe droughts. Drought‐weakened forests have often become more susceptible to pest outbreaks such as bark beetle infestations. Tree species diversity is expected to increase resistance to drought and pests, but evidence for a positive tree diversity effect on insect pest reduction is largely circumstantial. Here we tested the effects of tree diversity and biogeographic origin of tree species on bark beetle infestation in a large, young tree diversity experiment, with six broadleaved and six conifer species from Europe and North America. Lower infestation risk was expected for the exotic tree species in each congeneric pair (spruce, larch and pine) and for mixtures with higher species richness and higher broadleaf proportion. Following a severe drought in summer 2018, the conifer trees were attacked by the six‐toothed spruce bark beetle Pityogenes chalcographus. Bark beetle boreholes were recorded in winter 2018/2019 on all conifer species. Norway spruce Picea abies and European larch Larix decidua were the most infested species and thus considered main hosts of the bark beetle. For these two species, probability of infestation decreased with increasing tree diversity (although this was only significant for Larix). In contrast, Pinus, which were less infested overall, were more likely to be infested in plots with high tree diversity. Exotic trees tended to be less infested, with clearest support for enemy release found at the level of infestation intensity when considering pure conifer stands. Overall, the effects of tree diversity and tree species origin were not as strong as the effect of position within the experimental site, where higher rates of infestation were observed at the edge than in the centre. Synthesis. Increasing tree diversity may reduce the risk of bark beetle infestation for genera prone to high infestation rates (Picea and Larix), but risk for less preferred genera (Pinus, and to some extent the exotic tree species) may increase with tree diversity due to spillover from preferred hosts. In mixed forests, the risk of infestation, even by relatively specialized insect pests, may be redistributed among tree species rather than reduced for all.

Key message: Seven European beech provenances differing largely in growth performance were grown at two common garden sites in Germany and Slovakia. The intra-specific variability of most traits was explained more by phenotypic plasticity than inter-provenance variability, and efficiency-related traits showed a higher phenotypic plasticity than safety-related traits. Context: To maintain climate-resilient future forests, replicated common-garden experiments are suited for developing assisted migration strategies for key tree species. Aims: We analysed the magnitude of inter-provenance variability and phenotypic plasticity for 12 functional traits of European beech (Fagus sylvatica L.) and analysed whether the climate at the place of origin left an imprint. Moreover, we asked whether growth is unrelated to xylem safety and to what extent the foliar, xylem and growth-related traits are coordinated. Methods: Terminal branches were collected from 19-year-old and 22-year-old trees of seven European beech provenances planted at two common garden sites in Germany and Slovakia, respectively. Three hydraulic, three wood anatomical and four foliar traits were measured and related to two growth-related variables. Results: At the two sites, the same pair of provenances showed the highest and lowest growth. Nevertheless, a high degree of phenotypic plasticity was observed, as all traits differed significantly between sites after accounting for provenance effects, with hydraulic safety-related traits showing the lowest and efficiency-related traits the highest plasticity. There was no evidence for inter-provenance variability in xylem embolism resistance (P₅₀) or the foliar carbon isotope signature (δ¹³C), a proxy for intrinsic water use efficiency (iWUE), and both were unrelated to growth. P₅₀ was positively correlated with the lumen-to-sapwood area ratio and vessel density. Conclusions: Because of the lacking trade-off between embolism resistance and growth, highly productive provenances can be selected without reducing the drought tolerance of the branch xylem. However, as xylem safety is only one element of a trees’ drought response, it may be beneficial to select provenances with other more conservative drought adaptations such as smaller vessel lumen areas for increasing xylem safety and small supported total leaf areas for reduction of total transpiration.