Drought characteristics alters neighbourhood diversity effects on tree growth responses during extreme events 

Drought events of increasing frequency, intensity, and duration are driving widespread forest dieback and mortality worldwide. While mixed-species forests are promoted as a strategy to enhance resistance and resilience to drought, increasing species richness alone does not consistently improve tree growth responses. Moreover, the tree diversity effects under unprecedented multi-year droughts remain poorly understood.Here, we used a network of planted tree diversity experiments to assess how neighborhood tree diversity and species-specific hydraulic traits influence drought-induced growth responses. We analysed tree-rings on 948 trees from 19 species across nine experiments spanning Europe’s major climate zones. All sites experienced recent severe droughts, including the record-breaking 2018–2020 multi-year drought. Experimental gradients in tree species richness (1–6 species) allowed us to disentangle tree diversity effects while controlling for environmental heterogeneity.We quantified radial biomass growth using X-ray computed tomography, and assessed the physiological drought stress using the carbon isotope signal of tree rings in dry and wet years (∆δ13Cdry-wet). We used a functional trait framework to evaluate diversity effects at neighbourhood scale, using hydraulic safety margin (HSMTLP​) to characterise the species’ drought tolerance.Tree growth responses were driven by drought characteristics, species drought tolerance, and neighborhood functional diversity, but not by neighborhood species richness per se. Increasing drought duration within a growing season shifted neighborhood diversity effects on growth from beneficial to negative. Under consecutive drought years, diversity effects on growth responses depended on site context, but strengthened in sites showing positive or negative effects. Neighborhood diversity reduced physiological drought stress (lower ∆δ13Cdry-wet) for drought-susceptible species (low HSMTLP) growing alongside drought-tolerant neighbors. Yet such changes in carbon isotopic composition were not directly coupled to growth responses during the same drought year.Our results demonstrate that functional trait diversity—rather than species richness—determines how trees respond to extreme and prolonged drought. While mixing species with contrasting hydraulic strategies can alleviate physiological drought stress, increasing tree diversity does not always enhance growth resilience. The effectiveness of tree mixing is highly context dependent at neighborhood scale, and can shift with increasing drought duration and intensity. This underscores the need for trait-based approaches and locally-adapted solutions to make our forests more resilient to longer and harsher droughts. Keywords: tree diversity, mixed plantation trials, TreeDivNet, multi-year drought, drought tolerance, functional traits, tree rings, X-ray computed tomography, 13C isotopic composition