Tree size, neighbourhood composition and structure affect individual tree vitality of European beech following extreme drought

The severe drought of 2018/19 caused widespread early discoloration, defoliation and tree mortality in European beech (Fagus sylvatica L.), although local drought responses varied strongly among coexisting trees. To identify potential drivers, we investigated 520 mature beech trees across 20 forest sites dominated by European beech in Southern Germany, Central Europe. We recorded crown health conditions and described the small-scale variability in the topographic, edaphic, structural and competitional status of individual trees, complemented by dendrochronological sampling. Overall, tall trees growing on sites with more available soil water and higher predrought growth showed highest defoliation rates, supporting global findings that larger trees are more susceptible to drought-induced tree mortality. In the ordered beta-regression model, including forest sites as random effect, pre-existing canopy gaps significantly improved vitality during drought and beech tended to show lower defoliation in mixed stands with Scots pine. These findings highlight canopy and neighbourhood structure as a possible buffer against drought stress. By contrast, the influence of tree size and soil water availability could not be detected in the model due to a large variability across sites. A substantial portion of the remaining small-scale variation in defoliation could not be explained by our fixed effects, but rather by site effects, underlining the complexity of drought responses in beech forests. We conclude that for future-proofing our forests, selective thinning of large European beech trees in non-drought years appears to be an appropriate silvicultural measure to strengthen the drought resilience of neighbouring trees, which deserves further testing. Furthermore, silvicultural management actions should account for species mixtures while explicitly considering species-specific drought response strategies, for example by combining species with contrasting hydraulic strategies or rooting systems.

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