Drought-induced tree mortality of European beech affected by legacy effects and small-scale heterogeneity in soil properties and tree neighbourhood composition

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Forests are facing increasing pressure due to climatic stress and its influence on herbivore and pathogen dynamics, mainly through increased intensity and frequency of drought events. A recent example of how increasing climate stress can shape the future of forests is the regional die-back of European beech (Fagus sylvatica L.) after the severe Central European drought of 2018/2019. This drastic and unprecedented response of this major European broad-leaved species that would naturally dominate Central European forest vegetation suggests a downward spiral of decreasing vitality in combination with decreasing growth stability — questioning the envisioned prominent role of this species for climate change mitigation.The sites affected by beech die-back exhibit a high heterogeneity, where individuals with strong drought responses up to mortality occur next to vital and seemingly unaffected individuals. We propose that complex interactions of various abiotic and biotic factors might be responsible for this uneven distribution of drought response within beech stands: while small-scale heterogeneity in abiotic factors, predominantly soil properties, drives the variability of soil water availability within the stand, intra-specific competition as a critical biotic factor might further mediate legacy effects and drought-induced mortality.The proposed project, therefore, aims at disentangling the drivers of drought-induced tree mortality in European beech in response to the severe 2018/19 drought and heatwave by a joint analysis of growth-responses to historic climatic variability and fine-scale edaphic and structural factors along a climatic gradient. Specifically, we aim to i) confirm the visual categorization of tree vitality and crown condition by historical high-resolution remote sensing products, to ii) describe small-scale variability in the topographic, edaphic, structural and competitional status of each tree, to iii) quantify and compare the loss of vitality among trees through decline indicators derived from radial growth, to iv) develop a novel strategy for risk assessment through establishing the biogeographic context for drought-related patterns of growth decline and recovery across the distributional range of European beech, and (v) to provide a field-validated framework for predicting the drought-sensitivity of this economically and ecologically important tree species under given edaphic and structural conditions.