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

由于气候压力及其对食草动物和病原体动态的影响，森林面临着越来越大的压力，主要是由于干旱事件的强度和频率增加。气候压力的增加如何影响森林的未来，最近的一个例子是欧洲山毛榉的地区性死亡。在2018/2019年中欧严重干旱之后。这种主要的欧洲阔叶树种自然地主导着中欧森林植被，这种剧烈而史无前例的反应表明，活力不断下降，生长稳定性不断下降--质疑这种物种在缓解气候变化方面所设想的突出作用。受山毛榉枯萎影响的地点表现出高度的异质性，其中具有强烈干旱反应直至死亡的个体发生在生命和看似未受影响的个体旁边。我们认为，各种非生物和生物因素的复杂相互作用可能是造成山毛榉林内干旱响应不均匀分布的原因：虽然非生物因素(主要是土壤性质)的小规模异质性驱动了林内土壤水分有效性的变异性，但作为关键生物因素的种内竞争可能进一步调节遗传效应和干旱诱导的死亡。因此，拟议的项目旨在通过联合分析历史气候变异性和沿气候梯度的细尺度土壤和结构性因素的生长响应，来解开欧洲山毛榉因干旱导致树木死亡的驱动因素。具体地说，我们的目标是：i)通过历史高分辨率遥感产品确认树木活力和树冠状况的视觉分类；ii)描述每棵树木的地形、土壤、结构和竞争状况的小尺度变异性；iii)通过径向生长的衰退指标量化和比较树木之间的活力损失；iv)通过建立整个欧洲山毛榉分布范围内与干旱有关的生长衰退和恢复模式的生物地理背景，开发一种新的风险评估策略，以及(V)提供一个田间验证的框架，用于预测在给定的土壤和结构条件下这种重要的经济和生态树种对干旱的敏感性。

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.