• The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial.
• We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use.
• Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting.
• These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

中文翻译：

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干旱减少了山毛榉从干燥的表土中的水分吸收，但没有从较深的土壤层发生补偿性吸收

• 预计未来干旱事件的强度和频率将增加，预计会对森林产生不利影响。因此，有关干旱期间和干旱后不同土壤层的树木吸水动态的信息至关重要。
• 我们应用原位水同位素监测系统来确定欧洲山毛榉土壤和木质部水中的氧同位素组成，分辨率为 2 小时，同时测量土壤含水量、蒸腾作用和树木水分亏缺。使用贝叶斯同位素混合模型，我们推断出四种不同土壤层的水对树木用水的相对和绝对贡献。
• 在 2018 年干旱初期，山毛榉从最上层 5 厘米的土壤层吸收了超过 50% 的水分，但随后从干燥的表层土壤中吸收的绝对水分减少了 84%。这些树木无法通过从更深的土壤层额外吸收来定量补偿有限的表土水分可用性，这与细根深度分布有关。重新润湿后 3 周内，表层土壤的绝对吸水量恢复到干旱前的水平。
• 这些吸收模式有助于解释山毛榉的干旱敏感性及其在干旱释放后的高恢复潜力。