The role of carbon (C) and nutrient uptake, allocation, storage and especially their interactions in survival and recovery of trees under increased frequencies and intensities of drought events is not well understood. A full factorial experiment with four soil water content regimes ranging from extreme drought to well-watered conditions and two fertilization levels was carried out. We aimed to investigate whether nutrient addition mitigates drought effects on downy oak (Quercus pubescens Willd.) and whether storage pools of non-structural carbohydrates (NSC) are modified to enhance survival after 2.5 years of drought and recovery after drought relief. Physiological traits, such as photosynthesis, predawn leaf water potential as well as tissue biomass together with pools and dynamics of NSC and nutrients at the whole-tree level were investigated. Our results showed that fertilization played a minor role in saplings’ physiological processes to cope with drought and drought relief, but reduced sapling mortality during extreme drought. Irrespective of nutrient supply, Q. pubescens showed increased soluble sugar concentration in all tissues with increasing drought intensity, mostly because of starch degradation. After 28 days of drought relief, tissue sugar concentrations decreased, reaching comparable values to those of well-watered plants. Only during the recovery process from extreme drought, root NSC concentration strongly declined, leading to an almost complete NSC depletion after 28 days of rewetting, simultaneously with new leaves flushing. These findings suggest that extreme drought can lead to root carbon exhaustion. After drought relief, the repair and regrowth of organs can even exacerbate the root carbon depletion. We concluded that under future climate conditions with repeated drought events, the insufficient and lagged carbon replenishment in roots might eventually lead to carbon starvation and further mortality.

中文翻译：

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根碳和养分稳态决定软橡树树苗的存活和干旱恢复

在干旱事件频率和强度增加的情况下，碳 (C) 和养分吸收、分配、储存的作用，尤其是它们在树木存活和恢复中的作用尚不清楚。进行了从极端干旱到充足浇水条件和两种施肥水平的四种土壤含水量状况的全析因试验。我们的目的是调查营养添加是否减轻了对绒毛橡树（Quercus pubescens Willd. ）的干旱影响。以及是否对非结构性碳水化合物 (NSC) 的储存池进行了改造，以提高 2.5 年干旱后的存活率和干旱缓解后的恢复能力。研究了生理性状，如光合作用、黎明前叶水势以及组织生物量，以及全树水平的 NSC 和养分的池和动态。我们的研究结果表明，施肥在树苗应对干旱和缓解干旱的生理过程中发挥了次要作用，但在极端干旱期间会降低树苗死亡率。无论营养供应如何，Q . 毛茸茸的随着干旱强度的增加，所有组织中的可溶性糖浓度增加，主要是因为淀粉降解。干旱缓解 28 天后，组织糖浓度下降，达到与浇水良好植物相当的值。只有在极端干旱的恢复过程中，根部 NSC 浓度急剧下降，导致在 28 天再润湿后几乎完全耗尽 NSC，同时新叶冲洗。这些发现表明，极端干旱会导致根系碳耗竭。干旱解除后，器官的修复和再生甚至会加剧根系碳耗竭。我们得出的结论是，在未来反复发生干旱事件的气候条件下，