Under drought, potential plant death from depletion of carbon (C) stores, referred to as carbon starvation, is thought to result from negative carbon balance during (partial) stomatal closure. As evidence for C starvation is rare for mature trees, we investigated the C dynamics in mature beech and spruce under drought, focusing on non-structural carbohydrates (NSC) as an indicator of their C balance. Trees were exposed to complete exclusion of precipitation throughfall over two consecutive growing seasons. We assessed NSC concentrations during the early dormant season in leaves, twigs, stem phloem/xylem, coarse and fine roots. Up-scaling to whole-tree NSC pools was achieved using a process-based single-tree model (“BALANCE”), estimating tree biomass increment. While there were distinct differences in NSC concentration among different tissues in both beech and spruce (root < stem < twig < leaves and xylem < phloem), drought did not affect NSC concentrations. However, compared to controls, the whole-tree NSC pool size significantly decreased under drought in both beech (42 %) and spruce (36 %), in parallel to a significant growth decline of overall 52 % and 57 %, respectively. Nevertheless, drought-stressed beech and spruce invested almost twice as much C in reproductive structures relative to total C investment (i.e. 6.0 ± 3.3 and 52.3 ± 8.71 %) compared to control trees (3.1 ± 1.8 and 29.2 ± 7.8 %). This highlights the high priority of C investment into reproduction relative to growth under drought. Given that NSC concentrations are maintained even under severe drought over two growing seasons, NSC pool sizes appear to be a better proxy to assess whole-tree’s carbon status in mature trees. Overall, trees maintained NSC availability, avoiding carbon starvation, by downregulating a major C sink (i.e. growth) while upholding reproduction.

在干旱情况下，碳 (C) 储存枯竭导致的潜在植物死亡，称为碳饥饿，被认为是由（部分）气孔关闭期间的负碳平衡引起的。由于成熟树木缺乏 C 的证据很少，我们研究了干旱条件下成熟山毛榉和云杉的 C 动态，重点关注非结构性碳水化合物 (NSC) 作为其 C 平衡的指标。树木在连续两个生长季节完全没有降水。我们评估了早期休眠季节叶、树枝、茎韧皮部/木质部、粗根和细根中的 NSC 浓度。使用基于过程的单树模型（“BALANCE”），估计树木生物量增量，实现了对全树 NSC 池的扩展。虽然山毛榉和云杉不同组织的 NSC 浓度存在明显差异（根 < 茎 < 枝 < 叶和木质部 < 韧皮部），但干旱不影响 NSC 浓度。然而，与对照相比，山毛榉 (42 %) 和云杉 (36 %) 的全树 NSC 池大小在干旱条件下显着减少，同时总体增长分别显着下降 52 % 和 57 %。尽管如此，与对照树木（3.1 ± 1.8 和 29.2 ± 7.8 %）相比，干旱胁迫的山毛榉和云杉在繁殖结构中的 C 投资几乎是总 C 投资的两倍（即 6.0 ± 3.3 和 52.3 ± 8.71 %）。这凸显了相对于干旱条件下的增长，C 投资对再生产的高度重视。鉴于即使在两个生长季节的严重干旱下 NSC 浓度也能保持不变，NSC 池大小似乎是评估成熟树木中整棵树的碳状况的更好代表。总体而言，树木通过在维持繁殖的同时下调主要的碳汇（即生长），保持了 NSC 的可用性，避免了碳饥饿。