Understanding the vulnerability of trees to drought-induced mortality is key to predicting the fate of forests in a future climate with more frequent and intense droughts, although the underlying mechanisms are difficult to study in adult trees. Here, we explored the dynamic changes of water relations and limits of hydraulic function in dying adults of Norway spruce (Picea abies L.) during the progression of the record-breaking 2018 Central European drought. In trees on the trajectory to drought-induced mortality, we observed rapid, nonlinear declines of xylem pressure that commenced at the early onset of xylem cavitation and caused a complete loss of xylem hydraulic conductance within a very short time. We also observed severe depletions of nonstructural carbohydrates, though carbon starvation could be ruled out as the cause of the observed tree death, as both dying and surviving trees showed these metabolic limitations. Our observations provide striking field-based evidence for fast dehydration and hydraulic collapse as the cause of drought-induced mortality in adult Norway spruce. The nonlinear decline of tree water relations suggests that considering the temporal dynamics of dehydration is critical for predicting tree death. The collapse of the hydraulic system within a short time demonstrates that trees can rapidly be pushed out of the zone of hydraulic safety during the progression of a severe drought. In summary, our findings point toward a higher mortality risk for Norway spruce than previously assumed, which is in line with current reports of unprecedented levels of drought-induced mortality in this major European tree species.

了解树木对干旱引起的死亡的脆弱性是预测未来气候中森林命运的关键，干旱更加频繁和严重，尽管很难在成年树木中研究潜在的机制。在这里，我们探讨了垂死的挪威云杉（Picea abies ）的水分关系和水力功能限制的动态变化L.) 在破纪录的 2018 年中欧干旱的进程中。在干旱引起的死亡轨迹上的树木中，我们观察到木质部压力的快速、非线性下降，这种下降始于木质部空化的早期，并在很短的时间内导致木质部水力传导性完全丧失。我们还观察到非结构性碳水化合物的严重消耗，尽管可以排除碳饥饿是观察到的树木死亡的原因，因为垂死和幸存的树木都表现出这些代谢限制。我们的观察提供了惊人的实地证据，证明快速脱水和水力坍塌是成年挪威云杉干旱引起的死亡的原因。树木水分关系的非线性下降表明，考虑脱水的时间动态对于预测树木死亡至关重要。液压系统在短时间内崩溃表明，在严重干旱的过程中，树木可以迅速被推出液压安全区。总而言之，我们的研究结果表明，挪威云杉的死亡风险高于之前的假设，这与目前关于这一主要欧洲树种因干旱导致死亡率空前水平的报道一致。