Arend Matthias, Link Roman M, Patthey Rachel, Hoch Günter, Schuldt Bernhard, Kahmen Ansgar
Department of Environmental Sciences, University of Basel, 4056 Basel, Switzerland;
Department of Botany II - Ecophysiology and Vegetation Ecology, Universität Würzburg, 97070 Würzburg, Germany.
Proc Natl Acad Sci U S A. 2021 Apr 20;118(16). doi: 10.1073/pnas.2025251118.
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 ( 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.
了解树木对干旱导致死亡的脆弱性是预测在未来气候中干旱更频繁且更严重的情况下森林命运的关键，尽管其潜在机制在成年树木中难以研究。在此，我们探究了在破纪录的2018年中欧干旱过程中，挪威云杉（Picea abies (L.) Karst.）濒死成年树木水分关系的动态变化以及水力功能的极限。在走向干旱导致死亡轨迹的树木中，我们观察到木质部压力迅速且非线性下降，这种下降在木质部空化早期就开始了，并在很短时间内导致木质部水力导度完全丧失。我们还观察到非结构性碳水化合物严重消耗，不过碳饥饿可被排除为观察到的树木死亡原因，因为濒死和存活的树木都表现出这些代谢限制。我们的观察为快速脱水和水力崩溃作为成年挪威云杉干旱导致死亡的原因提供了显著的实地证据。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。水力系统在短时间内崩溃表明，在严重干旱过程中，树木会迅速被推出水力安全区。总之，我们的研究结果表明挪威云杉的死亡风险比之前设想的更高，这与目前关于这种欧洲主要树种前所未有的干旱导致死亡水平的报告一致。
