Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith NSW, Australia.
Department of Biology, University of North Florida, 1 UNF Drive, Jacksonville, FL, USA.
Tree Physiol. 2019 Jun 1;39(6):910-924. doi: 10.1093/treephys/tpz016.
Drought-induced tree mortality alters forest structure and function, yet our ability to predict when and how different species die during drought remains limited. Here, we explore how stomatal control and drought tolerance traits influence the duration of drought stress leading to critical levels of hydraulic failure. We examined the growth and physiological responses of four woody plant species (three angiosperms and one conifer) representing a range of water-use and drought tolerance traits over the course of two controlled drought-recovery cycles followed by an extended dry-down. At the end of the final dry-down phase, we measured changes in biomass ratios and leaf carbohydrates. During the first and second drought phases, plants of all species closed their stomata in response to decreasing water potential, but only the conifer species avoided water potentials associated with xylem embolism as a result of early stomatal closure relative to thresholds of hydraulic dysfunction. The time it took plants to reach critical levels of water stress during the final dry-down was similar among the angiosperms (ranging from 39 to 57 days to stemP88) and longer in the conifer (156 days to stemP50). Plant dry-down time was influenced by a number of factors including species stomatal-hydraulic safety margin (gsP90 - stemP50), as well as leaf succulence and minimum stomatal conductance. Leaf carbohydrate reserves (starch) were not depleted at the end of the final dry-down in any species, irrespective of the duration of drought. These findings highlight the need to consider multiple structural and functional traits when predicting the timing of hydraulic failure in plants.
干旱导致树木死亡会改变森林结构和功能，但我们预测不同物种在干旱期间何时以及如何死亡的能力仍然有限。在这里，我们探讨了气孔控制和耐旱性特征如何影响导致水力衰竭临界水平的干旱胁迫持续时间。我们研究了四个木本植物物种（三种被子植物和一种针叶树）的生长和生理响应，这些物种代表了一系列水分利用和耐旱性特征，经历了两个受控干旱-恢复循环和一个延长的干旱期。在最后一个干旱期结束时，我们测量了生物量比和叶片碳水化合物的变化。在第一和第二干旱阶段，所有物种的植物都因水势下降而关闭气孔，但只有针叶树物种由于早期气孔关闭相对于水力功能障碍的阈值，避免了与木质部栓塞相关的水势。在最后一次干旱下降过程中，植物达到临界水分胁迫水平所需的时间在被子植物之间相似（从 39 到 57 天到 stemP88），在针叶树中更长（从 156 天到 stemP50）。植物的干燥时间受到多种因素的影响，包括物种的气孔-水力安全裕度（gsP90 - stemP50），以及叶片多汁性和最小气孔导度。在任何物种中，无论干旱持续时间如何，最终干燥结束时叶片碳水化合物储备（淀粉）都没有耗尽。这些发现强调了在预测植物水力衰竭时间时需要考虑多个结构和功能特征。
