Department of Plant Biology, Ecology, and Evolution, Oklahoma State University, Stillwater, OK, 74078, USA.
Department of Biological Sciences, Macquarie University, Sydney, NSW, 2109, Australia.
Nat Ecol Evol. 2017 Sep;1(9):1285-1291. doi: 10.1038/s41559-017-0248-x. Epub 2017 Aug 7.
Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function.
与干旱相关的广泛树木死亡已在所有森林大陆上观察到,预计全球变化将加剧植被的脆弱性。森林死亡对未来的生物圈-大气相互作用的碳、水和能量平衡有影响,但在动态植被模型中表现不佳。减少不确定性需要基于稳健生理过程的改进死亡率预测。然而,干旱诱导死亡的拟议机制,包括水力衰竭和碳饥饿,尚未解决。越来越多的实证研究已经研究了这些机制,但数据尚未使用标准化生理框架在物种和生物群落之间进行一致分析。在这里,我们表明,木质部水力衰竭在多种树木类群中普遍存在于干旱诱导的死亡中。所有评估的物种的木质部导水率都有 60%或更高的损失,与提出的理论和模型生存阈值一致。我们在死亡率处发现了非结构性碳水化合物储备的多样化反应,表明支持碳饥饿的证据并不普遍。与木质部水力脆弱性相关的是,裸子植物的非结构性碳水化合物减少比被子植物更为常见,这可能在降低水力功能方面发挥作用。我们发现,干旱诱导的死亡率中的水力衰竭在物种间是持续存在的,这表明使用水力功能阈值可以大大提高植被模型的性能。