Department of Biology, Stanford University, Stanford, CA 94305, USA.
Glob Chang Biol. 2013 Apr;19(4):1188-96. doi: 10.1111/gcb.12100. Epub 2013 Jan 10.
Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and carbon-cycle feedbacks. Recent drought-induced, widespread forest die-offs highlight that climate change could accelerate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and precludes mechanistic modeling and prediction of forest die-off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die-off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumulated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2 years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavitation, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deterioration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vulnerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climate-vegetation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress-induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.
森林死亡构成了对陆地生态系统和碳循环反馈的气候影响预测的主要不确定性。最近由干旱引起的广泛森林死亡突显了气候变化可能加速森林死亡，这对全球碳循环、生态系统服务和生物多样性具有多样化和潜在的严重后果。树木在多年的干旱中是如何死亡的，在很大程度上仍然未知，这妨碍了对气候变化下森林死亡的机制建模和预测。在这里，我们研究了近年来北美西部大部分地区广泛发生的颤杨（Populus tremuloides）死亡的生理基础。利用正在死亡的本地树木的观测结果和对成熟树木的降雨排除实验，我们在多个季节和多年测量了水力性能，并评估了积累水力损伤的途径。我们检验了积累的水力损伤是否可以预测树木在 2 年内的生存概率。我们发现，在多年的死亡过程中，水力损伤在垂死的树木中持续并增加，并且几乎没有修复的迹象。这种积累的水力恶化主要是由对空化的脆弱性增加引起的，这一过程被称为空化疲劳。此外，这种水力损伤可以预测跨年度茎死亡率的概率。与幸存树木经受住严重干旱的预期相反，这里展示的水力恶化表明，由于木质部受损积累，这些森林中幸存的区域实际上对未来的干旱更加脆弱。作为北美的最广泛树种，这些森林对干旱的脆弱性增加对生态系统稳定性、生物多样性和生态系统碳平衡具有重要影响。我们的研究结果为将积累的干旱影响纳入气候-植被模型提供了基础。最后，我们的研究结果强调了干旱胁迫积累和对胁迫诱导损伤的修复在避免植物死亡方面的关键作用，为干旱对森林生态系统的影响提供了一个动态和有条件的框架。
