Biological and Environmental Sciences, University of Stirling, Scotland, FK9 4LA, UK.
CREAF, Campus de Bellaterra (UAB), Edifici C, Cerdanyola del Vallès 08193, Catalonia, Spain.
Glob Chang Biol. 2017 Sep;23(9):3742-3757. doi: 10.1111/gcb.13636. Epub 2017 Mar 3.
Ongoing climate change poses significant threats to plant function and distribution. Increased temperatures and altered precipitation regimes amplify drought frequency and intensity, elevating plant stress and mortality. Large-scale forest mortality events will have far-reaching impacts on carbon and hydrological cycling, biodiversity, and ecosystem services. However, biogeographical theory and global vegetation models poorly represent recent forest die-off patterns. Furthermore, as trees are sessile and long-lived, their responses to climate extremes are substantially dependent on historical factors. We show that periods of favourable climatic and management conditions that facilitate abundant tree growth can lead to structural overshoot of aboveground tree biomass due to a subsequent temporal mismatch between water demand and availability. When environmental favourability declines, increases in water and temperature stress that are protracted, rapid, or both, drive a gradient of tree structural responses that can modify forest self-thinning relationships. Responses ranging from premature leaf senescence and partial canopy dieback to whole-tree mortality reduce canopy leaf area during the stress period and for a lagged recovery window thereafter. Such temporal mismatches of water requirements from availability can occur at local to regional scales throughout a species geographical range. As climate change projections predict large future fluctuations in both wet and dry conditions, we expect forests to become increasingly structurally mismatched to water availability and thus overbuilt during more stressful episodes. By accounting for the historical context of biomass development, our approach can explain previously problematic aspects of large-scale forest mortality, such as why it can occur throughout the range of a species and yet still be locally highly variable, and why some events seem readily attributable to an ongoing drought while others do not. This refined understanding can facilitate better projections of structural overshoot responses, enabling improved prediction of changes in forest distribution and function from regional to global scales.
持续的气候变化对植物功能和分布构成重大威胁。温度升高和降水模式改变加剧了干旱的频率和强度，增加了植物的压力和死亡率。大规模的森林死亡事件将对碳和水文循环、生物多样性和生态系统服务产生深远影响。然而，生物地理学理论和全球植被模型对最近的森林死亡模式的代表性很差。此外，由于树木是固定的和长寿的，它们对气候极端事件的反应在很大程度上取决于历史因素。我们表明，有利于树木生长的有利气候和管理条件时期可能会导致地上树木生物量的结构性过度生长，因为随后水的需求和供应之间出现时间不匹配。当环境有利性下降时，水和温度压力的增加如果是持久的、快速的或两者兼而有之，会导致树木结构响应的梯度变化，从而改变森林自疏关系。从过早的叶片衰老和部分树冠凋落到整株树木死亡等反应会减少胁迫期内的树冠叶面积，并在随后的滞后恢复窗口内恢复。在整个物种地理范围内，从局部到区域尺度，都可能出现水的需求与供应之间的这种时间不匹配。由于气候变化预测未来干湿条件会有很大波动，我们预计森林在更具压力的时期会变得越来越不适应水的可用性，从而出现过度生长。通过考虑生物量发展的历史背景，我们的方法可以解释大规模森林死亡的一些先前存在的问题方面，例如为什么它可以发生在一个物种的整个范围内，但仍然在局部高度变化，以及为什么有些事件似乎很容易归因于持续的干旱，而其他事件则不是。这种更深入的理解可以促进更好地预测结构过度生长的反应，从而能够从区域到全球尺度更好地预测森林分布和功能的变化。
