Macalady Alison K, Bugmann Harald
University of Arizona, School of Geography and Development and Laboratory of Tree-Ring Research, Tucson, Arizona, United States of America.
Forest Ecology, Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland.
PLoS One. 2014 May 2;9(5):e92770. doi: 10.1371/journal.pone.0092770. eCollection 2014.
The processes leading to drought-associated tree mortality are poorly understood, particularly long-term predisposing factors, memory effects, and variability in mortality processes and thresholds in space and time. We use tree rings from four sites to investigate Pinus edulis mortality during two drought periods in the southwestern USA. We draw on recent sampling and archived collections to (1) analyze P. edulis growth patterns and mortality during the 1950s and 2000s droughts; (2) determine the influence of climate and competition on growth in trees that died and survived; and (3) derive regression models of growth-mortality risk and evaluate their performance across space and time. Recent growth was 53% higher in surviving vs. dying trees, with some sites exhibiting decades-long growth divergences associated with previous drought. Differential growth response to climate partly explained growth differences between live and dead trees, with responses wet/cool conditions most influencing eventual tree status. Competition constrained tree growth, and reduced trees' ability to respond to favorable climate. The best predictors in growth-mortality models included long-term (15-30 year) average growth rate combined with a metric of growth variability and the number of abrupt growth increases over 15 and 10 years, respectively. The most parsimonious models had high discriminatory power (ROC>0.84) and correctly classified ∼ 70% of trees, suggesting that aspects of tree growth, especially over decades, can be powerful predictors of widespread drought-associated die-off. However, model discrimination varied across sites and drought events. Weaker growth-mortality relationships and higher growth at lower survival probabilities for some sites during the 2000s event suggest a shift in mortality processes from longer-term growth-related constraints to shorter-term processes, such as rapid metabolic decline even in vigorous trees due to acute drought stress, and/or increases in the attack rate of both chronically stressed and more vigorous trees by bark beetles.
导致与干旱相关的树木死亡的过程尚不清楚，尤其是长期的诱发因素、记忆效应以及死亡过程和阈值在空间和时间上的变异性。我们利用来自四个地点的树木年轮来研究美国西南部两个干旱时期内矮松的死亡率。我们借助近期的采样和存档样本进行以下研究：（1）分析20世纪50年代和21世纪干旱期间矮松的生长模式和死亡率；（2）确定气候和竞争对死亡和存活树木生长的影响；（3）推导生长-死亡风险的回归模型，并评估其在空间和时间上的表现。存活树木的近期生长量比死亡树木高53%，一些地点表现出与先前干旱相关的长达数十年的生长差异。对气候的不同生长响应部分解释了活树和死树之间的生长差异，对湿润/凉爽条件的响应对最终树木状况影响最大。竞争限制了树木生长，并降低了树木对有利气候的响应能力。生长-死亡模型中最佳的预测指标包括长期（15 - 30年）平均生长速率，结合生长变异性指标以及分别在15年和10年内突然生长增加的次数。最简约的模型具有较高的判别能力（ROC>0.84），并正确分类了约70%的树木，这表明树木生长的各个方面，尤其是数十年间的生长，可能是广泛的与干旱相关的树木死亡的有力预测指标。然而，模型判别能力在不同地点和干旱事件中有所不同。在21世纪的干旱事件中，一些地点生长-死亡关系较弱，且在较低存活概率下生长较高，这表明死亡过程从长期与生长相关的限制转向了短期过程，例如即使是健壮的树木由于急性干旱胁迫也会迅速代谢衰退，和/或树皮甲虫对长期受压和更健壮树木的攻击率增加。
