Department of Natural Resources and Environmental Science, University of Nevada Reno, Reno, Nevada, 89557, USA.
Ecol Appl. 2019 Mar;29(2):e01831. doi: 10.1002/eap.1831. Epub 2018 Dec 13.
Severe drought has resulted in widespread tree die-off events in forests and woodlands globally and is forecast to become more frequent in coming decades. Tree mortality is a complex process influenced by climate, soils, characteristics of individual trees, interactions between trees, and the dynamics of pests and pathogens. The role of stand structure and stand density in mediating the resistance of trees to drought remains poorly understood, especially in semiarid woodlands, which are expected to be highly susceptible to future severe drought. We sampled permanent plots in central Nevada woodlands dominated by single-leaf pinyon pine and Utah juniper before and after a severe multi-year drought (2013-2015) to investigate the importance of climate, tree attributes, and local-neighborhood stand structure on tree mortality and canopy dieback at the level of individual trees and 0.1-ha plots. We observed widespread tree mortality of pinyon at approximately eight times the reported background mortality rate, and substantial canopy dieback in both pinyon and juniper. Both species were more prone to mortality and dieback in hotter, drier sites. Canopy dieback was associated with both long-term average climate and the severity of recent drought, with elevated mortality on sites with higher water deficits, average summer temperatures, and vapor pressure deficits. Soils also played a role in tree dieback, with greater mortality on deeper soils. While mortality was driven largely by climate at coarse scales, fine-scale stand structure interacted with climate to mediate mortality and dieback. Neighborhood statistics showed that trees were susceptible to competitive influence, and pinyon trees were especially sensitive to neighborhood density on drier sites. Mortality and dieback were associated with diverse, co-occurring insect and parasitic plant mortality agents. Canopy dieback prior to the drought was strongly associated with tree mortality during the drought, implying that current widespread defoliation caused by these agents may foreshadow future elevated woodland decline. Fine-scale influences such as stand structure and soil characteristics play a key role in the long-term dynamics of semiarid woodlands, and these factors should be considered in predictive models of forest and woodland susceptibility to drought.
严重干旱导致了全球森林和林地中广泛的树木死亡事件，预计在未来几十年内将更加频繁。树木死亡是一个复杂的过程，受到气候、土壤、树木个体特征、树木之间的相互作用以及病虫害动态的影响。林分结构和林分密度在介导树木对干旱的抵抗力方面的作用仍知之甚少，特别是在半干旱林地中，预计它们对未来严重干旱高度敏感。我们在内华达州中部的林地中采集了永久性样地，这些林地主要由单叶刺柏和犹他州 Juniper 组成，这些林地在经历了一次严重的多年干旱（2013-2015 年）之前和之后进行了采样，以调查气候、树木属性以及当地林分结构对树木死亡率和树冠枯萎的重要性，其研究范围涉及个体树木和 0.1 公顷的样地。我们观察到，刺柏的树木死亡率大约是报道的背景死亡率的八倍，而且刺柏和 Juniper 的树冠枯萎程度都很大。在较热、较干燥的地点，这两个物种的死亡率和枯萎率都更高。树冠枯萎与长期平均气候和最近干旱的严重程度有关，在水亏缺、平均夏季温度和蒸气压亏缺较高的地点，死亡率较高。土壤也在树木枯萎中起作用，在较深的土壤上，死亡率更高。虽然死亡率在粗尺度上主要受气候驱动，但细尺度林分结构与气候相互作用，介导死亡率和枯萎率。林分统计数据表明，树木容易受到竞争的影响，在较干燥的地点，刺柏对林分密度的敏感性更高。死亡率和枯萎率与多种同时发生的昆虫和寄生植物死亡因素有关。干旱前的树冠枯萎与干旱期间的树木死亡率密切相关，这意味着目前这些因素造成的广泛落叶可能预示着未来林地的衰退加剧。林分结构和土壤特征等细尺度因素在半干旱林地的长期动态中起着关键作用，在预测森林和林地对干旱的敏感性时应考虑这些因素。
