To explore the effects of multiple time-scales, climatic and stand factors on tree mortality in forests, we examined the changes in annual and inventory-cycle tree mortality patterns across 264 forest inventory plots in four national forests of eastern Texas. These data were obtained from the Forest Inventory and Analysis (FIA) Program and the plots had been individually surveyed in four inventory cycles over the past 20 years. The generalized linear mixed effects model (GLMM) was used to explore the effects of climatic factors (drought severity, duration of drought, mean annual temperature, and mean annual precipitation), tree size (diameter at breast height) and stand factors (basal area, stand density, and stand age) on tree mortality. The results showed that tree mortality rates increased by 151% in the particular year with severe drought and by 123% during exceptional inventory cycle during the inventory cycle with severe drought. The major cause of death was weather (exceptional drought and large hurricanes). Both drought severity as measured by standardized precipitation evapotranspiration index (SPEI) and the duration of drought had significant negative effects, whereas annual precipitation had a significant positive effect on tree survival. Tree basal area had a significant negative effect, while tree size, stand age and stand density had significant positive effects on tree survival. Trees with larger size (DBH) were more vulnerable to drought than smaller ones. During the exceptional drought, tree mortality rate of pine species (2.1%) was lower than that of hardwood species (3.9%), while tree mortality in the natural forests (3.0%) was higher than that in the pine plantations (1.9%). Our results suggested that it was essential to consider the relative importance of both intrinsic (tree size) and extrinsic (stand factors and climatic factors) factors in analyzing tree mortality.
为探讨不同时间尺度、气候因子及林分因子对森林中树木死亡的影响,本研究以美国德克萨斯州东部的4个国家森林中264个重复调查的森林样地为对象,使用近20年来美国森林清查4个周期的数据,估算其在清查周期和年度水平上的树木死亡率变化,并使用广义线性混合效应模型来分析气候因子(干旱强度、干旱持续时间、年均温和年降水量)、树木大小(胸径)和林分因子(树木胸高断面积、林分密度和林分年龄)对树木存活的影响。结果表明: 在重度干旱当年和重度干旱的清查周期中,森林的树木死亡率分别增加了151%和123%,天气干扰(干旱和飓风)和植物之间的竞争是其主要的影响因素;干旱强度(标准化降水蒸散发指数,SPEI)和干旱持续时间对树木的存活具有显著的负效应,年降水量对树木的存活具有显著的正效应;树木胸高面积对树木存活具有显著的负效应,树木大小、林分年龄和林分密度对树木存活均具有显著的正效应,但是大树比小树更容易受到天气影响而死亡;在重度干旱的清查周期中,松树种组的树木死亡率(2.1%)比阔叶树木种组(3.9%)低,天然林的树木死亡率(3.0%)高于人工林(1.9%)。在分析树木死亡率时,需同时考虑个体树木大小、林分因子与气候因子的相对重要性。.
Keywords: GLMM; SPEI; exceptional drought; tree mortality.