特大干旱对树木死亡的影响——以美国德克萨斯州东部森林为例

Abstract

美国德克萨斯州在2011年经历了史上最严重的干旱，这一事件造成约3亿多株树木死亡。在大时空尺度上（面积约9×10⁶ hm²，时间跨度近20年），基于近1800个森林样地，4次周期性调查中的约209663株树木，使用主成份分析（PCA）和广义线性混合效应模型（GLM）回归，对树木死亡的时空差异及其干旱强度与长度对树木死亡造成的中长期复杂影响进行了研究。采用树木密度、树木基面积、林地年龄、样地调查时间间隔作为树木间的竞争指标，分析了造成大旱前后周期水平和年度水平上的样地树木死亡差异的原因。综合分析了不同地理区域、树木种组、胸径大小和林地起源的4个划分标准下树木死亡对死亡率的相对贡献。结果表明：松属树木的死亡率最低（7.92%）；高度低、胸径小的树木的死亡率较大，分别为29.79%和26.00%。人工林的树木死亡率（10.26%）低于天然林（13.47%）；西海湾平原生态区树木的死亡率在干旱后达到最大（22.27%）；西南区的树木死亡率在干旱后也达到最大（13.78%）；海拔和纬度对树木死亡率影响不明显。德州东部森林整体死亡格局形成原因较为复杂，各地理区域、林地起源、树木大小和不同树种，对森林死亡的贡献率各不相同。四个区域中死亡贡献率较大的为天然林；无论是天然林还是人工林，其死亡贡献率最大的为小树；干旱对天然林的影响比人工林更显著；天然林和人工林中死亡贡献率情况各不相同，在天然林中松属和枫香属树种死亡贡献率低于人工林，其他种则是人工林高于天然林。林地密度和树木死亡的回归分析表明二者之间无显著关系。PCA结果显示干旱对树木死亡影响大于树木竞争和其它自然干扰的影响，干旱强度和干旱长度对树木死亡的影响基本相当。;In 2011, Texas experienced the most severe drought in its 100 years history, resulting in the death of more than 300 million trees. On a large spatial and temporal scale (an area of 9×10⁶ ha, a time span of nearly 20 years), this study is based on 1800 forest plots, with about 209,663 trees from four periodic surveys. This study utilized Principal component Analysis (PCA) and Generalized Linear Models (GLM) to characterize the spatio-temporal difference of tree mortality and drought severity and length effects. The causes of the differential tree mortality at cycle and annual level before and after the 2011 drought were analyzed using tree density, basal area, stand age, and survey interval as the competition indexes among trees. The relative contribution of tree death to mortality was analyzed according to four classification criteria, geographic region, tree species group, tree sizes (height and diameter at breast height-DBH) and forest stand origin. The results showed that the Pinus trees suffered the least, with a mortality rate of 7.92%. The mortality rates of trees with low height and small DBH were 29.79% and 26.00%, respectively. Natural forest trees witnessed a higher mortality rate (13.47%) than plantation forests (10.26%). The mortality rate of trees in the Western Gulf Coastal Plains ecological region and southwest region reached the highest after drought with a mortality rate of 22.27% and 13.78%. The effects of elevation and latitude on tree mortality were not apparent. The causes of overall tree mortality patterns in eastern Texas are complex, and differential mortality rates were observed across geographical regions, stand origin, tree sizes and different tree species. Among the four regions, the natural forest had a higher death contribution rate. Small trees contributed the most to the death in both natural and plantation forests. The effect of drought on the natural forest was more significant than that on plantation forests. The death contribution rates in natural forests and plantation forests are different. In the natural forests, the death contribution rates of Pinus and Liquidambar species were lower than those in the plantations. Regression analysis of forest density and tree mortality did not show a significant statistical relationship between them, suggesting tree mortality was triggered by density-independent factors. The results of PCA showed that the effect of drought on tree mortality was greater than that of tree competition and other natural disturbances, and the effect of drought severity and drought length on tree death was similar.