Stand structure and height-diameter relationship of a degraded Populus euphratica forest in the lower reaches of the Tarim River, Northwest China

Understanding stand structure and height-diameter relationship of trees provides very useful information to establish appropriate countermeasures for sustainable management of endangered forests. Populus euphratica, a dominant tree species along the Tarim River watershed, plays an irreplaceable role in the sustainable development of regional ecology, economy and society. However, as the result of climate changes and human activities, the natural riparian ecosystems within the whole river basin were degraded enormously, particularly in the lower reaches of the river where about 320 km of the riparian forests were either highly degraded or dead. In this study, we presented one of the main criteria for the assessment of vitality of P. euphratica forests by estimating the defoliation level, and analyzed forest structure and determined the height-diameter (height means the height of a tree and diameter means the diameter at breast height (DBH) of a tree) relationship of trees in different vitality classes (i.e. healthy, good, medium, senesced, dying, dead and fallen). Trees classified as healthy and good accounted for approximately 40% of all sample trees, while slightly and highly degraded trees took up nearly 60% of total sample trees. The values of TH (tree height) and DBH ranged from 0-19 m and 0-125 cm, respectively. Trees more than 15 m in TH and 60 cm in DBH appeared sporadically. Trees in different vitality classes had different distribution patterns. Healthy trees were mainly composed more of relatively younger trees than of degraded tress. The height-diameter relationships differed greatly among tress in different vitality classes, with the coefficients ranging from 0.1653 to 0.6942. Correlation coefficients of TH and DBH in healthy and good trees were higher than those in trees of other vitality classes. The correlation between TH and DBH decreased with the decline of tree vitality. Our results suggested that it might be able to differentiate degraded P. euphratica trees from healthy trees by determining the height-diameter correlation coefficient, and the coefficient would be a new parameter for detecting degradation and assessing sustainable management of floodplain forests in arid regions. In addition, tree vitality should be taken into account to make an accurate height-diameter model for tree height prediction.

了解林分结构以及树木的树高 - 胸径关系可为制定濒危森林可持续管理的适当对策提供非常有用的信息。胡杨是塔里木河流域的优势树种，在区域生态、经济和社会的可持续发展中起着不可替代的作用。然而，由于气候变化和人类活动的影响，整个流域内的天然河岸生态系统严重退化，特别是在河流下游，约320公里的河岸森林要么高度退化，要么死亡。在本研究中，我们通过估算落叶程度提出了评估胡杨林活力的主要标准之一，并分析了森林结构，确定了不同活力等级（即健康、良好、中等、衰老、濒死、死亡和倒伏）树木的树高 - 胸径（树高指树的高度，胸径指树木胸高直径）关系。被归类为健康和良好的树木约占所有样本树木的40%，而轻度和高度退化的树木占样本树木总数的近60%。树高（TH）和胸径的值分别在0 - 19米和0 - 125厘米之间。树高超过15米且胸径超过60厘米的树木零星出现。不同活力等级的树木具有不同的分布模式。健康树木主要由相对较年轻的树木组成，而非退化树木。不同活力等级树木的树高 - 胸径关系差异很大，系数在0.1653到0.6942之间。健康和良好树木的树高和胸径的相关系数高于其他活力等级的树木。树高和胸径之间的相关性随着树木活力的下降而降低。我们的研究结果表明，通过确定树高 - 胸径相关系数，或许能够区分退化的胡杨和健康的胡杨，并且该系数将成为检测干旱地区河漫滩森林退化和评估其可持续管理的一个新参数。此外，为了建立准确的树高预测树高 - 胸径模型，应考虑树木活力。