塔里木河上游胡杨种群结构与动态

采用样带调查法对塔里木河上游3个断面8.5 hm2天然胡杨林进行野外调查,绘制了胡杨种群年龄结构图和存活曲线,结合种群动态量化方法与时间序列预测,分析种群结构与动态特征。结果表明:塔里木河上游胡杨种群密度随径级增大而减小,自疏指数接近-3/2,个体胸径达24 cm后种群密度保持相对稳定。中、下段胡杨种群年龄结构呈金字塔型,幼龄级所占比例较大、林分年轻;存活曲线为Deevey C型,且动态指数0,为增长种群。受所处生境条件影响,上段不同生境胡杨种群年龄结构差异明显。阿瓦提县胡杨种群结构呈金字塔型,幼龄级比例高达77.2%,存活曲线属Deevey C型,为增长种群;南口镇与托海乡胡杨种群结构呈壶型,幼龄级比例极低、中大龄级比例高达84.7%,存活曲线属Deevey A型,为衰退种群;沙黑里克镇胡杨种群结构呈钟型,存活曲线属Deevey A型,为暂时稳定种群。种群数量动态与时间序列分析表明,中、下段胡杨更新资源丰富,未来种群能够稳定增长;上段南口镇、托海乡与沙黑里克镇幼龄级数量随时间推移减少,种群向大老龄级发展,未来种群将走向衰退。总之,上段胡杨种群因更新资源不足而趋于衰退,未来保持适当的人工辅助恢复措施和加强保护现有植株及其生境是保持胡杨种群自然更新和进行种群恢复的关键。

The population structure and dynamics of Populus euphratica at the upper reaches of the Tarim River

Desert riparian forest, an important vegetation type in the Tarim River Basin, plays a dominant role in the structure and function of the surrounding desert ecosystem and has a strong influence on the landscape vegetation patterns in the vicinity. Desert riparian forest dominated by Populus euphratica has not only protected the fragile desert ecological system but also served as an irreplaceable natural barrier. P. euphratica is highly prized in the Tarim Basin, Xinjiang Region, China, for its great ecological, economic and social benefits. Land opening campaigns resulted in the destruction of large areas of P. euphratica forests after the 1950s. The excessive use of water for irrigation created a severe threat to the remaining P. euphratica forests by robbing them of their source of water. Maintaining the stability of P. euphratica populations is important to local development. The objective of this study was to illuminate the current population structure, quantitative dynamics, forest health and the trends for development of the remaining P. euphratica forests, with the goal of ensuring the wise use (conservation) and protection (preservation) of the dominant forest tree population in the Tarim River Basin. Plots that covered 8.5 hm² were established in three (upper, middle and lower) sections in the upper reaches of the Tarim River; diameter at breast height (DBH), tree height, crown width and canopy density were measured for stems of all trees with a (DBH) > 2.5 cm, and trees were classified and identified to species using transect sampling. Graphs of the age structure and survival curves were created, and analysis was conducted of the quantitative population dynamics. Time sequence prediction of future forest conditions was conducted based on field data. The results showed that population density of P. euphratica tended to decrease as the size class increased; that is, the value of self-thinning was close to -3/2 and the population density would remain relatively stable when the DBH exceeded 24 cm. The age structure of P. euphratica populations had a positive pyramidal type, with a high percentage of younger age-class individuals. At the middle and lower sections, the survivorship curve of P. euphratica populations conformed to Deevey C type (concave-type) with the dynamic indices (V_pi) > 0 indicating that the P. euphratica populations were expanding. The environmental conditions in the upper section were much different from those of the middle and lower sections of the upper reaches of the Tarim River; populations in the upper section developed different characteristics and age structures from the other two sections. In Awti County, the age structure of the P. euphratica population was a positive pyramidal type and younger age-class individuals accounted for 77.22% of the total; the survivorship curve conformed to Deevey C and the structure was an expanding population. In Nankou and Tuohai towns, the age structure of the populations was a constrictive pyramid, with lower ratios of younger age-class to older individuals, while the ratio of adult individuals reached 84.7% of the population. The survivorship curves were classified as Deevey A (convex) type and structures indicated declining populations. In Shaheilike town, the age structure of P. euphratica populations was the bell type, the survivorship curve conformed to Deevey A type, and structure indicated a temporarily stable population. The analysis of the quantitative dynamics and time sequence prediction for P. euphratica populations in the next 10, 20 and 30 years indicated that P. euphratica populations would grow steadily in the future as a result of rich seedling resources in the middle and lower sections. The younger age-class individuals of P. euphratica populations would decrease in numbers and numbers of old individuals would significantly increase in the upper section. The populations were declining during ecological succession in Nankou, Shaheilike and Tuohai towns. Limited recruitment in P. euphratica populations, caused by habitat degradation and the withdrawal of water for irrigation, may affect the persistence of this species and the long-term dynamics of the forest in the upper section. Therefore, protection of existing living trees and their habitats is important to the sustainable development of P. euphratica forests in the upper reaches of the Tarim River.