依据塔里木荒漠河岸林胡杨群落的实地调查资料与地下水位监测数据,应用多元统计法、Hill与β多样性指数测度法,研究优势种胡杨种群数量特征沿地下水位梯度的变化规律及其相互关系,探讨群落物种多样性对地下水位梯度的生态响应.结果表明,运用系统聚类法将荒漠河岸林不同地下水位的胡杨群落划归为3类,优势种群胸径、冠幅、盖度、密度、死亡率与地下水位呈极显著相关(P＜0.01).随地下水位降低,优势种长势减弱,枯死率显著增大,种群退化.Hill多样性随地下水位梯度呈现3个明显不同的变化阶段,其中地下水位4-6 m时,多样性指数锐减与曲线渐趋平直,物种多样性开始受损.Cody(βε)、Whittaker(βw)指数均随地下水位梯度增大而增大,Sorensen(βs)指数则呈下降趋势;βs、βc、βw与地下水位高差间均呈极显著相关(P＜0.01),地下水位＞4m,βc、βw值显著增大而βs值显著降低.综合以上分析,地下水位＜4 m胡杨群落结构相似与共有种较多,优势种长势良好,其是优势种群生存的适宜生态水位;地下水位＞4 m,群落物种多样性与伴生种减少,物种变化速率增大,生境异质性增强,优势种群数量特征变化明显,群落结构简化;地下水位6 m左右,植被退化,物种多样性锐减,优势种稀疏衰败.因此,塔里木荒漠河岸林植被恢复的合理生态地下水位应维持在4m左右.%The largest area of desert riparian forest is distributed in the Tarim Basin,northwest China.It controls the structure and function of the desert ecosystem;determines landscape patterns,vegetation processes,and land desertification;and changes the ecological environment of oases in southern Xinjiang.Populus euphratica Oliv.is the dominant species and is distributed widely across desert riparian forests.It protects biodiversity and desert ecosystem stability,and serves as a natural barrier to curb desertification and sandstorms.Since the 1950s,land has been reclaimed and the water resources have been utilized in the area.This has resulted in a reduction in the groundwater level,weak vegetation growth,and high desert plant mortality,which has affected the sustainable development of agriculture and the economies of the affected oases.The aim of this study was to understand the ecological responses of P.euphratica communities to changes in groundwater levels (GWL),to determine the rational GWL needed for the survival of desert riparian forest,and to provide a theoretical basis for population conservation and vegetation restoration in the Tarim Basin.Nine belt transects and 45 plots were established to monitor the GWL and vegetation in the upper reaches of the Tarim River and the ecological responses of the P.euphratica community to the GWL.The relationship between the vegetation and the GWL gradient were analyzed using multi-analysis methods,and Hill andβ-diversity index analyses.The Ward cluster analysis divided the 45 sampling plots into three types and the quantitative characteristics (density,DBH,canopy diameter,coverage,mortality) of the P.euphratica population and the GWL gradients were significantly correlated (P ＜ 0.01).The P.euphratica growth vigor decreased and the population structure transitioned from expanding to declining as the GWL decreased.Species diversity varied considerably in the three different plot types as the GWL decreased.In particular,when the GWL ranged from 4 to 6 m,the Hill-diversity index decreased sharply,and the diversity curve became a smooth line.Species diversity began to decline at GWL ＞ 4 m.Furthermore,the Cody (βc) and Whittaker (βw) values increased and the Sorensen (βs) value decreased as the GWL fell.Theβs,βc,andβw indexes,and the differences in groundwater levels were significantly correlated (P ＜ 0.01).At GWL ＞ 4 m,theβc andβw values increased,but theβs value decreased considerably.The above analyses suggested that the P.euphratica communities had similar structures and many common species,and that the P.euphratica population grew most efficiently at GWL ＜ 4 m,which was the most suitable ecological water table for P.euphratica population survival.At GWL ＞ 4 m,species diversity and the common species in the community decreased rapidly,differences in species components and environmental heterogeneity increased,the quantitative characteristics of the P.euphratica population varied significantly,and the community structure became simplified.When the GWL was about 6 m,vegetation became degraded and the dominant population declined considerably.Therefore,the rational ecological GWL for restoring degraded desert vegetation is about 4 m in extremely arid areas.
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