Abstract: Based on the data of plant communities collected from 29 sampling sites,species diversities and structures of the natural plant communities in the lower reaches of the Tarim River were analyzed from the angle of different environmental gradients,using species diversity,richness and eveness indices.Results show that (1) the study area had a total of 14 genera of plants,belonging to 10 families,with Tamarix ramosissima,Lycium ruthenicum and Populus euphratica being the first three in frequency of occurrence and P.euphratica being the highest(0.384),T.ramosissima the next highest (0.230) and Halogeton arachnoideus the lowest(0.002) in the importance value,and P.euphratica being dominant species was mainly distributed in areas less than 500 m from the river.(2) Groundwater tables along the river lowered from the upper reaches to the lower reaches,expcept for the Yiganbujima Section,and so did the specties diversity on the whole and arbor-shrub-herb-dominated communities were gradually changed into arbor-shrub-dominated one;while getting farther away from the river,the communities first increased and then decreased in density and coverage,and the plant community structures gradually shifted from arbor-shrub and arbor-shrub-herb to shrub-herb or shrub.

Key words: community structure, species diversity, groundwater table, the lower reaches of the Tarim River