Abstract: Based on the data obtained from 21 plots of natural plant community and 16 plant species in the middle reaches of Tarim River of China, quantitative classification (TWINSPAN) and ordination (CCA) methods were used. The natural plant communities are classified and the major environmental variables that correlated with the natural plant community types are discussed. TWINSPAN results show that the natural plant communities in the middle and lower reaches of Tarim River can be divided into 15 groups which belonged to 4 Vegetation type groups, 4 Vegetation types, 6 Vegetation subtypes, 9 Formations. CCA was combined with 8 environmental variables. The results of CCA ordination reflect sufficiently the relationship between vegetation and groundwater depth, groundwater pH and groundwater degree of mineralization Five groups are identified from 16 plant species by their compatibility to endure drought and salt-alkali. The gathered groups of 21 plant plots in CCA ordination are consistent with the result of quantitative classification (TWINSPAN) in formation class. The results of the two analysis methods were in good agreement. Nine types of plant communities are Populus euphratica forest, Halimodendron harodendron shrubland, Calligonum Kuerlese forest, T amarix ramosissima shrubland, Lycium ruthenicum shrubland, Halostashys caspica shrubland, Karelinia caspica community, A lhagi sparsifolia community, Apocynum venetum community. Among nine types of plant communities Halostashys caspica shrubland can endure the highest salt stress, Halimodendrom halodendron shrubland can endure the highest drought stress and Lycium ruthenicum shrubland can endure the highest alkali stress of the groundwater.

Key words: Tarim river, Vegetation, Environmental variables, TWINSPAN, CCA