Abstract: Lancang River valley, with unique history of geological evolution and the longitudinal change of ecological environment, provides an ideal place for understanding the differentiation and assembling process of plant communities. In this study, we comprehensively used the methods of TWINSPAN quantitative classification, community ordination, and phylogenetic structure analysis to elucidate the community characteristics and maintenance mechanisms of the valley vegetation in the middle and upper reaches of Lancang River. The results showed that: (1) According to TWINSPAN classification, plant communities in the middle and upper Lancang River valley were divided into 28 types. The result of the classification was similar to the DCA twodimensional ranking map. CCA and DCCA ordination showed that elevation was the major environmental factor affecting vegetation distribution. (2) Among the 28 community types, the average net relatedness index (NRI) value of 19 communities was greater than 0. The environmental filtering effect of river valley habitat was obvious. The phylogenetic structure of the plant communities generally showed an aggregation pattern. (3) Dryhot scrubs and drywarm scrubs had a narrow range in the river valley, including Ass. Buddleja crispa,Ass. Sageretia horrida+Sophora davidii,Ass. Elsholtzia capituligera +Caryopteris forrestii,Ass.Osteomeles schwerinae–Gramineae sp., Ass. Elsholtzia fruticose +Artemisia gyitangensis,Ass. Vitex negundo f. laxipaniculata+Pistacia weinmannifolia. Community phylogenetic structure generally had a significant aggregation pattern (P<0.05). The phylogenetic structure of widespread plant communities, including Ass. Cyclobalanopsis glaucoides, Ass. Pinus yunnanensis, Ass.Picea likiangensisvar. rubescens, and Ass.Betula platyphylla, showed a significant divergent pattern (P<0.05). (4) Ass. Castanopsis delavayi+Quercus variabilis had the highest average of phylogenetic diversity (PD), but species abundance was not the highest. (5) Ten climatic and topographic factors could explain 40.8%, 38.59%, and 9.97% of the variation of species richness, PD and NRI, respectively. Environmental differentiation in the Lancang River valley was illustrated by numerical classification and ordination analysis of plant communities. Combined with the analysis of the phylogenetic structure, the environmental factors that lead to ecological differentiation of river valley vegetation were revealed, reflecting the diversity and complexity of the river valley habitats.

Key words: Lancang River valley, plant community, quantitative classification, phylogenetic structure.