Abstract: The Pinus densata alliance is one of the most widespread pine forests in the Mountains of Southwest China. Endemic to China, and the alliance is mainly found in western Sichuan, northwest Yunnan, and southeast Xizang. In this study, we defined the geographic distribution boundary and initially ascertained the geographic distribution area of Pinus densata alliance based on previous literature and field investigation sample sites. Based on data from 48 plots surveyed during 2020–2022, we proposed a preliminary scheme of vegetation classification and described the community structure characteristics for Pinus densata alliance, and analyzed the species composition characteristics combined with data from 11 plots surveyed in 2012. The results showed that: (1) The Pinus densata alliance covers 33 counties and cities, including Yajiang, Xiangcheng, Daocheng, Dêqên, Xamgyi’nyilha , Markam, Mainling, and so on, along altitude ranging from 2 300 to 4 000 m. The eastern, west-ern, northern and southern boundaries are Donggu Town in Danba County, Zengqi Township in Sangri County, Puxi Township in Zamtang County, and Yulong Snow Mountain in Yulong Naxizu Autonomous County. (2) 522 vascular plants belonging to 67 families and 222 genera were recorded, among which there were 500 seed plants belonging to 60 families and 209 genera, and 233 Chinese endemic plants belonging to 42 families and 114 gen-era. The north temperate element was the dominant areal-type. (3) Based on the differences of community struc-ture and species composition, Pinus densata alliance could be classified into 7 association groups and 20 associa-tions.

Key words: Pinus densata alliance, vegetation classification, community structure, species composition