The floristic characteristics in the source area of Changjiang (Yangtze) River

The source area of Changjiang (Yangtze) River is situated between latitude 31°35′~36°30′ N and longitude 89°35′~97°55′ E.The area belongs to the alpine and highland.Most of the area with altitudes ranging from 4 000 m to 5 000 m.The features of climate is the type with the alpine and continent.There are 1 377 species of seed plants that belong to 62 families and 321 genera in this region,and respectively has a percentage of 68.89% of total family,and of 63.19% of total genera and of 60.26% of total species of the Tanggute Region from the Qinghai Xizang (Tibet) Plateau Subdivision.The floristic characteristics in the area are as follows:(1) species are poorer.(2) The North Temperate Zone is superior to numbers of species in this area.The Temperate elements have a percentage of 96.81%.It shows the clear nature of the Temperate Zone that is the decisive factor of the floristic characteristics.(3) Most of the plants are perennial and poorest of trees.This flora is short of the ancient type and primitive species,and most of the Cosmopolitan elements and the endemic elements of China are developed from them relational genera that are wildly distributed.Shows the flora is a young and derviative flora.(4) The main vegetation such as alpine grassland,alpine meadows,alpine thickets and alpine moor meadows are all alpine type typical.Most of the constructive species of them are character species of typical alpine vegetation.Therefore,in this flora,the alpine vegetation are typical,and the characteristic species are typical,and the feature of alpine and plateau flora is obvious.(5) Its specially glaring the selection of ecological factor and the alpine specialization,and appearance to adapt hydro coldize,and coldize and droughtize in the flora.It depends on the ecological environment that is formed by the geographical position,the elevation and the climatic characteristics.In view of the main family and genera,and all the main vegetation,constructive species and character species are the typical representatives of the Tanggute Region are concentrated in this flora,and the genus Mecomopsis that is one of the features of the Tanggute Region widespread distributed in this area.More over,the floristic natures and characteristics are identical the Tanggute Region,so we consider this flora is one of the representatives flora of the alpine flora in Tanggute.However,by the analysis of the endemic species of China,the flora is close relationship with both the floras of Northeast Xizang,Hengduan Mountains and Southern Gansu.Its the richest common species between the three areas,and including 593 species and has a percentage of 74.69% of total endemic species of China in this area.About the southern dividing line of the Tanggute Region,we hold that the natural line on the distribution range of the genera Pomatosace,Przewalskia,Syncalathium and Xanthopappus etc.which should be considered.In other words,it tally with the natural line that “the subregion of Northwest Sichuan,Southwest Gansu,Southeast Qinghai” belong to the Tanggute Region.Because the center of distribution of above genera are in the Tanggute Region and the distribution extende to the neibouring area of the Tanggute Region.