| Alluvial fan as an important geomorphological unit Tibet,it is of important significance to understand the distribution of species composition,community types,species diversity and their relationships with environmental factors,for the restoration and reconstruction of the vegetation,the protection and utilization of plant resources,and the construction and improvement of the living environment of farmers and herdsmen.The vegetations on 12 alluvial fans in the Lhasa River basin were selected as research objects.The plant family and genus composition,growth composition,flora and geographic distribution,the basic characteristics of medicinal plant were analyzed on the basis of field investigation.The plant community was classified and named by the method of superior species and joint name,and the diversity of community species were studied by using Margelef index,Shannon-Wiener index,Pielou index and Simpson index.The relationship between the plant community and its species composition,species diversity index and environmental factors were analyzed by CCA ordination and correlation analysis respectively.The main conclusions are as follows:(1)A total of 162 species,135 genera and 43 families of plants were recorded in the survey area,of which Compositae,Gramineae,Labiatae,Leguminosae,Rosaceae,Cruciferae and Boraginaceae accounted for 56.17%.The plant growth types were distributed in all the 7 types,and perennial herbs was the most,and subshrubs was the least,accounting for 54.14% and 0.75% of the total plants,respectively.Flora and geographic distribution showed that the temperate floristic types were dominant and coexist with other floristic types.There were 155 medicinal plants,which were classified by the medicinal parts,medicinal properties,medicinal taste and other properties.It was found that the medicinal plants with all parts,plain medicinal plants and bitter medicinal plants were accounted for 45.81%,27.10% and 38.71% of the total medicinal plants,respectively.(2)The 120 plots were divided into 27 community types in the survey area.Among them,there were 12 herbal communities,and ASS.Eleusine indica,ASS.Gnaphalium affine and ASS.Scirpus validus were widely distributed.There were 6 shrub communities,among which ASS.Rosa omeiensis pteracantha-Eleusine indica was more distributed;There were 3arbor communities,most of which were "four side forest" and nursery.There were 6 crop communities,mainly ASS.Hordeum vulgare nudum.(3)The main environmental factors affecting the species composition,distribution pattern and species diversity index of herbal and shrub communities were average annual rainfall,altitude and soil pH in the survey area.Besides,the environmental factors also included slope,aspect and soil total nitrogen content for the distribution of herbal community and species composition,and included average annual temperature and soil organic matter content for the shrub community and species composition.(4)There was a certain correlation between different environmental factors of herbaceous and shrub communities in the survey area.It is mainly manifested as a significant positive correlation between soil total nitrogen content and organic matter content.There was a significant positive correlation between soil total nitrogen content,organic matter content and average annual rainfall,respectivelya.There was a significant positive correlation between average annual rainfall and altitude,and a significant negative correlation between soil total potassium and soil pH. |
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