Abstract:The article focuses on the enclosed grassland in sub-alpine meadow.We examined the composition of three different communities by the methods of quadrat survey,measured area-based leaf CO2 assimilation rate(Aarea),foliar nitrogen content(Nmass),leaf mass per area(LMA) and photosynthetic nitrogen-use efficiency(PNUE) of three common species Elymus dahuricus,Cirsium setosum and Medicago sativa to study the variation of plant photosynthetic and physiological characteristics in different communities.The results show that:(1)there is obvious composition difference between three sample areas,community composition,particularly leguminous plants can improve nutrient status.However,plant foliar nitrogen content is influenced by dominant interspecific competition;(2)the same species has different height in different communities and the different communities have different vertical light intensity as well,which lead to significant different of light intensity that same species can obtain in different communities;structure including plant height alter light intensity that plants enable harvested significantly;(3)the illuminance and nutrient are variant between communities,though the variation tendencies of Aarea,LMA and PNUE are not same to different plants.However,the strong relationship between Narea and Aarea is little changed in general,between communities and among species,basically shows a significant positive correlation.Thus it can be seen that the difference in nitrogen and light intensity caused by community composition and structure is an important factor which lead to different photosynthetic physiological characteristics with same species,while different species have different photosynthetic adaptive mechanism to changing environment.