| Gannan Tibetan Autonomous Prefecture is an important pastoral area of Gansu Province, which covers alpine meadows, sub-alpine meadows, and marsh meadows. However, in recent decades, due to the impact of human activities and natural disturbance the land of this region has been somewhat degraded, including the decrease of species diversity and productivity, and even the emergence of grassland desertification.The present thesis consists of three parts:the first is about the relationships betweenα,βdiversity and productivity, the second is regarding to the effects of fertilization ((NH4)2HPO4) on species diversity, productivity and community species composition, and the last one is about the impact of spatial structure and soil factors on community species composition. Our main concerns are as following:(1) Whetherα,βdiversity of alpine meadows and sub-alpine meadows in Gannan are related to community productivity? If so, how do these relationships change with spatial scales?(2) How does fertilization affect species diversity, productivity and community composition? What are the underlying mechanisms for these changes?(3) Among ten soil factors, including pH, total N content, total P content, available N content, available P content, organic content, Ca, K, Mg, and Na, which is/are important for community structure? Regarding to the niche theory and neutral theory, which one could better explain the patterns observed?The experiments were conducted on alpine meadows (Maqu and Oula) and sub-alpine meadows (Hezuo). Through field observations and nutrition addition experiments, using the software of OriginPro 7.5 and Canoco 4.5, the results we obtained were as follows:(1) At local spatial scale, two plots of three showed that the relationships between a diversity and productivity were positive at (sub-)alpine meadows. However, when the spatial scale was expanded to the whole landscape, the positive relationships were significant for all three plots.(2) At local spatial scale, two plots of three showed that the relationships between P diversity and productivity were negative in this region. However, when the spatial scale was expanded to the landscape level, the negative relationships were significant for all three plots.(3) Fertilization notably improved the productivity of grassland plant community, but the relationship of fertilizing levels with productivity was bell-curve, not linearly positive. Productivity was highest at the fertilizing level of 43.2gNm-2 (48gPm-2)(4) Fertilization significantly reduced species diversity of communities. The relationship of fertilizing amount with species diversity was clearly negative. The higher the fertilizing amount, the more the species diversity lost. The evenness of species distribution was highest at the fertilizing level of 14.4gNm-2(16gPm-2). Species of Gramineae and Cyperaceae were more and more dominant in communities with the fertilizing amount increasing.(5) On the sloping plots, for the soil layer of 0-20 cm, the main soil factors which affected community species composition were:Ca content, available P content, Mg content, organic content, and Na content, and the order of significance was:Ca content> available P content> Mg content> organic content> Na content; for the layer of 20-40 cm, the main factor was:available P content; for 40-60 cm, the main factors were:total P content, Ca content, and pH, and the order of significance was: total P content> Ca content> pH. On the bottomland of plots, for 0-20 cm soil layer, the main factors were:Na content, pH, K content, total P content, organic content, and Mg content, and the order was:Na content> pH> K content> total P content> organic content> Mg content; for 20-40 cm soil layer, the main factors were:available N content and pH, and the order was:available N content> pH; for 40-60 cm soil layer, the main factors were:Na content and total P content, and the order was:Na content≈total P content. For both sloping and bottomland plots, the number of the main soil factors notably affecting the community structure was maximum in the layer of 0-20 cm.(6) Regarding to the percentage of the variation for community species composition explained by the soil factors, in sloping plots the explained variation was 14.62%, and in bottomland was 18.67%. The percentage of the variation explained by the variables of spatial dispersal in sloping plots was 13.53%, and in bottomland was 14.01%. Based on this, we could conclude that both niche theory and neutral theory could correspondingly explain species composition in community of alpine meadow in Gannan.
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