Abstract: A two-year yak grazing trial was conducted in a two-seasonal grazing pasture in Kobresia parva meadow to study the variations of the quantitative characteristics of plant community in the pasture in warm season and cold season. Within the grazing periods of the two years, the coverage of high quality forage in the pasture decreased with increasing grazing intensity, while the coverage of weeds had an increasing trend. The dominant plant species under no grazing, light grazing, and moderate grazing in both warm and cold seasons were K. parva and Elymus nutans; whereas the dominant species under heavy grazing were Potentilla anserine and Pedicularis alaschanica in warm season, and K. parva and P. anserine in cold season. The larger the difference in grazing intensity, the lower the similarity of the community in the two seasons, indicating that grazing was the major factor leading to the difference in the community, and the predominant factor inducing the community change. The species richness, diversity index, and evenness index of the plant community in warm and cold seasons were all the lowest under no grazing, and the highest under moderate grazing, with the order of no grazing < light grazing < heavy grazing < moderate grazing, which supported the ‘hypothesis of medium disturbance’.

Key words: Climate changes, Spring wheat, Photosynthesis rate, Evapotranspiration, Water use efficiency