Abstract: The influence of grazing on the temperate steppe in Tibet is complex and sensitive. The changes of plant community characteristics under different grazing intensities were studied in order to provide scientific baseline for species diversity protection and grazing management. In this paper,the temperate steppe in Lhasa River Valley was taken as the research object,three grazing gradients were set,and the effect of different intensities of sheep grazing on the plant community and species diversity were analyzed by using the method of quadrat survey. The results showed that,the plant community height,Shannon-wiener index and Pielou index were significantly increased under grazing prohibited treatment,but Simpson index was significantly decreased with the lowest degree of species replacement. Shannon-wiener index,Pielou index and Simpson index were significantly decreased under moderate and heavy grazing treatments. Grazing can be an important factor that directly affects the important value of species and a direct driving force of species replacement. Enclosure increases vegetation community height and species diversity,reduces species turnover,and has a certain effect on repairing degraded grassland,but heavy grazing makes plants develop towards the direction of resistance to foraging,trampling and dwarfing,and reduces species diversity.

Key words: Grazing intensity, Temperature steppe, Community characteristics, Species diversity, Lhasa River Valley, Tibet

摘要： 放牧对西藏温性草原的影响敏感且复杂。本试验通过研究植物群落特征在不同放牧强度下的变化，以期为物种多样性保护和放牧管理提供科学依据。本文以拉萨河谷温性草原为研究对象，设置3种放牧梯度，采用样方调查法，分析不同放牧强度对植物群落和物种多样性的影响。结果表明，禁牧处理下植物群落高度、Shannon-wiener指数、Pielou指数显著升高，但Simpson指数显著降低，物种更替程度最低；中度、重度放牧处理下Shannon-wiener指数、Pielou指数和Simpson指数显著降低。放牧是直接影响物种重要值的重要因素，并且是物种更替的直接驱动力。围封使植被群落高度、物种多样性增大，物种更替率降低，对退化草地有一定的修复作用，但重度放牧使植物向耐啃食、耐践踏、低矮化方向发展，并且使物种多样性降低。

关键词: 放牧强度, 温性草原, 群落特征, 物种多样性, 拉萨河谷, 西藏