中文摘要：

草地的退化是当前畜牧生产面临的一个主要问题，本试验选取青藏高原高寒草地自然群落中等放牧强度和不放牧2个梯度进行对比研究，探讨当前的普遍放牧强度对群落生产力、不同功能群产量分配和群落结构的影响。结果表明，3年中放牧使群落总体生产力下降，其中不同功能群所占比例也有所变化，禾草和莎草比例由平均76．6％下降到57．4％，杂草、豆科和毒草的比例上升；群落高度降低，平均最大高度由46．8cm下降到22．4cm；物种多样性和多度增加，多样性由单位样方平均26．5种增加到30．9种，多度由单位样方215．7株增加到304．6株；总盖度并没有明显差异，禾草和莎草盖度减小，杂草盖度增加，群落结构由垂直分层为主演化为水平生态位分化为主。3年中放牧的影响没有显示出草地的进一步退化趋势。

英文摘要：

Degradation of rangeland is an important problem in stock breeding. We compared grazed and nongrazed grassland plots in the Qinghai-Tibetan Plateau. From September 2005 to September 2007, we sampled ten square （0.5 m×0.5 m） randomly arranged plots of each treatment： Long-term grazing and grazing exclusion for six years. We divided species into five functional groups： Legumes, cyperaceous, poisonous forbs, forbs and grasses. We measured species richness, abundance （number of individuals）, community height and cover in each square, and above-ground biomass according to group functions. Plant samples were dried at 80℃ and weighed with a sartorius balance to the nearest 10^-3 g. Grazing resulted in a significant reduction in community productivity. Functional groups had different responses to grazing from 2005 to 2007. Biomass of graminoids was significantly reduced from an average of 76.6% to 57.4%, while that of legumes, forbs and weeds increased. The largest plant community was reduced from 46.8 to 22.4 plants per square. Biodiversity and individual density both increased： Biodiversity from an average 26.5 species per square to 30. 9 and abundance from an average of 215.7 per square to 304.6. Total percentage cover did not differ significantly between grazed and non-grazed plots. We did not find any evidence that the grassland had become more degraded during the three experimental years.