Abstract: The difference in biomass allocation relationships among roots,stems and leaves of dominant species Stipa purpurea and associated species Oxytropis microphylla between fencing and grazing grasslands was investigated in the alpine steppe of northern Tibet. The results showed that the fraction of leaf,stem of Stipa purpurea and Oxytropis microphylla were influenced significantly by grazing. The allometric scaling exponents of leaf-stem and leaf-root of both species increased,while the scaling exponents of root-shoot reduced. The allometric relationship of leaf-stem and leaf-root performed convergent assimilation in grazing grassland. It showed significant difference between species on the fraction of organs. And the allometric scaling exponents of leaf-stem,leaf-root and stem-root of Oxytropis microphylla were greater than those of Stipa purpurea. The trade-off strategy among organs was changed by grazing. The trade-off between leaf-stem of Oxytropis microphylla was higher than Stipa purpurea under grazing conditions. Similarly,as for the trade-off leaf-root and stem-root,the benefit of Oxytropis microphylla was more inclined to leaf,while this of Stipa purpurea benefitted stem. Compared with the root,more resources were allocated to the aboveground parts of two species. The variation of biomass allocation among plant organs caused by grazing environments and species means a lot to the community composition succession.

Key words: Alpine grassland, Fencing, Grazing, Biomass allocation, Allometry

摘要： 以藏北高寒草地优势种紫花针茅（Stipa purpurea）和伴生种小叶棘豆（Oxytropis microphylla）为研究对象，探讨围封和放牧草地中两种植物在叶、茎、根间生物量分配的差异性。结果表明：放牧对紫花针茅和小叶棘豆的叶、茎的质量比影响显著，两者叶-茎和叶-根的异速生长指数均升高，根-冠的异速生长指数均降低，植株叶-茎和叶-根的异速生长关系出现趋同化。种间根、茎和叶生物量分配存在显著差异性，小叶棘豆叶-茎、叶-根和茎-根的异速生长指数均大于紫花针茅。放牧改变了植物各器官间的权衡策略，紫花针茅在叶-茎权衡关系中更益于茎的生长，小叶棘豆更益于叶的生长；在叶-根和茎-根权衡关系中，小叶棘豆更倾向于叶，紫花针茅则更倾向于茎；相对于根系，两者均在放牧后分配更多资源到地上部分。放牧环境和种间差异导致的植物各器官间生物量分配的变化对群落组成演替有着重要意义。

关键词: 高寒草地, 围栏, 放牧, 生物量分配, 异速生长