Grazing rate effect on major plant population niches in artificially mixed-sown alpine grassland

Dominance and polarizing rule of niche for twenty-two main plant populations in Elymus nutans/Puccinellia tenuiflora mixed-sown alpine grasslands with different stocking rates were studied. Results show that after three grazing seasons,Elymus nutans and Puccinellia tenuiflora are still the dominant plants in plots with different grazing rates, whereas sub-dominant and concomitant plants are highly variable if not vulnerable. Under ifferent grazing conditions, planted Elymus nutansplant is the competitive winner and restrains growth of Puccinellia tenuiflora due to its high height, deep and well-developed roots, hence widest niche breadth(0.956)，which closely followed by Puccinellia tenuiflora (0.821). Planted Elymus nutans and Puccinellia tenuiflora plants and invading Poa spp. have larger niche overlaps with other plants, except for Koeleria cristata, Festuca rubra, Anaphalis lacteal and among each other. There are also larger niche overlaps among Potentilla anserina, P. multifida and P.nivea; and Kobresia humilis, K. parva and Carex ivanavoe,which exhibit similar phenology. This indicates that larger niche overlaps exist among plant species with similar morphology and phenology, and between species with greater niche breadths over others.Species distributing in two extremities along stocking rate gradients show narrower niche overlaps.