Abstract: Plant density gradient can restrict the distribution pattern of plant biomass, and thus, affect the performances of mixedseeding grassland in ecology and production. From June to September 2010, a replacement series experiment was conducted to establish a piece of grassland with mixedseeding of Avena sativa and Vicia villosa in the upper reaches of Shiyang River, Gansu Province of Northwest China. Five treatments were installed, according to the relative density of A. sativa to V. villosa, i.e., 8∶2, 6∶4, 5∶5, 4∶6, and 2∶8, with the effects of the density gradient on the distribution patterns of the biomass of A. sativa and V. vilosa studied. With the decreasing relative density of A. sativa and the increasing relative density of V. villosa, the root-biomass of A. sativa decreased first and increased then, and decreased lastly, while that of V. villosa was in the contrary. The stem and leaf biomass of A. sativa and V. villosa increased after an initial decrease. With the growth of the grasses, the allocation ratio of the stem biomass of A. sativa and V. vilosa in the five treatments increased gradually, while that of the leaf and root biomass was in adverse. The adjustment of the resource allocation strategies of the mixedseeding grasses under the restriction of plant density gradient demonstrated the asymmetry of the light competition of plant aboveground parts and the symmetry of the resource competition of underground parts. The climbing growth of V. vilosa and its pressure on A. sativa stem made the plants have higher allocation ratio of stem biomass, and achieve the maximum resource use.

Key words: Root cutting, Photosynthetic rate, Aboveground biomass, Water use efficiency (WUE), Maize