Abstract: Taking the common perennial grass species Elymus nutans, Festuca sinensis, and Festuca ovina widely distributed over the alpine grassland of QinghaiTibetan Plateau as test materials, and through establishing different mono and mixed cultured species combinations, a fiveyear continuous measurement of the plant community productivity was conducted, aimed to understand the effects of species composition, richness, sowing density, and soil nutrients on the  complementary effect of plant communities, and how the  complementary effect changed with time. The results indicated that species composition had significant effects on the  complementary effect of plant communities. When the soil nutrients were richer, the plant communities with higher species richness had higher  complementary effect. With increasing sowing density, the  complementary effect had a decreasing trend at the initial stage, but did not vary significantly in subsequent years. Fertilization increased the  complementary effect significantly, and this effect tended to be increased with the increasing years of plant growth.

Key words: nutrient structure, principal component analysis, water quality, Tangshan Bay.