Effect Of Grazing On The Plant Communities And The Soil Seed Banks And Their Relationship In Inner Mongolia Steppe

Four typical grassland communities, Stipa klemenzii, Stipa grandis, Leymus chinensis and Leymus chinensis + forbs, had been chosen along precipitation gradients from 200, 290, 350 to 450 mm respectively in Inner Mongolia Steppe. The objectives of this study were to determine the effects of grazing on plant community composition, aboveground and belowground biomass, species and functional group diversities and the relationship between diversity and productivity, the effects of grazing on soil seed bank species composition, seed density and diversity along the transect. Based on these, this study was to determine the relationship between standing vegetation and soil seed bank. The main results are as follows: 1 Effects of grazing on plant communities The grazing successions were Stipa klemenzii → Salsola collina + Stipa klemenzii → Salsola collina community in the desert steppe; Stipa grandis or Leymus chinensis → Cleistogenes squarrosa + Stipa krylovii or Stipa grandis → Potentilla acaulis + Cleistogenes squarrosa community in the typical steppes and Leymus chinensis + forbs →Leymus chinensis + Stipa baicalensis community in the meadow steppe. There were marked differences in the complementary effect patterns of the life form functional groups responses to different grazing intensities in different communities: in Stipa klemenzii community, the perennial bunch grasses decreased while annuals and biennials increased along the grazing intensity increased; in Stipa grandis and Leymus chinensis communities, the perennial bunch and rhizome grasses decreased while the perennial forbs increased along the grazing intensity increased; and in Leymus chinensis + forbs community, perennial rhizome grasses decreased while perennial bunch grasses increased when grazing intensity ranged from un-grazed to moderate grazing. There were same complementary effect in the patterns of water ecological groups and photosynthetic pathway groups responses to different grazing intensities in different communities: grazing made the cool season plants and non-xerophytes decreased while the warm season plants increased. Results on the effects of grazing by sheep on species and functional group diversities had not led to consensus in different communities: species richness, species diversity, diversities of life forms and water ecological type groups decreased with grazing intensity increased except in the Leymus chinensis + forbs community. Grazing reduced aboveground biomass of the communities except in the Stipa klemenzii community. In this case annual warm season plant, Salsola collina, became absolutely dominant species in the community under heavy grazing intensity. There was not significant change in 0~10 cm belowground biomass except in the Leymus chinensis community and grazing reduced 0~30 cm belowground biomass except in Stipa grandis. The relationship between diversity and biomass in Inner Mongolia grazing steppe can be best described as Y = 809 + 774x(r2=0.84, P<0.001), where Y is the sum of aboveground and belowground biomass and x is water ecological type diversity. 2 Effects of grazing on the soil seed banks The soil seed bank species composition of different communities was dominated by perennial forbs in different grazing intensities during the sampling time. The seed amount of different life form functional groups had great difference: annuals and biennials seed density was highest except in the Leymus chinensis + forbs community. Moderate grazing increased perennial grasses seed density expect in the Stipa klemenzii community and annuals and biennials seed density in the Stipa klemenzii and Leymus chinensis + forbs communities. There was not significant different about the perennial forbs seed density along grazing intensities expect in the Leymus chinensis + forbs community. The seed amount of shrubs and semi-shrubs was relatively low, grazing intensity or sampling time had no significant effect on theseed intensity of shrubs and semi-shrubs. Moderate grazing increased the total density of the seed banks except in the Stipa klemenzii community. The total density of the seed banks in Inner Mongolia grazing steppe ranged from 20.8 m-2 to 3819.2 m-2. The highest species richness of seed banks was in October and grazing deceased it expect in the Leymus chinensis + forbs community. Moderate grazing increased the seed bank species diversity in Stipa klemenzii and Stipa grandis communities of July and Leymus chinensis + forbs community during the sampling time. 3 Relationship between standing vegetation and soil seed bank The dominant species of seed bank and standing vegetation were same in the definite sampling time and grazing intensity, but only 23.3% of total sampling time and grazing intensity had the same dominant species with the standing vegetation. The Sorensen similarity index was influenced by sampling time and grazing intensity. In October, the Sorensen similarity index was higher. Moderate grazing increased the index in Leymus chinensis and Stipa grandis communities of April and Leymus chinensis + forbs and Stipa klemenzii communities during the sampling time. There was not significant relationship between the aboveground biomass of standing vegetation and the density of soil seed banks during the sampling time. 4 In order to estimate the seed bank species composition and seed density and study the relationship between standing vegetation and soil seed bank, after the direct germination, protecting the sampling and one year later germinated again maybe is a better method. Keywords: Inner Mongolia Steppe, precipitation gradient, grazing, functional...