Reproductive Characteristics Of Dominated Population In Stipa Breviflora Desert-steppe Under Different Grazing Intensities

The experiment was conducted in Siziwang desert grassland in Inner mongolia which were divided into 3 different grazing intensities and a check treatment, i. e. Heavy Grazing(HG), Moderate Grazing (MG), light-grazing (LG), and check (CK), with 0.93,1.82,2.71, 0 sheep units/hm2/half year respectively. The reproductive characteristics of the main plants in Stipa breviflora desert-steppe from 3 aspects of vegetative propagation, soil seed bank and the correlation between aboveground vegetation were systematically investigated in period of from June to September, 2007. Influence of grazing on their productive behavior the main plants were discussed from the angle of plant adaptation to the local environment. The main research results were as follows:1 LG treatment was more benefit to vegetative propagation of all species, and CK treatment took the second place. The Artemisia frigida had a strong vegetative propagation capability. It could produce plentiful branches and adventitious roots in grazing treatments. That occupied largest creeping areas in LG. Rhizomes length of Leymus chinensis decreased by increasing the grazing intensity. And the number of node of the rhizome was largest in HG, least in light-grazing, middle position in CK and MG. The number of germination branches of Leymus chinensis was highest in LG, second in CK and lowest in HG, but all were not significantly difference. The tiller number of Stipa breviflora and its coverage in LG were higher than in other treatments. The big bunches of Stipa breviflora were broken into many small bunches in HG that increased the density of bunches.2 In all treatments the seed percentage of perennial plant in soil seed bank was predominate, and the percentage was up to 43.48%. The percentage of the seed of annual and biannual plant was quite high, up to 39.71%. Semishrub up to 16.06%. The effect of grazing on the density of soil seed bank was significant. The density of soil seed bank was highest in check plot, 839.4±218.08 grain/m2 and significantly difference from HG , MG and LG(P﹥0.05).3 The similarity between aboveground vegetation and the seed composition in soil was lowest in HG, with 18.2% similarity coefficient. The density of soil seed bank was highest in August , second in July and lowest in September.4 Spatial distribution of the soil seed bank differenced significantly, especially the vertical distribution of the soil seed bank was more significantly. The density of soil seed bank in different plot decreased sharply with the deepening of layer thickness, there was significant difference between layers. The density of soil seed bank in 0 ~ 10 cm layer was 58.9%, very significant from 10~20 cm (P<0.01). The density of soil seed bank in 10~20 cm layer was 30.9%, no significant from 20~30 cm(P﹥0.05). There still a certain amount of seed in 20~30 cm layer. There were still some changes in the horizontal distribution of the soil seed bank , especially 0 ~ 10 cm layer influenced strongly by grazing.