The Effects Of Steppe Degradation And Long-Term Grazing Exclusion On The Soil Seed Bank In Xilingole Steppe

Soil seed bank is an important research area in plant ecology. The Xilingole steppe has great representation in the Mongolian Plateau grassland. It is important in theoretical value and practical significance to reveal the characteristics of the seed bank in grassland degradation process and the long-term grazing exclusion Four series were chosen in the study areas in the Baiyinxile Ranch of the Xiiingole steppe. The four series are as below: 1) the first series which includes light-degraded, moderate- degraded, heavy-degraded and extreme degraded sites; 2) the second series which includes degraded sites with different distances from herdsmen residential; 3) the third series including enclosed and over-grazed sites; 4)site with Caragana microphylla shrub. In order to reveal the total soil seed banks and soil seed bank which after seed germinated in spring and before new seed rain added to, and the vertical distribution of soil seed bank, soil samples were collected in early April and late June 2006. Soil seed banks composition and structure were identified using germination method. Furthermore, the relationship between soil seed bank and standing vegetation were discussed. Our results are described as follows:1. Characteristics of the soil seed bankand standing vegetation of degradation grassland.The results of the first series showed: The number of the total species and the perennial species and the density of soil seed banks in April and June are negative correlation with the degeneration degree. The variances of the density of soil seed banks between sites are significant both in April (F=33.749, P＜0.001, df. =15) andJune (F=12.578, P＜0.05, df. =15). Light-degraded, moderate-degraded and heavy- degraded sites all have leymus chinensis, Stipa krylovii and Cleistogenes squarrosa distribution. The density of standing vegetation is highest in moderate- degraded site. The dry biomass is highest in light-degraded site. They are both lowest in heavydegraded site.The results of the second series showed: The composition and size characteristics of soil seed bank have no significant changes as the distance from the herdsmen residential increased. The variances of the density of soil seed banks between sites are not signifacant in April (F=3.739, P=0.066, df. =11) but signifacant in June (F=10.096, P＜0.05, df. =11). The ratio of perennial density in the density Of soil seed banks is positive correlation with the distance from the herdsmen residential.Density of standing vegetation decreases with the distance from the herdsmen residential increased, but dry biomass of standing vegetation increases.The distribution of soil seed banks in the two series are mainly in the layer of 0～3cm in April and in 0～6cm in June, and the density of soil seed banks decrease with the depth of soil generally.2. Characteristics of the soil seed bank and standing vegetation of long-term grazing exclusion and over grazed grasslandIn April, the density of soil seed bank in over-grazed degradation site only accounted for 19.3% and 18.0% of the sites enclosed for 23 years and 10 years respectively, but 33.3% and 34.5% in June. The variances of the density of soil seed banks between sites are obvious in April (F=3.739, P=0.066, df.=11) and June (F =10.096, P＜0.05, df.=11).The soil seed banks of most species mainly distribute in the layer of 0～3cm, density of soil seed banks decrease with the depth of soil.The density of soil seed banks in sites in April were lower than they were in June obviously. For the site enclosed for 23 years, the density of soil seed bank in April accounts for 52.6% of that in June, half of seed germinate in the spring nearly. Plant species of standing vegetation increase significantly in the site enclosed for 23 years and the site enclosed for 10 years, especially some mesophytes forbs and grasses, such as Saposhnikovia divaricata and Serratula centauroides. Both density and dry biomass of standing vegetation increase with the years enclosing. Long-term grazing exclusion can restore the species composition and soil seed bank of degraded leyrnus chinensis+ Stipa grandis steppe.Annuals including Artemisia scoparia, Chenopodium album and C. aristatum constitute a major component of the soil seed bank.3. The impact of thicketization on soil seed bankThe study on effects of Caragana microphylla shrub on the soil seed banks showed that the density of soil seed banks has no obvious correlation with crown-width. But the relationship between the densities of soil seed banks among Caragana microphylla shrubs and under Caragana microphylla shrubs is impacted by crown-width.4. Generally, the similarity between soil seed banks and standing vegetation, are middle similary or not similar in all series both in April and in June. However, the similarity coefficients in the first series are highest, followed by that in the second series and that in the third are lowest. Meanwhile, it was observed that the similarity coefficients of soil seed banks in all sites between April and June are higher than those between soil seed banks and standing vegetation, indicating that the stability of soil seed banks is higher than standing vegetation.