| IIIVegetation coverage, necessary for scientific research, can provide basic data for quantitative study on ecology, water reserve, soil, water conservancy, plant, etc., making the research results and model theories more scientific and credible. As an important changing marker of ecosystem, it can offer scientific indication for the study of frontier problems, such as the change of regional or global surface coverage, landscape differentiation, and accelerate the development of study on natural environment. And, as the important parameter of the study on quaternary geology and quaternary environmentology, it takes a significant role in monitoring and prevention for secondary geological disasters.Qomolangma Nature Reserve mainly protect the unique alpine natural ecosystem of Tibetan Plateau, including the extremely alpine landscape, the humid mountain forest ecosystem and semiarid plateau shrub-grassland ecosystem in the south and north slope of Himalayas respectively. The eco-environment there is so fragile that it is desiderate to approach effective monitoring and managing methods for this area.This paper introduced the basic theories of vegetation coverage, its research progresses and the developing trends. Then we analyzed and discussed the methods of measuring vegetation model. We take the Qomolangma Nature Reserve as study area, based on the dimidiate pixel model, with the support of RS and GIS, the vegetation coverage information of this area in 1990, 2000 and 2005 is extracted through improved pixel dichotomy model. In this paper, five crucial ecosystems in different altitude are studied specifically. They are evergreen broad-leaved forest, evergreen coniferous forest, evergreen shrub, Kobresia meadow and alpine talus vegetation. By approaching the changing process and trend of their coverage with the change of time and space, we analyzed the reason of the differentiation. The study results are as follows:(1) All the evergreen broad-leaved forests were distributed in the south slope of the study area, and the changing of the area of evergreen broad-leaved forest between the year of 1990 and 2000 relied mainly on stability, which accounted for 78.56%, the vegetation degradation region accounted for 10.53%, and the recovery region, however, which is a little larger than the degradation one accounted for 78.56%. The year between 2000 and 2005 when the area was mainly on stability, however, the vegetation degradation region accounted for 21.84% comparing significantly with the recovery one for 12.65%. Some herdsmen'pursuit of economic interests, building and heating through deforestation may be the cause of vegetation degradation.(2) The evergreen coniferous forests also distribution in the southern slope, and the changing in this area relied mainly on stability between either 1990 and 2000 or 2000 and 2005. The area of degradation region is larger than of the recovery region during the first 10 years. The vegetation coverage improved significantly between 2000 and 2005, the degradation region reduced from the past 203.64km2 to 64.78km2, with the recovery region increasing from 23.96km2 to 94.42km2, what's more, the stability region also increasing from the former 312.06km2 to 380.46km2. The data showed that some people did excessive chopping only for economic interests in the first 10 years, making the evergreen coniferous forests degenerated greatly. During the year between 2000 and 2005, however, the vegetation coverage changed to the recovery way because of the project of natural forests protected and the policy of conversion of cropland to forest. The government also carried some measures of chopping forbidden and increased the management power, which promoted the forests recovery.(3) The stable region of evergreen shrub which is the primary area in coverage changing reached 76.42% between 1990 and 2000, the vegetation recovery region accounted for 10.34%, and the degradation region, however, which is a little larger than the recovery one accounted for 13.24%. The recovery region increased appreciably from former 10.34% to 12.28% from 2000 to 2005. We can draw a conclusion that the vegetation types degenerated in a certain degree in the first 10 years, but it recovered slightly after 2000 because of the grazing forbidden measures and the stronger protection by the government, however, the area of degradation region was still a little larger than the recovery one as a whole. By the condition of the stability on spatial distribution, the area of degradation region both in the northern and southern slope was larger than which of the recovery region, and the southern condition was more serious than the north.(4) The stable region in the changing of kobresia meadow held advantage above 80% whatever between 1990 and 2000 or between 2000 and 2005. The area of degradation region accounted for 8.92%, comparing by the recovery region for 9.52% between 1990 and 2000. From 2000 to 2005, the area of degradation region accounted for 8.77%, which is a little smaller comparing by the recovery region for 10.68%. Kobresia meadow in Qomolangma Nature Reserve was a stable—recovery style in both slopes, the recovery region, especially, was in the northern slope by large. So the change of this ecosystem type was stable, and the system was protect well. (5) The degradation region of alpine talus vegetation accounted for 10.96% between 1990 and 2000, and the recovery region accounted for 11.22%. Between 2000 and 2005, the degradation region accounted for 10.09%, which was a little smaller than the recovery one for 10.61%。So we can draw a conclusion that it was hard for people to get in this ecosystem which was hardly influenced by human beings. The main changing on each slope was stable style, sometimes was slight recovery. The area of degradation region on southern slope was equal to the recovery region, and the recovery region was a little larger than degradation one on northern slope.
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