LIANG Si-hai, WAN Li, LI Zhi-ming, CAO Wen-bing
In the recent 20 years,the vegetation degradation and the decrease of height, output and coverage of grazing in the source regions of the Yellow River,located at the edge of permafrost in the Tibetan plateau,is one of the most serious problems scientists have to face.The purpose of this study is to discover the relation between alpine meadow and permafrost environment through quantitative analysis.From five-year field work,including making 11 profiles cutting across 49 investigation holes,many investigation data are got,such as vegetation coverage, earth temperatures and percentages of moisture content.The research findings indicate that the depth of seasonal thawing affects the moisture content percentage of shallow soil,where major plant root exists,then affects the vegetation coverage.There are two perfect correlations between the depth of seasonal thawing and the moisture content of shallow soil and between the depth of seasonal thawing and the vegetation coverage.When the depth of seasonal thawing is less than 2.0 meters,the depth of seasonal thawing,which determines the moisture content in shallow soil,affects the alpine meadow’s growth.As a result,only some plants with long-root and able to bear drought can exist with lower coverage,less than 35 percent.The depth,less than 2.0 meters from the surface, is named ecological depth of seasonal thawing,where the capillary water does not climb up to the shallow soil layer to eliminate dryness,and the growth of majority of alpine meadow becomes impossible.Ground temperature has increase by more than 1.2 ℃ for twenty years at the depth of 3.2 meters,as measured at Madoi Meteorological Station.As a result,permafrost table drops down,depth of seasonal thawing increases and ice-short permafrost appears,even permafrost disappears,making soil much dryer with smaller water supply.Sequentially,moderate hygrophilous plants are replaced by xerophious plants,or even rodent damage.In conclusion,the condition of alpine meadow is changing,the former stable relation between the vegetation and the depth of seasonal thawing is breaking,and a series of ecological degradation can be seen in the source regions of the Yellow River.
