黄河源区冻土对植被的影响

黄河源区由于近年来气候变化的影响,打破了高寒植被与冻土环境之间稳定的适应性关系,由此引发了一系列生态环境退化的现象.在黄河源区多年野外工作的基础上,定量分析了冻土与植被之间的关系.研究表明:多年冻土埋深通过影响浅层土壤含水量影响植被生长的,多年冻土的埋深与浅层土壤含水率和植被的覆盖率具有良好的相关性规律.冻土埋深<2 m时,冻土埋深决定浅层土壤含水率,成为影响植被的生长主要因素;埋深>2 m时,冻结层上水水位低、补给量少,冻结层上水水量小,毛细上升高度不能达到植被根系分布的浅层土壤中,植被生长环境干旱化,多数植被生长受限制,这时只有少量根系发达的耐旱植被存活,覆盖率小,一般不超过35%.因此,2 m的多年冻土埋深为“生态冻土埋深”.近20 a来,黄河源区地温长期处于增温状态,多年冻土出现表层融化,形成深埋的或少冰的冻土等现象;部分地带完全融化消失,连续多年冻土变成不连续冻土或岛状冻土.多年冻土退化后,土壤含水量减少,导致植被物种更替、“黑土滩”等退化现象.

The Effect of Permafrost on Alpine Vegetation in the Source Regions of the Yellow River

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.