The Change Of Permafrost And Its Eco-environmental Effect In The Qilian Mountains

The Qilian Mountains are an important ecological security barrier and biodiversity conservation area in western China,and are also a frequent area for geological disasters.In recent decades,more and more ecological and environmental problems have emerged in the Qilian Mountains,such as grassland vegetation degradation,lake shrinkage,rising snow lines,melting ice and snow,wetland reduction,and land desertification and so on.As a regulator of climate change and a special type of aquifer,permafrost affects the exchange of greenhouse gases,vegetation growth and the entire ecosystem between the surface and the atmosphere,and is also a sensitive indicator of environmental change.Therefore,under the background of global warming,the study of permafrost change is of great scientific significance for revealing the changes of the ecological environment of the Qilian Mountains.Based on the site temperature observation data and vegetation remote sensing data,the nonparametric Mann-Kendall test method,Sen’s slope estimation method,trend analysis method and correlation analysis method were used to analyze the changes of climate,freezing and thawing index and vegetation index in the Qilian Mountains.The spatial-temporal changes of permafrost area and active layer thickness were simulated based on the ground freezing index model and Stefan equation.Based on the above research,the correlation and sensitivity of normalized difference vegetation index（NDVI）with temperature,rainfall and active layer thickness,as well as the significance level of each influencing factor,were analyzed from the temporal and spatial scales using the numerical analysis software MATLAB.The main conclusions of this paper are as follows:（1）From 1961 to 2014,the average annual temperature in the Qilian Mountains was in an overall rising period,which was spatially controlled by elevation.The overall precipitation showed a fluctuating and slow upward trend,with a decreasing spatial gradient from southeast to northwest.The overall evaporation showed a fluctuating downward trend.This indicated that the climate in mountainous areas is moving in the direction of warmth and humidity.（2）The results showed that in the past 54 years,the freezing index had a significant downward trend,and the thawing index had a significant upward trend.The annual average air freezing index,air thawing index,ground surface freezing index,and ground surface thawing index were roughly distributed between 994.3℃·d and 1540.9℃·d,1828.2℃·d and2376.6℃·d,744.7℃·d and 1287.3℃·d,2706.0℃·d and 3542.6℃·d,respectively.The climatic tendency rates were-6.5℃·d·a^-1,6.5℃·d·a^-1,-7.7℃·d·a^-1,and 9.1℃·d·a^-1,respectively.From northwest to southeast,the freezing index showed the distribution characteristics of high in the middle and gradually decreasing in the east-west direction,while the thawing index was opposite;in addition to altitude and latitude,freezing and thawing index was also affected by slope aspect,surrounding topography,snow depth and human activities.（3）Permafrost is widely present in the Qilian Mountains and is mostly in a stable state,which mainly concentrated on high altitudes.The average distribution area of simulated permafrost was 9.28×10⁴ km²,accounting for 48.76%of the total area of the mountainous area.In the past 15 years,the permafrost area of the Qilian Mountains had decreased by about1.70×10⁴ km²,and the degradation rate was 0.113×10⁴ km²/a.The degraded areas were mainly distributed at the junction of permafrost and seasonal permafrost.Altitude is the main factor controlling the overall distribution of permafrost,and other factors such as topography,landform,hot lakes and human factors also affecting it.The impact of human activities on frozen soil in the eastern human activity area was extremely significant.（4）Under the background of climate warming,in the past 15 years,the seasonal melting depth of permafrost in the Qilian Mountains has shown an increasing trend,with an increase rate of 1.0 cm/a,while the maximum freezing depth of seasonal frozen soil was the opposite,with a decrease rate of 1.2 cm/a.In the permafrost area,the multi-year variation of thaw depth ranged from 0.03 to 3.90 m,with an average value of 2.58 m,which was larger at the junction of permafrost and seasonal permafrost,and smaller in high-altitude mountainous areas.Spatially,the melting depth of the permafrost area was centered on Hala Lake,and the seasonal permafrost area gradually increased,and its change was controlled by the elevation.In the seasonal permafrost zone,the maximum freezing depth varied from 1.36 to 3.15 m over multiple years,with an average value of 2.55 m.Spatially,the closer to the permafrost boundary,the greater the maximum freezing depth,and gradually increased from south to north,and its change was controlled by both elevation and latitude.（5）In the past 15 years,the NDVI of vegetation in the Qilian Mountains had shown an overall increase trend.The area of vegetation improvement and degradation accounted for 34.85%and 6.07%of the total area,respectively.The area of vegetation improvement was concentrated in the central and western regions,especially the high-altitude mountainous areas.The decrease of NDVI was mainly concentrated in mountainous areas and gully areas in eastern China,which was greatly affected by human activities.The root cause was that in the past,people overgrazing and extensive land reclamation and logging activities had seriously accelerated the degradation of vegetation.（6）In the time scale,rainfall had the most significant effect on NDVI,followed by the thickness of active layer.In terms of spatial scale,the active layer thickness had the most obvious effect on NDVI within the height range of 3200-4750 m,which meant that the upper buried depth of frozen soil was the main factor affecting the change of NDVI.When the height was greater than 4750 m,rainfall had the most significant effect on NDVI,followed by air temperature,and the active layer thickness had the least effect.