Study On Vegetation Variation And Its Response To Climate Factors And Human Activities In The Qilian Mountains

As one of the most important components of the terrestrial ecosystem on Earth,vegetation bears the heavy responsibility of connecting soil,atmosphere,hydrology and other ecological elements,which is of great practical significance for improving and protecting the fragile ecosystem of the Qilian Mountains.The systematic study of vegetation variation in the Qilian Mountain at different temporal and spatial scales and its response mechanisms to climate factors and human activities can provide scientific basis for the rational formulation of ecological protection and restoration measures and policies in the Qilian Mountains.Based on GIMMS NDVI,MODIS NDVI,contemporaneous meteorological data and land use data,this study used Theil-Sen trend analysis,Mann-Kendall significance test,Hurst index,partial correlation analysis and residual analysis to analyze the characteristics of vegetation coverage variation in the Qilian Mountains at different temporal and spatial scales from 1982 to 2020,to explore the response of vegetation coverage variation to climate change and human activities,and to quantify the impact of climate change and human activities on vegetation coverage variation.The main results are as follows:(1)The intra-annual variation of vegetation coverage in the Qilian Mountains from1982 to 2020 is obviously cyclical,and the inter-annual variation shows an increasing trend.The intra-annual variation of NDVI is clearly seasonal,peaking in July,and the intra-annual variation pattern of NDVI at the regional scale is consistent with the pixel scale.The NDVI of the Qilian Mountains in interannual(0.0007/a),growing season(0.0010/a),spring(0.0006/a),summer(0.0014/a),autumn(0.0009/a),winter(0.0002/a)and eastern(0.0006/a),middle(0.0006/a),western(0.0008/a),mountains(0.0004/a)and valleys(0.0011/a)all showed increasing trends.(2)The overall vegetation coverage of the Qilian Mountains improved and locally deteriorated from 1982 to 2020.At the pixel scale,NDVI showed a gradual increase in the improvement area(72.81%→87.57%),a slight increase degradation area(2.80%→3.55%),and a gradual decrease unchanged area(24.38%→8.88%).At the regional scale,the percentage of NDVI trends in the eastern,middle,and western sections of the Qilian Mountains,mountains and valleys,and various vegetation types varied and changed significantly before and after 2000,and the areas showing improvement increased significantly.(3)There are differences in the responses of vegetation growth to co-seasonal climate factors and similarities in the responses to pre-season climate factors at different scales in the Qilian Mountains from 1982 to 2020.At the pixel scale,vegetation growth is controlled by co-season temperature in 63.81% of the areas and by co-season precipitation in 29.95% of the areas.NDVI in spring,autumn,and winter is mainly positively correlated with co-season temperature,and negatively correlated with coseason precipitation.Vegetation growth is affected by temperature for a longer period of time in a year,and the temperature is the dominant climatic factor affecting the vegetation growth in Qilian Mountain.At the regional scale,the correlations of NDVI with autumn temperature and spring precipitation in the eastern part of the Qilian Mountains is opposite to that in the middle and western parts.The response of NDVI to co-season temperature is similar in the mountains and valleys,and the response to co-season precipitation is different.The correlation between NDVI and co-season temperature is higher than that of precipitation in most vegetation types(except summer).The NDVI of the Qilian Mountains is positively correlated with pre-season climate factors at both pixel and regional scales,and negatively correlated with summer temperature only,indicating that warming and rainfall in the previous season have a lagging promoting effect on vegetation growth.The maximum response of NDVI to temperature and precipitation in the Qilian Mountains lags about 2-3 months and 0-1 month,respectively.The lag period of response to temperature is longer than that of precipitation,and the promotion effect of temperature rise in the previous season on vegetation is stronger than that of precipitation.(4)Climate change and human activities jointly affect vegetation growth in the Qilian Mountains from 1982 to 2020,with climate change having a greater impact but a gradually decreasing contribution(73%→60%)and human activities having a smaller impact but a gradually increasing contribution(27%→40%).The area with higher climate change contribution gradually transfers from the central part of Qilian Mountains to the peripheral areas around the Qilian Mountains,and the area with higher human activities gradually transfers from the central part of Qilian Mountains to the southeastern part of Qilian Mountains.Before 2000,the eastern part of Qilian Mountains had the highest climate change contribution,followed by the central and western parts;while after 2000,the western part had the highest contribution rate of climate change.The climate change contribution rates in the mountains and valleys were not very different,and the climate change contribution rates in both regions decreased after 2000.Most vegetation coverage variation of most vegetation types are dominated by climate change drivers.Human activities have both positive contribution and negative inhibition to vegetation coverage variation.