Spatial And Temporal Evolution And Analysis Of Driving Factors For Vegetation NPP In Qilian Mountains

The Qilian Mountains region located in the northwestern part of the Qinghai-Tibet Plateau,which not only responsible for maintaining the balance of the mountain-oasis-desert ecosystem,but also for ensuring the ecological security barrier and water conservation in the western part of China.In recent years,under the influence of both climate change and human activities,a series of ecological problems such as soil erosion,vegetation degradation,permafrost melting and glacier retreat have emerged in some areas of the Qilian Mountains,seriously limiting the ecological security and green sustainable development of the region.The degradation of ecological environment will break the stability and balance of the ecosystem and make the ecosystem service function weaken,which in turn threatens the sustainability of economy and society.Based on MOD17A3 remote sensing data in Qilian Mountains,this paper analyzes the driving factors affecting the spatial distribution pattern of vegetation NPP in Qilian Mountains using geographic probes,and explores the effects of climate change and human activities on the spatial and temporal changes of vegetation NPP in Qilian Mountains using partial correlation analysis,residual analysis and land transfer matrix.On this basis,the area share of climate change and human activities on vegetation restoration and degradation areas in Qilian Mountains region from 2000—2019 was quantified to provide a scientific basis for ecological environmental protection management and sustainable economic and social development in Qilian Mountains.The main conclusions are as follows:（1）Based on MOD17A3 remote sensing data in the Qilian Mountains,it was found that the overall NPP of vegetation in the Qilian Mountains showed a fluctuating upward trend from 2000 to 2019,with a growth rate of 1.67 g C·m^-2·a^-1.The spatial distribution pattern was high in the east and low in the west,and the multi-year mean vegetation NPP was 138.54 g C·m^-2·a^-1.In the interannual variation,the mean vegetation NPP was 126.22 g C·m^-2·a^-1 in 2000—2005,and the mean vegetation NPP in 2005—2010 vegetation NPP mean value increased rapidly to 138.32 g C·m^-2·a^-1,2010—2015 vegetation NPP mean value was 140.93 g C·m^-2·a^-1,2015—2019vegetation NPP mean value increased to 150.62 g C·m^-2·a^-1,and the growth area is mainly concentrated in the east-central part of Qilian Mountains.（2）The spatial distribution of vegetation NPP in the Qilian Mountains was influenced by each driver in descending order of multi-year average q-values:growing season average temperature>land use>growing season precipitation>vegetation type>soil type>GDP>soil erosion>slope orientation>slope gradient.The influence of the driving factors on the NPP of vegetation in Qilian Mountains was synergistically enhanced among them,specifically in two types of non-linear enhancement and two-factor enhancement.Among the natural factors,the growing season average temperature and growing season precipitation have a dominant effect on vegetation NPP,while among the economic and social factors,land use change also plays an important role.Therefore,the growing season average temperature,growing season precipitation and land use change were selected to analyze the spatial and temporal effects of climate change and human activities on vegetation NPP in Qilian Mountains.（3）The correlation coefficient between vegetation NPP and growing season precipitation in Qilian Mountains from 2000 to 2019 was 0.6（P<0.05）,and the partial correlation coefficient with growing season mean temperature was 0.57（P<0.05）,both growing season precipitation and growing season mean temperature had a greater influence on vegetation NPP changes in Qilian Mountains Further analysis revealed that the vegetation NPP in Qilian Mountains was less influenced by temperature in areas with high influence of precipitation,and less influenced by precipitation in areas with high influence of temperature.The area of grassland（3561.95 km²）grew the fastest during the land use change in Qilian Mountains from 2000 to 2018,and the rest were in the order of watershed（1128.78 km²）,cultivated land（205.79km²）and construction land（80.79km²）,while the area of unused land（4965.90km²）decreased the most and forest land（11.42km²）also shrank.The contribution of land use change to the change of vegetation NPP in Qilian Mountains is about 4.9%,which indicates that human activities have a low and positive effect on the change of vegetation NPP in Qilian Mountains.（4）NPPa,NPPp and NPPh were all dominated by an increasing trend in 2000—2019,with an annual increase of 2.5 g C·m^-2·a^-1 for vegetation NPPa,1.61 g C·m^-2·a^-1for vegetation NPPp,and 0.89 g C·m^-2·a^-1 for vegetation NPPh in Qilian Mountains.The NPPa and NPPp growth areas are widely distributed,the lowering areas are less distributed and scattered,and the NPPh distribution is smaller and mostly in the low elevation and human activity areas.Based on the above study,we found that climate change and human activities explained 89%and 11%of the vegetation recovery in the vegetation recovery zone,respectively.Climate change explained more than 80%of the scrub,desert,grassland,bog and alpine vegetation in the recovery zone,and human activities explained more than 20%of the coniferous and broad-leaved forests in the recovery zone.In the degraded vegetation zone,climate change and human activities explained 22%and 78%of the vegetation degradation,respectively.Human activities explained more than 80%of the broadleaf forest,scrub,desert,grassland and bog vegetation in the degraded zone,and climate change explained more than 20%of the coniferous forest and alpine vegetation in the degraded zone.