Vegetation Change And Its Driving Forces In The Tibetan Plateau From 2000 To 2020

Global climate change and the role of human activities have profoundly impacted terrestrial ecosystems,accelerating their complexity and vulnerability.Tibetan Plateau,as a typical fragile ecosystem area,has a terrestrial ecosystem that is very sensitive in responding to climate change and human activities.And global warming and increased anthropogenic disturbances in recent decades have threatened the ecological security in parts of the Tibetan Plateau.As an important component of terrestrial ecosystems,vegetation is very sensitive and highly indicative to climate change.Therefore,detecting changes in vegetation cover in the Tibetan Plateau and identifying its drivers are important for the determination of ecological management policies and thus the maintenance of a healthy and stable ecosystem in the region.Based on the datasets of vegetation cover(MODIS EVI),land use,vegetation type,Digital Elevation Model(DEM),temperature and precipitation,this study analyzes the temporal and spatial variation patterns of vegetation cover in the Tibetan Plateau from 2000 to 2020,explores the lagging response of plateau vegetation to temperature and precipitation,and uses the residual trend analysis method(RESTREND)to identify the main drivers of vegetation change and quantify the contribution of each driver to vegetation change.Finally,the following conclusions were obtained.(1)From 2000 to 2020,the vegetation mean EVI in the growing season in the Tibet Plateau varied between 0.194 and 0.213,with a fluctuating slow growth trend and a growth rate of 0.006/10a.The interannual variation of EVI varied somewhat among different vegetation types.The trend values was the highest for cultivated vegetation(0.01/10a),followed by meadows,steppes,alpine vegetation,forests,scrub and grasses,with annual growth rates of 0.006/10a,0.006/10a,0.005/10a,0.004/10a,0.003/10a and0.003/10a,respectively.The interannual variation of vegetation EVI also varies within the different altitude gradients.Annual decreasing trend occurred in the low-altitude zone(-0.004/10a),while an growth trend was observed in the mid-altitude,highaltitude and extremely high-altitude zones,with values of 0.012/10a,0.007/10a and0.006/10a,in that order.(2)The annual mean values of vegetation EVI in the growing season showed a spatial pattern of "low in the northwest and high in the southeast".The analysis of EVI annual trends at the pixel scale showed that: 66.87% of the areas have no significant changes in vegetation;30.75% of the areas showed significant increase in vegetation cover,mainly in the central and northern parts of the plateau;and only 2.38% of the areas showed significant degradation in vegetation cover.The area of degraded trend vegetation was greater than the area of improved trend vegetation in the grasses,while the rest of the vegetation types were mainly improved trend.The statistics by altitude gradient showed that only in the low altitude zone the degraded trend area of vegetation was greater than the improved trend area,while in the middle,high and extremely high altitude zones the vegetation was predominantly improved trend.(3)The annual variations in both temperature and precipitation on the Tibetan Plateau showed an increasing trend.There were spatial differences in the changing trend of temperature and precipitation,in which the annual increases of temperature was mainly distributed in the central and southern parts of the plateau,while the annual increases of precipitation was mainly distributed in the eastern and western parts of the plateau.The temporal lagged response of vegetation to temperature and precipitation was more than 0 months and 7 months.The climate factors that have considered temporal lags improve the explanatory power on vegetation variation by 6%.In addition,it was found that the temperature time lag of each vegetation type on the plateau was mainly short-term,while the short-term and long-term lag of precipitation existed.(4)Based on the newly constructed vegetation change driving factor classification framework,the vegetation changes influenced by human activities are analyzed,and it is found that human activities mainly play a role in improvement,resulting in a small area of degradation;In the vegetation change area driven by climate,vegetation changes dominated by temperature and precipitation alone have obvious spatial aggregation,and vegetation changes in areas with abundant plateau water resources are mainly dominated by temperature,while vegetation changes in arid and semi-arid areas are dominated by precipitation.(5)The driving factors of vegetation change in different regions of the Tibet Plateau are quite different,but overall,the influence of human activities on the areas of significant vegetation changes has become stronger and stronger.