Abstract:

Climate change and human activities are the two main factors affecting vegetation restoration and degradation. How to quantitatively evaluate the impacts of climate change and human activities on vegetation restoration and degradation is of great significance for addressing climate change, construction of ecological restoration and economic development along the Silk Road, In this paper, net primary productivity (NPP) is used as an indicator to evaluate vegetation restoration and degradation, and climate productivity and productivity affected by human activities are used as indicators for assessing climate change and human activities for vegetation restoration and degradation. Xinjiang is located in the arid region of northwestern China, with a typical temperate continental climate and extremely fragile ecological environment. The annual average precipitation is less than 200 mm. The main vegetation types are desert and grassland, accounting for 52% and 28% of the total vegetation area. Based on the MODIS spatial resolution of 500 m NDVI data from 2001 to 2016 and 99 meteorological data and vegetation type data, the CASA model and Zhou Guangsheng model were used to calculate the actual productivity and climate productivity of vegetation, and the difference between climate productivity and actual productivity was defined the impact of human activities. Using the methods of linear regression analysis and F-test, the effects of climate change and human activities on vegetation productivity and productivity of different vegetation types were separated by analyzing the trends of vegetation productivity, so as to evaluate the effects quantitatively of both on vegetation restoration and degradation. The results showed that the area where vegetation NPP has a significant change trend accounts for 34.02% of the total vegetation coverage area from 2001 to 2016 in Xinjiang, only 3.44% area underwent degradation, whereas 30.58% area experienced restoration. The area of vegetation restoration caused by human activities and climate change accounts for respectively 42.03% and 30.58% of the total area of vegetation NPP change, the vegetation NPP was increased by 319 Gg C·a^-1 and 59 Gg C·a^-1, respectively. The area of vegetation degradation caused by human activities and climate change accounts for 57.63%和19.45% of the total area of vegetation NPP change, the vegetation NPP decreased 319 Gg C·a^-1, 59 Gg C·a^-1, respectively. Different vegetation types are subject to different dominant roles in the process of restoration and degradation. Restoration was domination by human activities in crops, desert, grassland, alpine vegetation, whereas degradation was domination by human activities in forest, shrub, and swamp. Except for swamp, restoration was domination by climate in seven vegetation types. In general, NPP in different vegetation types is increasing, especially crops, deserts and grasslands. The NPP was increased by 293 Gg C·a^-1,171 Gg C·a^-1 and 148 Gg C·a^-1 respectively. The results of these studies indicate that the vegetation in Xinjiang is recovering, and human activities are the main influencing factors of vegetation restoration. In particular, crops contribute the most to Xinjiang vegetation productivity, and the climate change generally promoted the increase of vegetation NPP in Xinjiang. Through the correlation analysis between vegetation NPP and climate factors, it showed that precipitation has a significant positive correlation with vegetation NPP. There is a weak negative correlation between temperature and vegetation NPP, and water conditions have become the main limiting conditions for vegetation growth.

Key words: net primary productivity, climate change, human activities, arid areas