Spatial-temporal pattern and gravity center change of fractional vegetation cover in Xinjiang, China from 2000 to 2019

Abstract: Vegetation has become the main producer to serve as an integral part of natural ecosystem, as well as perform various vital ecological functions. Vegetation coverage has some direct effects on the structure, functions, and health of an ecosystem. The vegetation coverage can also be one of the key ecological parameters that can reflect the level and quality of an ecosystem, particularly on Xinjiang, China, characterized by shortage of water resources and fragile ecological environment in the arid hinterland of Eurasia. New challenge has posed on the ecological system in Xinjiang, as "the Belt and Road" initiative is being promoted. Furthermore, the area and quality of vegetation coverage is degrading, as adverse effects increased from the climate change and human activities in the last 20 years, leading to severe negative impacts on the ecological environment and social economy. Therefore, it is necessary to carry out the long-term sequence monitoring on the vegetation coverage at the regional scale, in order to promote the eco-friendly development model, and further to ensure the sustainable development of the "Silk Road of Green Development". In this study, MODIS/EVI data was employed to obtain the Fractional Vegetation Coverage (FVC) based on pixel dichotomy, thereby to explore the change in vegetation coverage and its driving mechanisms. The variation of FVC was revealed using the percentage of deviation from normal and linear regression analysis. The driving mechanism of FVC change was also analyzed by the gravity center model and correlation analysis. The results showed that: 1) The average vegetation coverage was 0.255 in Xinjiang from 2000 to 2019, where the area with an very low coverage accounting for 63.53%. The vegetation coverage was higher in Yili River Valley, where the area with an extremely high coverage accounting for 77.9%, whereas that in southern Xinjiang only accounted for 6.94%. 2) The FVC was on the rise in Xinjiang as a whole, with a stable variation. The percentage of abnormal change was within the normal range. However, the fluctuation of FVC was relatively large in eastern Xinjiang. In 2015, the FVC value was nearly 20% higher than the average level, with other regions fluctuating in the range of ±5%. The reduction area of vegetation coverage was larger than that of the increasing area, where 53.04% of the area tended to decrease. At the level of significance less than 0.05, the area with increasing trend was large. 3) In the driving mechanisms for the variation of vegetation coverage, the spatial position of vegetation centers, temperature and precipitation were calculated to analyze the change and relationship between the gravity centers of various elements from the spatial level. The vegetation center of gravity fluctuated to high longitude and high latitude. In terms of longitude change, temperature was the main driving factor of vegetation spatial change in Xinjiang, indicating a positive correlation between the center of vegetation and temperature. There was a negative correlation between the vegetation center and precipitation center in Yili River Valley, with R value of -0.492. However, there was no significant relationship between the vegetation center and the center of temperature and precipitation in eastern Xinjiang. The significant change of vegetation coverage was less than 0.05 under the level of low vegetation coverage and uneven distribution. The increased area was greater than the reduced area, indicating an obvious effect of ecological restoration. A recommendation can be to strengthen the vegetation protection, especially in the area of reduced vegetation coverage. The evaluation of vegetation change and its driving mechanisms in different regions of Xinjiang can provide a scientific basis to protect the fragile ecological environment.