Responses of NDVI evolution to climate change and human activities in the Hunshandake Sandy Land and its ecotones

[Objective] Understandings of the ecological evolution in the Hunshandake Sandy Land are divergent duo to the differences in study area, data used, and period of study. How the vegetation in the hinterland and marginal ecotones of the Hunshandake Sandy Land interacts with regional climate and responds to human activities remain to be addressed. [Methods] In this study, the Gobi desert area was set as the background area (CK), and the Hunshandake Sandy Land and its surrounding areas were divided into a core area (CA) and four ecotones. Based on the long-term GIMMS NDVI3g v1 (1982-2015) dataset, combined with climate data (annual temperature and precipitation) from 1982 to 2015, nighttime light dataset (1984-2015), and population and herbivore data of each banner/county (1986-2015), the correlations between NDVI and climate change and human activities in the CK, CA and the ecotones were analyzed. [Results] The results show that: (1) Vegetation phenology was regionally heterogeneous. The length of growing season (LOS) was between 158 and 196 days, and the phenological difference within the region reached 38 days. (2) The change rate of NDVI was spatially different. Total NDVI increased at a rate of 0.00021/10a, while the rate of change in each region varied, with a maximum difference of 17 folds. (3) The response of vegetation to climate variability was zonal. In these ecotones, NDVI change was more sensitive to monthly temperature than to monthly precipitation. (4) The coefficient of variation (Cov) of vegetation went through several stages. The Cov fluctuated little in the 1980s, was relatively stable in the 1990s, and had higher positive variation in the first 15 years of this century. (5) The response of vegetation cover to human activities was not significant. While NDVI was not correlated with population density and nighttime light intensity per square kilometer, it was locally positively correlated with sheep and total herbivorous livestock density per square kilometer. [Conclusion] These results suggest that the NDVI of the Hunshandake Sandy Land and its ecotones fluctuated in response to climate change, and although it has improved in this century, it has only reached the level of the mid- to late 1980s.