China’s deserts greening and response to climate variability and human activities

The annual NDVI showed a significant increasing trend at a rate of 0.0018/yr, with values of 0.094 in 2000 and 0.126 in 2017. At the seasonal scale, the mean values of the NDVI were higher in summer and autumn, and lower in spring and winter, which showed significant increases in all four seasons, also higher in summer and autumn, and lower in spring and win- ter. The monthly NDVI showed an inverted U-shaped curve during the year, which was the lowest in January (0.0539) and the highest in August (0.0989). The variation of the monthly NDVI trends was similar. In terms of the climatic factors, the variations in monthly, seasonal, and annual temperature and precipitation did not show significant trends from 2000 to 2017. A total of 62.8% of the desert area showed greening trends, and only 0.78% showed brow- ning trends in spatial. The greening trends were mainly distributed on the southern edge of the Gurbantunggut Desert, in the northwestern part of the Taklimakan Desert, and in the Kubuqi Desert. The browning trends were distributed in the northwestern part of the Takli- makan Desert and the eastern part of the Qaidam Basin Desert. In terms of longitude and lati- tude, the area with greening trends was positively correlated with the desert area. The Badain Jaran Desert showed the highest percentage of greening (88.26%), followed by the Kumtag Desert (75.26%) and the Tengger Desert (70.07%); however, the Qaidam Basin Desert showed the highest percentage of browning (1.25%). In terms of seasons, spring had the largest propor-

tion of greening area, followed by summer, winter, and autumn. The spatial distribution of the greening and browning trends showed seasonal differences, and the spatial pattern in summer was the most similar to that at the annual scale, followed by autumn, winter, and spring. At the annual scale, climatic factors had limited explanatory power for NDVI variations, and only precipitation had a significant effect on NDVI, and the effect of precipitation was stronger than that of temperature. At the seasonal and monthly scales, temperature and precipitation were the main factors affecting NDVI variations. Furthermore, the lagged effects of climate fac- tors on NDVI at monthly scales were not observed because the time scale was not fine enough. Human activities played a critical role in the greening of China’s deserts from 2000 to 2017. Specifically, oasis expansion and sand stabilization measures were the two major reasons that led to desert greening. The former occurred mainly in the southern part of the Gurbantunggut Desert, the northwestern edge of the Taklimakan Desert, and patches at the edges of the Qai- dam Basin Desert, the Badain Jaran Desert, and the Tengger Desert, while the latter occurred mainly in the Kubuqi Desert. Supporting information S1 File. Image evidence of human activities. (PDF) S2 File. Data set (part 1). (ZIP) S3 File. Data set (part 2). (ZIP) S4 File. Data set (part 3). (ZIP) S5 File. Data set (part 4). (ZIP) S6 File. Data set (part 5). (ZIP) PLOS ONE China’s deserts greening and response to climate variability and human activities

PLOS ONE | https://doi.org/10.1371/journal.pone.0256462 August 30, 2021 15 / 20