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

and the greater effect of precipitation compared to temperature were also observed in related studies [34,36,56,77,78], mainly because the vegetation growth in arid regions is more limited due to the lack of moisture. The lagged effects of climatic factors on NDVI were not observed at the monthly scale, which may be because the time scale was not fine enough that the lagged effect might be less than one month. Related studies show that in temperate grassland deserts, the average lag time between NDVI and temperature and precipitation is 16.4 days and 11.8 days, respec- tively [79]. In general, the correlations at the seasonal and monthly scales were significantly strong but weaker at the annual scale. Spatially, the correlations with the NDVI at the monthly scale were stronger, and those at the annual scale were weaker and showed a completely different spatial distribution. These results confirm that climate variability is the main explanation of intra- annual NDVI variations, but could not effectively explain the greening and browning trends of China’s deserts from 2000 to 2017 reflected by NDVI at the annual scale. Human activities The results show that the majority of the desert area showed a significantly increasing trend from 2000 to 2017. By comparing remote sensing images of all desert areas that had turned green with those that had not, we found that there are two principal human activities contrib- uting to the greening of deserts in China: oasis expansion and sand stabilization measures (Fig

7). These two anthropogenic impacted desert areas differ. Oasis expansion, especially in the form of cropland expansion, led to considerable greening areas that were mainly distributed in the southern part of the Gurbantunggut Desert, the northwestern edge of the Taklimakan Des- ert, patches at the edges of the Qaidam Basin Desert, the Badain Jaran Desert, and the Tengger Desert. The expansion of crops in the desert region can be connected to inadequate transfer of rural labor forces and grain subsidies, which can lead to excessive consumption of water and disrupt the natural balance between the oasis and the desert [80]. Policies to control the mod- erate expansion of oasis need to be strengthened like facilitating the transfer of agricultural labor to non-agricultural labor and reasonably controlling the grain subsidies in these areas to effectively control the expansion of oasis. Similarly, agricultural practices are the primary cause of greening trends in most regions of the world [22,36]. Sand stabilization measures such as using straw checkerboards and planting grass, shrubs, and trees contributed to quite a few greening areas in the Kubuqi Desert. This result provides evidence of the effectiveness of desertification control in China [81]. By employing sustainable grazing practices, sustainable agriculture practices, water use quotas, and planting wind-shelter forests, China has paid great attention to desertification management [82]. These efforts in China provide a valuable exam-

ple for sustainable desertified land management throughout the world [83]. Since 2000, desert- ification in China has been implemented through national projects (e.g., the Sandstorm Source Control project and the Three-North Shelterbelt System Construction) [82,84]. Despite great progress in the greening of desert areas in China, there is still a long way to go, indicating the need for increased efforts by the government and funding for effective policies and projects [85]. Conclusions This study analyzed greening and browning trends in China’s deserts from 2000 to 2017 shown in the monthly, seasonal, and annual NDVI both temporally and spatially and mea- sured the effects of climate variability and human activities by correlation analysis and residual analysis. The following conclusions were obtained. 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 14 / 20