Response of Vegetation Cover Change to Drought at Different Time-scales in the Beijing-Tianjin Sandstorm Source Region, China

Dominated by an arid and semiarid continental climate, the Beijing-Tianjin Sandstorm Source Region (BTSSR) is a typical ecologically fragile region with frequently occurring droughts. To provide information for regional vegetation protection and drought prevention, we assessed the relations between vegetation cover change (measured by the Normalized Difference Vegetation Index, NDVI) and the Standardized Precipitation Evapotranspiration Index (SPEI) at different time-scales, in different growth stages, in different subregions and for different vegetation types based on the Pearson's correlation coefficient in the BTSSR from 2000 to 2017. Results showed that 88.19% of the vegetated areas experienced increased NDVI in the growing season; 48.3% of the vegetated areas experienced significantly increased NDVI (P < 0.05) and were mainly in the south of the BTSSR. During the growing season, a wetter climate contributed to the increased vegetation cover from 2000 to 2017, and NDVI anomalies were closely related to SPEI. The maximum correlation coefficient in the growing season (Rmax) was significantly positive (P < 0.05) in 97.84% of the total vegetated areas. In the vegetated areas with significantly positive Rmax, pixels with short time-scales (1-3 mon) accounted for the largest proportion (33.9%). The sensitivity of vegetation to the impact of drought rose first and then decreased in the growing season, with a peak in July. Compared with two subregions in the south, subregions in the north of the BTSSR were more sensitive to the impacts of drought variations, especially in the Xilingol Plateau and Wuzhumuqin Basin. All four major vegetation types were sensitive to the effects of drought variations, especially grasslands. The time-scales of the most impacting droughts varied with growth stages, regions, and vegetation types. These results can help us understand the relations between vegetation and droughts, which are important for ecological restoration and drought prevention.