Soil drought thresholds of alpine grassland deceased rapidly under the influence of extreme low temperature in northeastern Qinghai-Tibet Plateau

Abstract Background Droughts likely lead to the decrease of vegetation coverage and plant productivity. Due to climate change, more extreme climatic events, including soil droughts and extreme temperatures, may occur both independently and simultaneously. Therefore, it is important to understand the thresholds of soil drought in order to avoid various undesired transitions of alpine grassland. Methods Soil droughts were identified based on the change of soil moisture, and vegetation coverage decline events were identified by the abnormal decrease of normalized difference vegetation index (NDVI). Three thresholds based on two curves of vegetation responses to soil drought illustrated the tipping points that reflected the rapid loss of ecosystem resistance (T p1 ), the complete loss of ecosystem resistance (T p2 ) and the amplified magnitude of NDVI decrease (T m ). The influences of legacy effects and extreme temperatures were also considered. Results Alpine grasslands in northeastern Qinghai-Tibet Plateau had mean T p1 , T p2 and T m of 1.25, 1.98 and 1.93, respectively, indicating low resistance and high vulnerability. T p1 was low for the most study area, and T p2 and T m varied with elevation and vegetation types. Besides the low occurrence of soil droughts coupled with high temperatures, the extremely low temperatures decreased 36% T p1 , nearly all of T p2 and 30% T m . Different grassland types showed varying responses to soil droughts, with alpine sparse vegetation having the lowest resistance and alpine wetland having the highest. However, the responses according to soil types did not show obvious differences. Conclusions Alpine grasslands in northeastern Qinghai-Tibet Plateau were sensitive to soil droughts, and extremely low temperatures intensified the decrease of ecosystem resistance to soil drought. The identification of soil drought thresholds advances the understanding of how alpine grassland responds to soil droughts, and helps the restoration of alpine grasslands when faced with soil droughts and benefits climate adaptation.