Greening but enhanced vegetation water stress in the Yellow River Basin: A holistic perspective

The Yellow River Basin (YRB) has emerged as a focal point of global vegetation greening due to climate change and human activities. Given its ecological vulnerability and intense human activities, environmental sustainability has become an urgent concern for scholars. Current research on the hydrological effects of vegetation greening, from a reductionist perspective, still struggle to answer the crucial question that whether vegetation water stress is increasing or decreasing. Towards that, we adopt a holistic perspective to explore the relationships between monthly vegetation dynamics and multiple water stress indicators in the YRB from 1982 to 2018. Using statistical methods and the random forest model, we revealed that both gross primary productivity and water use efficiency showed an increasing trend, with rates of 5.83 g Cm^-2 and 0.01 g Cmm^-1m^-2 per year, respectively. We identified that with increasing climatic aridity, the water stress factors for vegetation transition from monthly scale water conditions (vapor pressure deficit, VPD) to 1-2 months scale (soil water content, SWC) and seasonal scale (standardized precipitation evapotranspiration index-3, SPEI-3) water balance status. And with an aridity index of 0.35 as the threshold, the response of vegetation to water stress factors exhibits marked spatial differentiation. Furthermore, since 2000, despite a persistent greening trend in the YRB, there has been a noticeable expansion in the spatial range of intermediate and long-term water stress factors (SWC, SPEI-3), indicating an enhancing vegetation water stress. This suggests that a serious attention should be paid to the future ecological security of the YRB under the intensified climate change.