Reduction in soil moisture dominates progressive tree mortality in semiarid areas

Increasing drought stress has triggered various negative impacts on forests worldwide, including growth reduction, defoliation, crown dieback, and even tree mortality, with unavoidable consequences for forest ecosystems. However, how reductions in both precipitation and soil moisture progressively lead to tree mortality remains largely unknown. Here, we define relative soil water (RSW) as the ratio of the actual soil moisture to the field capacity, which can reflect the fraction of water in the root zone, to reveal how soil moisture reduction leads to progressive tree mortality. Based on field measurements of tree behaviors, including transpiration, tree growth, defoliation, crown dieback and other behaviors, before, during and after an extreme drought in the Larix principis-rupprechtti plantations in 2021, we found that the variability in precipitation and soil moisture affect tree behaviors, but soil moisture is the dominant driver of drought stress on progressive tree mortality, with prolonged and severe soil moisture reduction leading to widespread tree mortality. RSW thresholds for different stages of progressive tree mortality and drought stress levels are identified as follows: Level I (RSW > 0.7), no detectable hydraulic limitations; level II (0.7 to 0.45), persistent stem shrinkage and onset of transpiration reduction; level III (0.45 to 0.35), onset of slight discoloration and defoliation; level IV (0.35 to 0.25), onset of crown dieback and tree mortality; and level V (< 0.25), severe defoliation, 20% crown dieback and tree mortality. Our results shed light on predicting tree mortality and distribution in forests under increasing climate warming, particularly in semiarid areas with warming-induced tree mortality.