Enhanced Variability and Declining Trend of Soil Moisture Since the 1880s on the Southeastern Tibetan Plateau

Historical soil moisture (SM) variations, trends, and their driving factors remain scarce for the area of Tibetan Plateau (TP), which hinders putting a sensible assessment of the current and future ecological drought risk into perspective. Here, we report the first three century-long regional summer (July-August) SM reconstruction for the southeastern TP during 1691-2007 CE using a paleoclimate proxy, that is, tree-ring delta O-18. The SM reconstruction, which explained 60.9% of the actual variance, revealed that an abrupt wet-to-dry change occurred in 1884. After 1884, SM exhibited a decreasing trend and enhanced variability, and dry summers occurred more frequently. In particular, the variability in SM reached an unprecedented level after the 1950s relative to that during the past three centuries. A structural equation model and running correlation analysis revealed that SM variation was mainly controlled by precipitation rather than temperature. This indicates that the anthropogenic-related weakening of the Indian summer monsoon played a more dominant role in SM changes after 1884 than the increase in temperature. If the SM variability is to be further increased in the future, it may undermine ecosystem stability and forest health. The results of this study are significant for predicting ecological drought in ecologically vulnerable regions such as the High Asia.