Increased soil moisture dominates the prolongation of leaf longevity for alpine grasslands in the Three-River-Source Region

Study regions The Three-River-Source Region Study focus This study integrated multi-source remote sensing and flux data from 2003 to 2022 to analyze the spatiotemporal dynamics of grassland leaf longevity (LL) and moisture conditions in the Three-River-Source Region. By applying a binning-based decoupling approach, we successfully isolated the independent impacts of soil moisture (SM) and vapor pressure deficit (VPD). New hydrological insights for the region Nearly half the region experienced intensified water stress, expanding warm-season compound droughts. Initially, LL correlated positively with SM and negatively with VPD in most regions, masking their distinct roles. Decoupling revealed SM as the dominant regulatory factor for LL, contributing approximately threefold more than VPD. The widely reported inhibitory effect of atmospheric dryness on grassland phenology was strongly amplified by its collinearity with soil aridity, with the role of VPD became more prominent when SM exceeded the non-restrictive threshold. Spatially, cold-region LL was more sensitive to soil-atmosphere moisture shifts, while in semi-humid regions, SM change still dominated, but VPD stress grew. These findings clarify the distinct mechanisms of soil-atmosphere moisture impacts, highlighting the importance of decoupling methods for ecological regulation analysis and guide adaptive ecological management.