Estimation model and future projections for leaf longevity in alpine grasslands based on XGBoost-SHAP algorithm:A case study of the Three-Rivers-Source Region,ChinaOA

Alpine grasslands are highly sensitive to environmental changes,with leaf longevity crucially modulating carbon cycling.Addressing uncertainties in long-term leaf longevity dynamics,driving mechanisms and its interplay with net primary productivity(NPP),we analyzed the spatiotemporal changes in leaf longevity and NPP of the Three-Rivers-Source Region(TRSR)from 2003 to 2022 using multi-source remote sensing data.Key drivers of leaf longevity were identified using XGBoost-SHAP algorithm and lasso regression,while a causality-based model projected future trajectories.Results showed that over 81%of the study area exhibited a significant leaf longevity extension(9.32 days decade-1),mainly due to delayed leaf senescence date.Concurrently,regional NPP increases were dominated by summer gains.There was a non-linear positive correlation between leaf longevity and NPP,confirming that longer leaf longevity enhanced carbon uptake by prolonging photosynthesis.However,this marginal gain declined once leaf longevity surpassed the ecological threshold(about 150 days),indicating that after summer vegetation activity peaks,relying solely on extending the growing season does not lead to substantial net carbon gains,and the carbon sink becomes saturated.Temperature consistently drove leaf longevity variation,while enhanced solar radiation exerted increasing influence,highlighting the greater importance of photothermal resources for foliar phenology.Projections suggested continued leaf longevity extension under SSP245 and SSP585 climate scenarios,with short-term NPP increasing but long-term stagnating or declining.These findings emphasize that alpine grassland management should prioritize ecosystem sustainability and adaptive resilience over maximizing leaf longevity,especially under extreme climate stresses,offering key insights for carbon sequestration optimization and restoration strategies in global alpine ecosystems.