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, China

● Alpine grasslands leaf longevity (LL) extended due to delayed autumn senescence. ● LL extension enhanced NPP but followed a non-linear threshold effect. ● Temperature was the key LL driver, with solar radiation impact growing. ● LL will keep rising, but NPP growth will plateau by century’s end. ● Optimizing LL is more sustainable than pursuing maximum extension. Alpine grasslands are highly sensitive to environmental changes, with leaf longevity (LL) crucially modulating carbon cycling. Addressing uncertainties in long-term LL dynamics, driving mechanisms and its interplay with net primary productivity (NPP), we analyzed the spatiotemporal changes in LL and NPP of the Three-Rivers-Source Region (TRSR) from 2003 to 2022 using multi-source remote sensing data. Key drivers 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 LL 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 LL and NPP, confirming that longer LL enhanced carbon uptake by prolonging photosynthesis. However, this marginal gain declined once LL 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 LL variation, while enhanced solar radiation exerted increasing influence, highlighting the greater importance of photothermal resources for foliar phenology. Projections suggested continued LL 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 LL, especially under extreme climate stresses, offering key insights for carbon sequestration optimization and restoration strategies in global alpine ecosystems.