Aridity Thresholds and Vegetation Sensitivity Influence Carbon–Water Trade‐Offs in the Qilian Mountains

ABSTRACT Dryland ecosystems face increasing pressure from climate change, making understanding the carbon–water relationship crucial for sustainable development. This study investigates how this relationship, represented by the trade‐off between water conservation (WC) and net primary productivity (NPP) (TWC‐NPP), responds to aridity gradients in the Qilian Mountain Area (QLMA). Using data from 2000 to 2018, we employed a moving window method, generalized additive models, and XGBoost–SHAP to quantify the contributions of climate and vegetation cover to TWC‐NPP variability. Our results showed that although both WC and NPP improved across the QLMA, TWC‐NPP also increased, indicating a growing trade‐off between these ecosystem services, especially in the eastern region. TWC‐NPP responded nonlinearly to aridity and peaked at an aridity index (AI) of 0.39, located in the dry sub‐humid zone near the humid boundary, indicating a dry–wet transition belt where carbon–water coupling shifts most strongly. Fractional vegetation cover (FVC) was identified as the dominant factor influencing TWC‐NPP, surpassing the role of snowmelt under higher aridity conditions. Furthermore, the FVC threshold for influencing TWC‐NPP decreased from 0.55 to 0.38 with increasing aridity, signifying heightened vegetation sensitivity under drier conditions. These findings highlight the crucial role of vegetation in mediating carbon–water trade‐offs along aridity gradients and underscore the importance of considering these complex interactions when developing ecosystem management strategies in the QLMA under a changing climate.