Nonlinear carbon–water coupling in terrestrial ecosystems: Insights from China’s Three-North Shelterbelt Forest region

Understanding the coupling between carbon and water in terrestrial ecosystems is essential for achieving sustainable development. Net primary productivity (NPP), carbon use efficiency (CUE), and water use efficiency (WUE) are key indicators for assessing carbon balance and carbon–water interactions. However, knowledge gaps remain regarding how these indicators respond to climate change and interact with one another. This study examined the spatial and temporal dynamics of NPP, CUE, and WUE, as well as their interrelationships, within the Three-North Shelterbelt Forest region of China. Furthermore, the study investigated the driving mechanisms of these indicators using Extreme Gradient Boosting (XGBoost), SHapley Additive exPlanations (SHAP), and Partial Least Squares Structural Equation Modeling (PLS-SEM). The results revealed that from 2000 to 2020, both NPP (2.69 g C/(m 2 •a); P