An XGBoost–SHAP-Based Interpretable Analysis of the Driving Factors of Carbon Storage in the Tumen River Basin

Balancing terrestrial carbon-storage conservation with land development is a central sustainability challenge in transboundary river basins, yet the consequences of alternative land-use pathways in the Chinese portion of the Tumen River Basin remain insufficiently understood. Using land-use datasets from 1990, 2000, 2010, and 2020, the InVEST and PLUS models were employed to quantify historical carbon-storage change and projected land-use and carbon-storage outcomes under three scenarios for 2050. Separately, an XGBoost–SHAP framework was applied to the 2020 spatial data to interpret the spatial heterogeneity of carbon storage and identify nonlinear thresholds. Forest remained the dominant land-use type between 1990 and 2020, whereas the area of Built-up expanded by 78.7%. Over the same period, total carbon storage decreased from 153.07 × 108 t to 151.03 × 108 t, following a decline–recovery–decline trajectory. Among the three 2050 pathways, only the Ecological protection scenario produced a net increase in carbon storage relative to 2020 (+3.82 × 106 t), whereas the Urban development scenario resulted in the largest loss (−1.65 × 108 t). For the 2020 spatial pattern, the XGBoost–SHAP analysis identified NDVI and slope as the leading explanatory variables, with model-derived thresholds for slope (6.18°), precipitation (666.98 mm), NDVI (0.92), elevation (442.65 m), GDP (CNY 8725), and distance to railways (10,379.04 m). These results therefore support a spatially differentiated land-use strategy that prioritizes forest-patch continuity and restricts the replacement of carbon-dense land by Built-up within the basin.