Forest restoration regulates the ecosystem service supply-demand relationship in a mountainous small watershed.

Clarifying the ecosystem service supply-demand relationship (ESSD) in response to environmental change forms the scientific foundation for ecological restoration strategies. Although ecological restoration effectively enhances ecosystem stability and promotes the sustainable provision, its impacts on ESSD remain unclear under coupled complex topography and diverse human activities. Focusing on the Jiuwanxi small watershed (JSW) in China's Three Gorges Reservoir Area, we quantified the spatiotemporal dynamics of five key ESSDs: grain production (GP), water yield (WY), soil retention (SR), water purification (WP), and carbon sequestration (CS), from 2001 to 2021. We integrated resident surveys with multi-source remote sensing data (Landsat 7 and Sentinel-2), and applied XGBoost-SHAP models to characterize the impacts of forest restoration and other variables on ESSD in a mountainous watershed. The results indicate that the forest area in the JSW increased by 24.82 % during 2001-2021. The supply of most ecosystem services improved across the watershed, with SR and WY exhibiting the most significant enhancement. In contrast, both GP supply and the demand for all ecosystem services declined substantially. The ESSD trends exhibited variations between watershed and village scales: at the watershed scale, all ecosystem service supply-demand ratios (ESDR) increased, with SR increasing by up to 591.69 %; whereas at the village scale, ESDR declines occurred for GP and CS in 28 % and 16 % of villages, respectively. Precipitation and forest area proportion were primary ESDR drivers, but cross-sectional analysis overestimated meteorological factors and ignored forest restoration's time-lag effects. Dynamic analysis showed forest area proportion changes most strongly correlated with ESDR variations, demonstrating restoration's efficacy in ESSD regulation. Consequently, the time-lag effects and spatial heterogeneity of forest restoration on ESSD in mountain watersheds must be comprehensively considered. Differentiated restoration strategies should be implemented based on topographic and socioeconomic factors to optimize the"high-altitude conservation, mid-altitude restoration, and low-altitude development" vertical spatial pattern. These findings provide a reference for sustainable ecological restoration in similar mountainous regions.