Threshold effects of vegetation increase on ecosystem services based on the constraint line method in the Loess Plateau gully Zone

• Quantifying the spatiotemporal changes in FVC and three ESs from 2000 to 2020; • Identifying the nonlinear impacts and threshold of vegetation increase on ESs. • Revealing intrinsic mechanisms underlying the constraint effects of FVC on ESs; Numerous ecological restoration projects (ERPs) in China aim to reverse ecosystem degradation and enhance human well-being through vegetation increase. However, the nonlinear impacts (e.g., threshold effects) of vegetation recovery on ecosystem services (ESs) remain inadequately explored. This study quantitatively assessed the spatiotemporal evolution of fractional vegetation cover (FVC) and three ESs in the Loess Plateau Gully Zone from 2000 to 2020, examining their spatial correlations across multiple scales. Using the constraint line method, we simulated ES response curves to FVC changes and identified the threshold of FVC influence. The results showed that: (1) from 2000 to 2020, FVC increased significantly, with noticeable variations in ESs changes: carbon sequestration rose markedly, soil conservation showed moderate improvement, and water yield declined. (2) over 95% the area exhibited a significant positive correlation between FVC and both carbon sequestration and soil conservation, whereas 43.95% of the area displayed a strong negative correlation between FVC and water yield; (3) FVC demonstrated a threshold effect on ESs, with vegetation coverage thresholds for carbon sequestration, water yield, and soil conservation identified at 77.64%, 66.67%, and 71.45%, respectively. Additionally, we reveal the intrinsic mechanisms underlying FVC’s constraint effects on ESs and its practical applications. These findings offer novel insights for policy formulation on regional ecological restoration.