Environmental Dependence of Nonlinear Relationships between Fractional Vegetation Cover and Ecosystem Services Based on the Constraint Line Approach

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Abstract Afforestation is often portrayed as a panacea for ecosystem service (ES) degradation and climate change, yet it is criticized for overlooking potential risks, highlighting the limited understanding of its effects across contexts. This study aimed to reveal the environmental dependence of nonlinear ES responses to fractional vegetation cover, in order to provide guidance for landscape management and decision-making. We quantified the spatiotemporal changes in FVC and three ESs, as well as the correlations between them, on the Loess Plateau from 2000 to 2022, using various quantitative assessment models, the Mann–Kendall test, and Pearson correlation analysis. We further used constraint-line methods to identify nonlinear FVC–ES relationships and threshold effects across precipitation gradients and geomorphological types. FVC is strongly positively associated with carbon sequestration and soil retention (in > 80% of the region), but shows a positive association with water yield in only 36.5% of the region. Plateau-wide FVC thresholds are substantially higher than those in arid and semi-arid zones and in plains. Ignoring this discrepancy risks setting unrealistically high restoration targets in drylands. Precipitation exerts a much stronger control on FVC thresholds than geomorphology, and its marginal effect declines with increasing precipitation. These findings clarify how vegetation restoration affects ecosystem services and provide a basis for ecological restoration strategies on the Loess Plateau and in arid and semi-arid regions worldwide. Graphical abstract Under the backdrop of global greening, how differing climatic and geographic conditions modulate the nonlinear responses between afforestation and ecosystem services (ESs) remains poorly understood. Using the Loess Plateau—an emblematic region for global afforestation—as the study area, this study employed various quantitative assessment models, the Mann–Kendall test, and Pearson correlation analyses to characterize the spatiotemporal dynamics and spatial correlations between fractional vegetation cover (FVC) and ESs over 2000–2022. We further applied constraint-line methods to identify nonlinear FVC–ES relationships and threshold effects along precipitation gradients and across geomorphological types. The results demonstrate that the nonlinear ES responses to FVC are strongly environment-dependent, and that precipitation exerts a substantially greater control over FVC threshold behavior than geomorphology. Moreover, we elucidate the potential risks associated with neglecting natural environmental conditions. Collectively, these findings provide critical scientific evidence for ecosystem management and ecological restoration policies on the Loess Plateau and in arid and semi-arid regions worldwide.

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