Identifying the thresholds of site condition and stand structure for optimizing multiple ecosystem services in the Liupan Mountains, Northwest China

Sustainable forest management is critical for promoting multiple ecosystem services (ESs), yet the thresholds of site condition and stand structure on the comprehensive benefit of multiple ESs remain unclear across forest types at regional scales. We took the Liupan Mountains as a case study and evaluated four key ESs (carbon storage, timber production, water yield, and soil conservation) using biomass models, timber volume equations, and the InVEST model at pixel scale. We then developed a comprehensive index (CI) integrating supply and trade-off indices to assess multiple ES benefits, and quantified the constraining effects and thresholds of site condition and stand structure on CI. The results showed that natural forests provided higher carbon storage, water yield, and soil conservation than planted forests, except for timber production. The supply index (SI), trade-off index (TI), and CI exhibited clear spatial patterns, with higher values in the south and lower values in the north. The SI (0.46 vs. 0.28), TI (0.24 vs. 0.23), and CI (0.89 vs. 0.83) showed differences between natural and planted forests. Mean annual precipitation, stand age, tree height, and DBH showed significant positive correlations with CI, while stand density had a strong negative effect—all with nonlinear relationships and critical thresholds. Thresholds for planted forests (mean annual precipitation = 590 mm, stand age = 28 years, stand density = 1,524 trees/ha, tree height = 12 m, DBH = 14 cm) differed from those of natural forests (651 mm, 54 years, 405 trees/ha, 21 m, 36 cm). Under similar precipitation and stand age, planted forests could be thinned toward natural forest density benchmarks to enhance their comprehensive benefits. Our findings provide regional-scale evidence-based forest management practices for achieving high comprehensive benefits of multiple ESs.