Thresholds of ecosystem service trade-offs and synergies at different spatio-temporal scales in hilly region of Southern China

Global climate change and intensified human activities drive rapid land use and land cover (LULC) changes, particularly in ecologically fragile regions like China's Southern Hilly Region (SHR), affecting ecosystem services (ESs) trade-offs/synergies. However, scale-dependent thresholds governing these relationships remain poorly quantified. We analyzed ESs dynamics (water yield, soil conservation, carbon storage, nutrient retention, habitat quality) across regional, watershed, and sub-watershed scales (1990–2020) using the InVEST model, sensitivity indices, and piecewise linear regression. ESs responses exhibited significant scale effects, with sub-watersheds showing the highest sensitivity to LULC changes and representing the most stable management unit. Particularly, we found that critical LULC thresholds regulate trade-offs/synergies: forest cover exceeding ∼70 % strongly enhanced synergies among multiple ESs, while cropland proportions between 30 and 65 % intensified trade-offs (e.g., between soil conservation and water yield). Impervious expansion consistently degraded ESs. Our results demonstrate that optimizing LULC patterns-prioritizing forest conservation (>70 % cover), limiting cropland (<65 %), and controlling urban sprawl-at the sub-watershed scale minimizes ESs trade-offs. This study establishes quantitative thresholds to guide targeted land-use planning and ecological restoration policies in hilly regions globally, supporting sustainable landscape governance.