Ecosystem functions and network complexity do not increase linearly with restoration levels on China's loess plateau

Ecological restoration efforts on China's Loess Plateau have significantly enhanced ecosystem functions (EFs) over recent decades. Nevertheless, whether the indiscriminate pursuit of higher restoration levels necessarily enhances EFs and network complexity remains empirically unresolved. Here, we develop a restoration level-function response framework that integrates ecological network construction with threshold modeling. This study quantifies how the enhancement of EFs responds to restoration levels and identifies key nodes and optimal threshold ranges for observed variables across restoration levels. We find that EFs increased by 80.27% across restoration levels; however, the rate of enhancement diminished. Compared with natural ecosystems, farmland ecosystems exhibited relatively limited potential for functional enhancement. Network cohesion and stability exhibited nonlinear relationships with restoration levels across the region and ecosystem types, peaking at moderate restoration levels. Climate and soil factors dominated at a slight restoration level, whereas intensive land use increasingly impaired water conservation and soil retention as restoration levels increased. Critically, different restoration levels required specific threshold ranges. Exceeding optimal values for certain variables paradoxically weakened functional enhancement, highlighting the need to shift from extensive intensity control to precision threshold management for restoration strategies. These findings elucidate the nonlinear mechanisms underlying the over-restoration paradox and offer a transferable methodological framework for regions facing similar restoration challenges.