Optimizing dryland ecological restoration: Unveiling the lagged and nonlinear threshold effects on carbon-water compound use efficiency

Ecological restoration regulates carbon water processes in arid and semi-arid ecosystems, yet its lagged and threshold effects remain unclear. Using multi-source remote sensing data from 2000 to 2022, we identified forest restoration regions, shrubland–grassland restoration regions, and gradual vegetation restoration regions ( VRRs ) in the Ningxia–Inner Mongolia Reach of the Yellow River and quantified their lagged effects on carbon use efficiency ( CUE ), water use efficiency ( WUE) , and carbon-water compound use efficiency ( COM) using distributed lag nonlinear models. VRRs dominated restoration, with restoration occurring earlier in the semi-arid area. CUE showed a dominant response at lag 5, whereas WUE and COM showed lagged responses at lag 3–5 in the arid area but immediate responses in the semi-arid area. Restoration thresholds varied by climate. In the arid area, SGRRs and VRRs had a safe restoration range of 23%–48% for annual newly added restoration proportion at the 1-km grid-cell scale, while negative WUE and COM responses emerged when restoration exceeded approximately 60%, associated with lag 3–5 effects. In the semi-arid area, restoration could exceed 60% without adverse effects. FRRs showed a low threshold of 10%–15%, SGRRs an optimal threshold of approximately 52%, and VRRs a higher threshold of 49%–61%. These findings provide a basis for climate-specific ecological restoration that maximizes long-term carbon–water synergies.