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.