Nonlinear Grassland Degradation and Recovery Benchmarking Potential Productivity: Evidence from the Agro–Pastoral Ecotone of Northern China

Grassland ecosystems form a cornerstone of terrestrial ecological security and support the livelihoods of millions of people worldwide. Under the combined influences of climate change and human activities, grassland degradation unfolds with pronounced spatiotemporal heterogeneity and marked nonlinearity, features that are particularly evident in ecological transition zones. Here, we focus on the agro–pastoral ecotone of northern China and integrate multi-source remote sensing and geospatial datasets to develop a grassland degradation assessment framework benchmarked against potential maximum net primary productivity(NPPmax). Adopting a change-pathway perspective, we identify long-term trajectory types of grassland degradation and recovery and quantitatively examine their underlying drivers. Our analyses reveal that degradation and recovery processes across the region are largely nonlinear, with abrupt, threshold-like shifts being spatially widespread. Although recovery trajectories dominate at the regional scale, a considerable fraction of grasslands remains locked in persistent moderate to severe degradation, and clear spatial differentiation emerges among trajectory types. Climatic factors primarily shape long-term trends in grassland productivity, while human activities play a pronounced amplifying role: they can accelerate rapid recovery under favorable climatic conditions, yet also precipitate sudden, localized degradation. By moving beyond single rates of change to emphasize dynamic pathways, this study deepens understanding of grassland degradation processes in agro–pastoral ecotones. Our findings underscore the importance of simultaneously accounting for climatic context and human regulation in grassland management and ecological restoration. The proposed framework and insights provide a strong scientific basis for zoned management, risk early warning, and adaptive strategies in ecologically vulnerable regions, and hold broad relevance for ecological transition zones worldwide.