Trade-Offs and Synergies of Ecosystem Services and Spatial Zoning Optimization in Shandong Province from a Linear–Nonlinear Coupling Perspective

Rapid urbanization has profoundly reshaped land use patterns and intensified pressures on ecosystem structures, thereby exacerbating trade-offs and synergies among ecosystem services (ESs). Understanding ecosystem service trade-offs, synergies, and their attribution mechanisms is critical for balancing ecological conservation and regional sustainable development in rapidly developing regions. This study quantified provisioning, regulating, supporting, and cultural ecosystem services in Shandong Province from 2000 to 2020 using the InVEST model and spatial analysis. An integrated framework combining Pearson correlation and bagplot analysis was developed to identify linear and nonlinear ES trade-offs and synergies, while the XGBoost–SHAP model was applied to quantify the relative contributions of natural and socioeconomic drivers. Ecosystem service bundles were further identified using a self-organizing map to delineate spatially functional zones. The results showed that: (1) Provisioning and cultural services increased markedly, whereas regulating and supporting services generally declined. Spatially, provisioning services were concentrated in the western plains, regulating and supporting services in the central mountains and eastern hills, and cultural services in urban areas. (2) Strong trade-offs emerged between provisioning services and most regulating/supporting services, while regulating and supporting services exhibited pronounced synergies. Cultural services reflected a generally compatible relationship with other ESs. (3) Regulating and supporting services were primarily shaped by natural conditions and land use patterns, whereas provisioning and cultural services were more strongly driven by socioeconomic factors. (4) SOM clustering identified four major functional zones, the ecological core zone, the ecological degraded zone, the food production zone, and the urban composite zone, each corresponding to differentiated ecosystem functions and development trajectories. The integrated framework provides a scientific basis for ecosystem-service-oriented spatial zoning and targeted management strategies to reconcile ecological protection and urbanization in rapidly developing regions.