Nonlinear relationship and threshold-based zones between ecosystem service supply-demand ratio and land use intensity: A case study of the Beijing-Tianjin-Hebei region, China

Achieving the United Nations' Sustainable Development Goals requires a healthy ecological environment, which is fundamental to human well-being. Although ecosystem services (ESs) exhibit a close relationship with land use intensity (LUI), most studies examining this relationship focus on the aspect of ESs supply. There is still sparse research on understanding this relationship from the perspective of ecosystem service supply-demand ratio (ESSDR) that can reflect ESs supply-demand equilibrium state. Particularly, the nonlinear interaction and threshold between ESSDR and LUI remain underexplored. To address the research gap, this study quantified ESSDR and LUI in the Beijing-Tianjin-Hebei region (BTH) of China, and identified their nonlinear relationship and threshold using constraint line and the restricted cubic spline method. Results revealed the following findings: (1) During 2000-2020, the supply-demand ratios of grain production (GP-SDR), carbon sequestration (CS-SDR), and habitat quality (HQ-SDR) decreased, whereas supply-demand ratios of water yield (WY-SDR) and leisure recreation (LR-SDR) increased. (2) The average LUI increased slightly from 2.91 in 2000 to 3.02 in 2020, with areas of high LUI clustered in southeastern central urban areas, illustrating a core-edge distribution pattern. (3) ESSDR and LUI displayed complex nonlinear relationships, with the constraint lines showing inverted U-shaped between WY-SDR, GP-SDR, and LUI, steep descent curves between CS-SDR, HQ-SDR, and LUI, and U-shaped between LR-SDR and LUI, respectively. Certain thresholds were observed, such as GP-SDR dropping notably as LUI exceeded 3.5, while LR-SDR increased around LUI 3.7. Finally, the BTH region was divided into core control areas, general control areas, and maintenance areas based on identified thresholds, with tailored suggestions being provided to facilitate human-environment coordination within different threshold areas. This study can deepen the comprehension of the intricate interplay between ESSDR and LUI, and offer scientific reference for sustainable land use management and eco-environment conservation.