Urban growth boundaries for ecologically fragile cities: A simulation-constraint approach for sustainable land use planning in mountainous regions

Rapid urban expansion in mountainous cities exacerbates ecological fragility and intensifies tensions between development and environmental sustainability. Achieving a dynamic balance between ecological protection and urban growth boundaries (UGBs) is therefore essential. However, existing UGB delineation methods rarely account for the unique ecological indicators and spatial complexities of mountain regions, limiting their effectiveness in guiding sustainable land use planning. To address this gap, we propose a framework that integrates ecological constraints (ECs) into the PLUS model for UGB delineation in mountainous cities. Key ecological indicators are systematically evaluated and embedded as model constraints, while mathematical morphology techniques are applied to optimize UGB boundaries. Using Chongqing, a representative mountainous city in China, as a case study, we identified 1, 641 km² of EC zones. Results indicate that soil and water conservation, water retention, and biodiversity maintenance are the primary ecosystem service functions in mountainous urban settings. Validation of the PLUS model demonstrated robust simulation performance (overall accuracy = 0.86, Kappa = 0.79, FoM = 0.34). Elevation, residential areas, and water bodies were identified as the main drivers of urban expansion. The EC-based approach yielded 1, 280 km² of UGBs—slightly less than the 1, 305 km² projected under the natural growth scenario—effectively curbing encroachment into ecologically sensitive areas while accommodating urban growth and reducing landscape fragmentation. The proposed framework provides practical guidance for sustainable land use planning and offers transferable insights for UGB delineation in other ecologically fragile mountainous cities.