Nature-based solutions for urban expansion: Integrating ecosystem services into the delineation of growth boundaries

Unplanned urban expansion poses grand social and environmental challenges worldwide. The concept of Natural-based solutions (NbS) has been recently proposed as new guidance to address challenges brought by urban expansion. However, how to effectively embody the role of nature in urban planning implementation remains largely unexplored. This paper aims to develop an NbS-based framework for addressing urban expansion challenges by integrating ecosystem services (ESs) into the urban growth boundaries (UGB) delineation. We tested this framework in the city of Hohhot, Inner Mongolia, a specific ecologically fragile region in Northwest China. We found: (1) The ESs-NbS framework for UGB delineation can reflect the interactive process of ecological security and social/economic development and thereby provide decision-makers a tool for adaptive management. (2) By considering ESs in the procedure of UGB delineation, the eight principles of NbS can be integrated into the framework to coordinate urban expansion and ecological protection. (3) Applying this framework, the predicted urban area of Hohhot under different ecological security scenarios would increase to 550.52 km2 in 2035, which would not exceed the proposed "safe" size of 607.18 km2 of the UGB that accounts for 3.54% of the city's total area. Our results demonstrated that such a framework has the potential as a planning tool to facilitate spatial planning in Chinese cities, as well others worldwide, and can promote the practice of NbS for more effective urban growth management.