A critical role of the capital green belt in constraining urban sprawl and its fragmentation measurement

Urban green belts (UGBs), an important green infrastructure of cities, have the functions of constraining urban sprawl, which are of great significance to urban green, resilient and sustainable development. However, it remains unclear how to quantify the constraining effect of UGBs on urban sprawl and the degree of fragmentation after UGBs constrain urban sprawl. Here, we proposed a UGBs extraction method to explore the spatial evolution of UGBs in the rapidly expanding megacity Beijing from 2000 to 2020. We also quantitatively investigated the role of UGBs in constraining urban sprawl and measured the degree of fragmentation of the UGBs after constraining urban sprawl with the help of the constraint assessment index, the Urban Annual Ring model, and the landscape pattern index. Results showed that: (1) Beijing’s UGBs area showed a “V”-shaped trend of first decreasing and then increasing from 2000 to 2020, decreasing from 332.61 km2 in 2000 to 303.41 km2 in 2015, and then increasing to 369.42 km2 in 2020, which is equivalent to about 54 Beijing Olympic Forest Parks. (2) The UGBs’ role in constraining urban sprawl was characterized by gradual enhancement in three periods from 2005 to 2020. The boundary containment rate (BCR) of urban sprawl was 0.48, 0.22, and 0.14 respectively. After the construction of the newly planned UGBs in 2005, the speed of urban sprawl from 2010 to 2015 decreased by 10.90%. (3) The fragmentation of UGBs showed an upward trend and connectivity showed a downward trend. The patch density (PD) characterizing fragmentation increased from 17.22 in 2000 to 31.60 in 2020, and the patch aggregation index (AI) characterizing connectivity decreased from 86.92 to 79.73. The research can provide a reference for other similar sized cities in UGB planning and government decision-making.