The direct and lag effects of administrative division adjustment on urban expansion patterns in Chinese mega-urban agglomerations

Administrative division adjustment (ADA) is the highest-level design policy for China's spatial management. Nevertheless, its impacts on urban land use change are still ambiguous, especially in mega-urban agglomerations. This study aims to quantitatively evaluate the mechanism and effects of ADA on the urban expansion patterns of three mega-urban agglomerations in China, i.e., the Pearl River Delta, the Yangtze River Delta, and Beijing-Tianjin-Hebei region, by using geographic detector in conjunction with landscape pattern index. The results showed that ADA has a 10-year "significant impact" for urban expansion: a 0-5 year lag promoted outlying and edge-expansion and a 6-10 year lag accelerated infilling expansion. The ADA determined an average of 12.79%, 7.76%, and 5.04% of outlying, edge-, and infilling expansion during 2000-2018 by stimulating industrial and infrastructure development, guiding population movement and government investment, and accelerating urban renewal and public services equalization, respectively. Moreover, the promotional impacts of ADA gradually shifted from peripheral sprawl during 2000-2012 to infilling expansion during 2012-2018. Due to differences in industrial structure, urbanization, and development degree, ADA's urbanization effects were heterogeneous in three mega-urban agglomerations. This study helps decision-makers evaluate and formulate differentiated urban planning and land use policy to improve regional urban-rural integration and achieve sustainable development.