Has urban form become homogenizing? Evidence from cities in China

Although the evolution characteristics and drivers of urban form have received significant attention in the past, the homogenization trend of urban form has been neglected. This study examined the changing urban form of a large cohort of 1525 Chinese cities over an extended, 25-year period of rapid growth from 1990 to 2015. Landscape indices and principal component analysis were used to quantify urban form in terms of morphological and spatial complexity, as well as spatial agglomeration. Then the cosine similarity index was employed to measure the homogenization degree of urban form (HDUF) of the entire cohort and of four tiers of these cities that had been previously defined by the city tier system of China. The K-means algorithm and trajectory analysis were subsequently used to describe the homogenizing orientation of urban form. Our results indicate that the HDUF within tiers is increasing along similar trends that are offset over time. The highest tier cities began to homogenize after 1995, followed by intermediate and lower tiered cities, which began to homogenize from 2000 to 2005. The lowest tier cities began to homogenize after 2010. The homogenization trends were also apparent when analysis was conducted between different tiers. As of 1990, the orientation of urban form of more than 99% of the cities was characterized by regular form with low patch density, but by 2005, almost half of the cities were undergoing transition toward either a compact from, which was characterized by low patch number and shorter distance patch distribution, or a fragmented form, which was characterized by elevated patch number and longer distance patch distribution. The temporal pattern of homogenization and the observed bifurcated orientation of form indicate (i) that challenges currently faced by higher tier cities will confront lower tier cities in the future and (ii) that it is important to understand the mechanisms that govern the alternative trajectories toward compactness and fragmentation. Our study deepens the understanding of the urban form evolution and its driving mechanisms and provide a guiding framework to predict future urban development.