2. Materials and methods

2.1 Study area

China’s territory is vast, with a large north-south span, and is about 9.6\(\times 10^{6}\) km2. China has the complex and diverse climate types, among which the continental monsoon climate is prominent. Its annual precipitation decreases from southeast coast regions to northwest inland regions. The elevation is high in the west and low in the east, showing three ladder-like distributions in space. Moreover, China boasts a diverse topography, with mountains, plateaus and hills accounting for about 67%, and basins and plains accounting for about 33%. In addition, China is the world’s most populous country, the total population of which was 1,411.78 million by the end of 2020 according to the seventh national census. Among them, 63.89% of people lived in urban areas, and the urban population proportion increased by 14.21% compared with 2010. The Hu Line, first proposed by Hu Huanyong, also known as the “Heihe-Tengchong” Line. It was initially used to demonstrate the population density differences of the areas either side of the line, and later also proved to be the natural and ecological boundary (Chen, 2016; Kong, 2022). In this paper, 368 cites in China were chosen, and Taiwan Province, Hong Kong and Macau were excluded due to the inaccessibility of data. To highlight regional differences in urbanization and ecosystem health, the entire country was divided into seven sub-regions: North China (NC), Northeast China (NE), East China (EC), Central China (CC), South China (SC), Northwest China (NW), and Southwest China (SW) (Fig 1).