## 1 Introduction  
Globally, urban expansion has progressed rapidly over recent decades, and with population growth and the steady transition of human residence from rural to urban areas, urban areas are expected to absorb approximately 68% of the world's population by 2050 [1], most of them in developing regions. China, one of the biggest developing nations, has experienced urban expansion at an unparalleled rate and scale against the background of social transformation during the past 60 years [2]. This trend will continue, and China's urbanization level is estimated to reach nearly 75%, with an urban population of 2.55 billion by 2050 [1, 3]. Urbanization has triggered the soaring national economy, and plays an important role in poverty reduction, safety and health, and employment, which is an inevitable requirement for the development of China. However, urbanization is often accompanied by the expansion of built-up land and the appropriation of physical landscapes (e.g., forest, grassland, cropland), which leads to land use/cover change and landscape fragmentation [4]. Additionally, the continuing population growth of cities has accelerated the consumption and exploitation of natural resources [5] and induced many serious environmental issues, such as increased carbon emissions, air and water quality pollution [6], total climate change [7], and thus, ecosystem service degradation and loss threatening urban sustainable development [8; 9]. These issues have had complicated impacts on the modernization development of China, have attracted the attention of experts and scholars, and generated demands for urban greenery to abate environmental problems and improve the quality of urban life. Extensive studies have shown that vegetation in cities is critically important because it provides a wide diversity of ecosystem services that benefit people, for example, mitigating the impact of urban heat islands [10], reducing noise [11], increasing terrestrial carbon sequestration and storage [12], reducing air pollution [13] and stormwater runoff-induced surface erosion [14], and strengthening resistance to natural disasters (e.g., flood control) [15]. Moreover, urban green spaces provide significant well-being benefits to local urban residents, including reducing stress, improving cognitive ability and physical and mental health, and increasing property value [16]. The enhancement of city green space to improve environmental quality has received increased attention from urban management and planning departments, and a series of policies have been implemented, which resulted in highly dynamic and patchy urban green spaces [17]. In the face of increasing and rapid environmental change in urban areas, land managers and policymakers urgently need a comprehensive understanding of urban green space change and urban expansion processes in order to make informed decisions. Therefore, quantifying the process and extent of urbanization and greening has been a vital task for urban management, which is critical for China to implement the 2030 Agenda for Sustainable Development [18], and has been elevated to the forefront of research and practical applications to support an effective urbanization process in China. Remote sensing typically serves as a key source in determining the status and dynamics of urban information attributes due to its extensive spatiotemporal coverage and data availability. Currently, remote sensing monitoring of urban expansion and greening mainly focuses on area changes, land cover type conversions, and range changes at different periods. [PERSON] et al. [19] used the principal component analysis and a hybrid classifier based on SPOT and Sentinel-2A images to monitor the land use change in Hangzhou City, China from 1996 to 2016. [PERSON] et al. [20] extracted information on urban expansion in China using the Defense Meteorological Satellite Program's Operational Line-scan System (DMSP-OLS) nighttime light data from 1993 to 2008.