## 1 Introduction  
Land use change induced by urbanization has greatly changed the physical conditions of urban ecosystems. Meanwhile, external risks faced by urban systems, such as climate change, require resilient urban ecosystems to provide a solid urban foundation to support urban production and livelihood [1]. Measures such as emission reduction are taken in urban systems to adapt to environmental change along with urbanization, for example, China is taking the lead in committing to achieving carbon neutrality in 2060 [2]. These measures would further change urban ecosystems with the goal of enhancing resilience. Unlike the sustainability concept, which is too comprehensive and hard to quantify in social-ecological systems, the resilience concept is derived from ecosystems and proves to be a good analytical tool for urban systems [3]. To quantitatively assess urban ecological resilience and elucidate its driving mechanisms is of great importance to guide sustainable urbanization, mitigate climate change impact and achieve high-quality development. The concept of ecological resilience is a measure of the persistence of an ecosystem and refers to the ability of a system to absorb natural and anthropogenic disturbances while still maintaining the interplay between population or ecosystem state variables and maintaining system function [4]. Ecological resilience in urban systems is a complex of socio-economic human activities and biophysical habitats [5]. An urban ecosystem is a specific type of ecosystem that integrates human activities into the biophysical sphere. In recent years, there has been much research on urban resilience that originates from social-ecological resilience. The urban system itself is a more complex system than an ecosystem, as an ecosystem is only one of the subsystems within the complex urban system, which includes society, economy, culture, ecology and environment, etc. [6; 7] Urban resilience refers to the capability of urban systems to remain intact or quickly recover to desired levels in the face of shocks or stresses [8; 9]. As resilience in different scales embeds different connotations and mechanisms, this research focuses on urban resilience to theoretically conceptualize and empirically explore resilience in urban systems. Utilization of the resilience concept must be based on the characterization and measurement of resilience. According to its concept, resilience can be measured by the rate of return of ecosystem state after change or disturbance, and the measurement of resilience is, in effect, the measurement of threshold-crossing [10]. Therefore, detecting an ecological threshold along disturbance gradients is essential to protecting the threshold from being crossed [11]. Previous research selects a key indicator or index to evaluate ecosystem resilience [12; 13; 14; 15] or quantifies the economic value of resilience [16]. For urban resilience, a popular measurement is to build a comprehensive evaluation index with indicators representing the resilience of urban elements [17; 18; 19] or resilience process [20; 21; 22]. However, there are also scholars who believe that instead of seeking accurate metrics to measure resilience or trying to develop a general resilience index, it might be better to use surrogates or proxies [23; 24]. As a matter of fact, evaluation of the previously mentioned resilience indicators is resilience surrogates in the sense that they demonstrate the impact factor of resilience rather than resilience (threshold-crossing) itself. However, many resilience assessments tools, especially comprehensive evaluation indexes, tend to use state variables or cross-section data to measure resilience [17; 25; 26], while resilience is a process concept that encompasses two processes, namely resistance and recovery [27], making its measurement difficult as threshold-crossing often does not happen [24].