## 1 Introduction  
The new EU Biodiversity Strategy for 2030, \"Bringing nature back into our lives\", has identified a comprehensive, systemic, and ambitious long-term plan for protecting nature and reversing the degradation of ecosystems, driving the recovery from the urban crisis, and helping to strengthen the resilience of cities against future crises. These recommendations aim to halt the loss of biodiversity and Ecosystem Services (ES) by 2020 and enable transformative change. These challenges require new approaches and tools that support the aim to make ES mapping, monitoring, and assessment mandatory within European countries [1]. The last report of the OECD estimated that 125-140 trillion US dollars per year of benefits are provided by ES at the global scale, while the cost of global policy inaction on biodiversity loss per year between 1997 and 2011 was 4-20 trillion US dollars, underlining the need to identify suitable tools to assess risks and opportunities related to ES, as well as their environmental impacts and dependencies, with specific attention at the local-site level [2]. As the definition of ES is context-dependent, the concept identifies a _boundary object_ for sustainability--namely, an idea embedding different points of view--which preserves a sense of continuity, engages different disciplines and non-scientists in shaping and achieving societal goals, and is instrumental in facilitating, implementing, and assessing transformative processes for more sustainable human-nature relations [3; 4; 5; 6; 7; 8; 9]. According to [PERSON] [10], sustainability valuation for ES requires an integrated and whole-system approach at appropriate spatial and temporal scales that is able to interpret the complexity of interactions related to ES production. Indeed, the Millennium Ecosystem Assessment (MEA) has adopted a definition that includes the spatial pattern and the social dimension as results of human-nature interactions in the provision of services and benefits for human beings [11]. Therefore, attention to the specific dimension of the context, the scale of change and urban characteristics, and the level of perceived influence are some of the most significant issues that need to be considered when identifying and assessing ES [12]. Nevertheless, when dealing with the urban dimension--as the place in which the built environment prevails over natural features and the population density is high--the specification of \"Urban Ecosystem Services\" (UES) needs to consider the actual role of ES within the city context [13; 14; 15; 16]. At the same time, some authors have interpreted the concept of UES according to the different relevance attributed to the definition of \"urban _ecosystem services_\" or \"_urban ecosystem_ services\" [17]. The first concept is related to ecosystem services as analogues of natural and semi-natural ecosystems within urban boundaries; it is prevalent in the UES literature and deals with the natural and semi-natural spaces within cities [18]. The second definition expresses a much broader term, including the former group as well as urban settings in a city, covering the full range of services produced by humans, including housing, transport, education, entertainment, or medical care, with attention to any service relating to urban areas useful for urban dwellers [19]. Moreover, [PERSON] et al. [17] pointed out that the definition of UES and its application are context-dependent and able to express the interpretations of relationships that link UES to natural and human-derived capital, exploring how a broader interpretation of UES might advance its implementations. In this context, UES can be considered as a pattern of multidimensional services locally produced in cities to guarantee human wellbeing in both tangible and intangible ways [17; 20], which is able to support policies and programs for planning and managing cities. The city, as a complex socio-ecological system [5; 21], needs suitable methods and tools that aim to:  
1.