Javier Babí Almenar
email: javier.babialmenar@list.lu

Thomas Elliot

Benedetto Rugani

Bodénan Philippe

Tomas Navarrete Gutierrez

Guido Sonnemann

Davide Geneletti

Babí Almenar

; Elliot

; Rugani

; Bodénan

; Navarrete Gutierrez

; Sonnemann

; Geneletti

Nexus between nature-based solutions, ecosystem services and urban challenges

Keywords: , ecosystem services and urban challenges Nature-based solutions, Ecosystem services, Urban challenges, Urban sustainability, Urban resilience

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The global trends of increasing urbanisation, urban population and their associated environmental impacts are expected to continue over the coming decades (Keivani, 2009). These trends are likely to intensify existing urban challenges (UC) for sustainability and resilience, as well as generate new ones. In terms of sustainability, UC include all factors that limit the capacity of urban areas to protect and conserve the environment, minimise environmental impacts and enhance resource-efficiency, human health, social inclusiveness and equality, as well as harness the productivity of local economies and value-added activities (United Nations, 2017). In terms of resilience, UC relate to those factors that limit the capacity of urban areas (including their inhabitants, institutions and inner systems) to resist and adapt to environmental, social or economic chronic stresses, and acute shocks (Meerow et al. 2016;Marron Institute of Urban Management, 2018). In many cases, UC for sustainability and UC for resilience (hereafter referred generically to as UC) overlap and also share limiting factors. The nature of these limiting factors, can be biophysical (e.g. a lack of woody vegetation can contribute to the presence of heat islands), technological (e.g. insufficient technological development for achieving universal access to certain goods or services), human-social (e.g. the current human, institutional or social structure act as barriers for adapting to new situations), or financial (e.g. limited amount of money restricts access to products of a certain quality). UC can therefore be determined by many types of limiting factor, which need to be understood before strategies and interventions can be developed to mitigate or address those UC.

Strategies and interventions to mitigate or address UC should acknowledge that urban areas have the potential to buffer their own impacts and enhance the quality of life of their inhabitants, since they are capable of fostering not only local, but also global sustainability and resilience (Elmqvist et al., 2015;Luederitz et al., 2015). Framing such strategies and interventions with specific solutions, requires a good understanding of the local environmental, social and economic conditions of urban contexts. Its acquisition would provide information about the suitability of the proposed solutions and the adequacy of their transfer and replicability in other urban contexts. As potential solutions, those that use natural systems and rely on concepts such as natural capital (Guerry et al., 2015), ecosystem services and green infrastructure are becoming increasingly popular among built-environment professionals, policy-makers and researchers (Cortinovis & Geneletti, 2018;Geneletti et al., 2020;Nesshover et al., 2017;Pauleit et al., 2017). Among these concepts that of nature-based solutions (NBS) has emerged recently.

NBS are defined by the European Commission (EC) as "solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social and economic benefits and help build resilience. Such solutions bring more, and more diverse, nature and natural features and processes into cities, landscapes and seascapes, through locally adapted, resource-efficient and systemic interventions" (European Commission, 2016). Further supported by several scholars (Faivre et al. 2017;Raymond et al. 2017;Dorst et al. 2019), an expert report for the EC emphasises that NBS are able to address multiple challenges simultaneously, as well as to provide additional co-benefits (European Commission, 2015). Similarly, the International Union for Conservation of Nature (IUCN) defines NBS as "actions to protect, sustainably manage, and restore (create) natural or modified ecosystems that address societal challenges (including urban ones) effectively and adaptively, simultaneously providing human well-being and biodiversity benefits" (Cohen-Shacham et al., 2016).

EC, IUCN and many other scholars relate NBS to the concept of ecosystem service (ES) and natural capital (Cohen-Shacham et al., 2016;Eggermont et al., 2015;European Commission, 2015;Joachim Maes & Jacobs, 2017;Nesshover et al., 2017;Potschin et al., 2016). For example, Eggermont et al. (2015) classify three types of NBS: i) better use of ecosystems; ii) sustainable and multifunctional management of ecosystems; and iii) design and management of new ecosystems. These types are organised based on their contribution to an increased supply of ES (i.e. linking ES and NBS) and the level of engineering to be applied to ecosystems to achieve this supply. ES are considered the outputs (flows) derived from natural capital stocks (Dominati et al., 2010). They are accounted for to estimate the contribution of natural capital to human well-being from a social, economic and environmental point of view (Turner et al., 2016).

As implied by IUCN and EC, and stated by other authors (Albert et al., 2019;Bush & Doyon, 2019), NBS contain natural capital stocks or are actions to maintain and enhance the flow of ES. Hence, as part of urban planning strategies and interventions addressing different UC, NBS can help to operationalise the concepts of natural capital and ES (Potschin et al., 2016).

In terms of conceptualisation, several authors propose framing NBS as an umbrella concept under which other ecological concepts such as ecological engineering, ecosystem-based approaches, green infrastructure or ecological restoration could be integrated (Cohen-Shacham et al., 2016;Dorst et al., 2019;Nesshover et al., 2017;Pauleit et al., 2017). Compared to these other ecological concepts, NBS emphasise the value of nature to address societal (urban) challenges (Kabisch et al. 2016), the connection to policy and the relevance of implementation aspects (Pauleit et al. 2017). However, NBS is still a very open concept (Potschin et al., 2016) and this vagueness hampers its mainstreaming into urban planning strategies and interventions (Dorst et al., 2019). Then, to facilitate its operationalisation, the concept itself and the added value of NBS (i.e. the fact that they provide multiple benefits) compared to other solutions needs to be easily understood by practitioners and decision-makers.

In practical terms, practitioners and decision makers need further studies relating specific urban NBS to particular benefits (e.g. Cortinovis & Geneletti, 2019;Frantzeskaki et al., 2019;Keeler et al., 2019). For this, first they need a clear NBS classification shared among different professionals. They also need to know when specific NBS are not suitable due to the specificity of the context (Albert et al., 2019), or when are not enough as a stand-alone solution, e.g. to address social related UC (Haase et al., 2017;Kotsila et al., 2020). More research also needs to show which attributes of urban NBS affect the supply of specific ES, to consider those during their planning and design. There are previous systematic reviews on factors influencing ES supply (e.g. Bordt & Saner, 2019;Smith et al., 2017). However, these are mainly focused on spatial levels such as entire ecosystem or landscape mosaics, rural contexts, and in general factors (e.g. population dynamics), which might not help to define individual urban NBS. In synthesis, there is a need i) to define and classify urban NBS in a form that is suitable for scientists, decision-makers and builtenvironment professionals; ii) to understand the causal relationships between different types of NBS, ES and the UC that they are supposed to address; and iii) how the attributes of NBS and the contexts where these are placed influence the provision of ES.

This paper aims to identify the nexuses between UC, ES and NBS, discuss their plausible causal relationships, and how these relationships can be affected by urban context conditions. Accordingly, the term "nexus" is understood as a qualitative link identified between a UC and an ES, as well as between a specific NBS and an ES. These nexuses are disclosed through a two-step systematic review plus a complementary non-systematic review. The identification of the nexuses is followed by a critical analysis of the collected evidence in order to assess which nexuses can be considered plausible causal relationships. We refer to "plausible" (i.e. likely) causal relationships according to a precautionary principle, because through a literature review a causal relationship cannot be confirmed with full certainty. In order to fulfil the aim, the following objectives are established:

• identify and classify UC, ES, NBS and their relationships based on the current scientific, policy and urban planning literature;

• determine attributes of NBS (e.g. soil properties, height of vegetation)and their contexts influencing social and biophysical processes, which lead to the generation of specific ES relevant for mitigating or addressing UC;

• identify similarities and differences in the UC, ES and NBS emphasised across urban case studies due to their specific socio-economic and environmental conditions.

Ultimately, the identification of nexuses as well as the discernment of plausible causal relationships was guided by the conceptual framework presented in Section 2.

Characteristics of the conceptual framework

The proposed framework aims to relate UC, ES and NBS in a simple form as outlined in Figure 1, which illustrates the overall structure and characteristics guiding the establishment of plausible causal relationships. A detailed description about the type of UC-ES-NBS causal relationships found is provided in Section 4.3.

First, specific nexuses are identified through their reiterated occurrence in the empirical results of the works included in the different literature reviews. Then, further analysis is required to understand if those nexuses are in fact plausible causal relationships. In particular, the analysis takes into consideration that urban areas are hybrid systems where natural and artificial components, as well as social and ecological processes are blurred and interconnected (Albertí et al., 2017).

Regarding the UC-ES nexuses, not all UC are rooted in biophysical limiting factors or could be mitigated with natural capital, and consequently their mitigation might not be achieved by increasing ES provision.

In these cases, plausible causal relationships between UC-ES do not exist or they are not relevant enough to be acknowledged.

In terms of ES-NBS nexuses, each NBS provides only a specific set of ES. Then, a plausible causal relationship between NBS and ES does not exist when the NBS does not contribute to the socio-ecological processes that generate the specific ES. In the case of NBS that are biophysical structures (i.e. created ecosystems), the contribution occurs if the abiotic and biotic attributes of the NBS are involved in the socio-ecological processes. In the case of NBS as actions applied on ecosystems (e.g. management and restoration actions), the contribution occurs if these actions modify the attributes of the ecosystems involved in the socio-ecological processes. In addition, the extent to which a particular ES-NBS plausible causal relationship not only exists, but it is relevant enough to be acknowledged also depends on the role of the abovementioned attributes in the performance of the socio-ecological processes, i.e. the lesser or greater generation of that ES. Besides the NBS itself, its implementation model (i.e. the combination of governance, business, and financial models under which the NBS is planned, developed and managed) might also influence its capacity to provide ES, and mitigate UC.

Methods

The method is composed of three main steps, namely a two-step systematic literature review on UC and ES, a complementary non-systematic review and a posteriori integrated analysis of the data (Figure 2). The systematic review adapts the review protocol of Luederitz et al. (2015) and Brink et al. (2016).

Operationally, the review was conducted by: i) identifying an initial list of articles based on a broad search string that encompasses UC and ES topics;

ii) preselecting the articles if their abstracts meet specific criteria (as reported in Figure 2, Data screening & cleaning);

iii) selecting the articles and conducting a critical analysis if their complete text fulfils the screening criteria (see Figure 2, Article appraisal & Analysis).

Figure 2. Methodological steps of the literature review including the criteria that the selected documents fulfilled to be kept in the review.

Systematic literature review

For the two-step systematic literature review, the search of peer-review papers was limited to the last 20 years, from 1998 to early 2019. The concept of NBS and similar concepts (e.g. green infrastructure), as well as the study of ES in regard to them, are very recent, making it unnecessary to account for a longer time period. In addition, UC evolve over time, hence limiting the temporal extent of the search ensures that only currently-relevant challenges are included in the analysis.

The papers were retrieved from Web of Science at the end of February 2019 using the search strings included in Figure 3. The screening phase (see the criteria in Figure 2) was performed to retain only papers describing an assessment of ecosystem services or ecosystem functions in urban contexts. The literature review of UC for sustainability and resilience included peer-reviewed papers, reports from international public institutions (e.g. United Nations, FAO) and local urban planning documents, thus integrating science, policy and local urban planning perspectives. The collection of policy reports and urban planning documents was completed in February 2019. The policy reports were selected from a preestablished list of well-known international institutions and related initiatives (Supplementary Material A). The local urban planning documents were reports selected from various global inter-city scale initiatives, such as Emerging and Sustainable Cities (Inter-American Development Bank), 100 Resilient Cities Initiative (Rockefeller Centre) and the C40 Cities. These initiatives include a comprehensive list of cities around the world that prove to be active in sustainable and resilient urban planning (Supplementary Material A). In the case of public institutions and local urban planning reports, the stages of data gathering, screening and scoping were done simultaneously.

The lack of a recognised exhaustive classification of UC in the literature made it necessary to develop an original one based on existing frameworks. The UC, and their sub-challenges, obtained from the selected literature were organised by combining the classifications of UC proposed in the Emerging and UC literature review 2016;Raymond et al. 2017;IADB 2019). These classifications well complement each other well and are already used in Latin America and Europe. New sub-challenges mentioned in the papers reviewed and not considered in the original classifications were also incorporated into the UC classification (See Supplementary Material B for a description of the classification). In the reviewed literature, multiple terms were used for the same UC or sub-challenge. This made a harmonisation of the terms necessary as part of the development of the classification system. In addition, when a paper or document referred to a UC or a sub-challenge using a vague terminology, and there was no definition and/or clear description to help its classification, this non-distinguishable UC or sub-challenge was disregarded from the review.

The papers of the UC review were analysed making use of nine categories that included the type of of some types of NBS (those with a biophysical structure) or the urban contexts where they are placed.

Non-systematic literature review

A complementary non-systematic review was performed to help establish the classification of NBS types. It was also used to fill gaps in the identification of social and biophysical processes and attributes involved in the generation of ES already identified. This was necessary because many papers from the systematic literature review did not clearly identify the factors influencing the supply of specific ES classes, as also raised by Luederitz et al. (2015) in their review.

The non-systematic review was supported by land management and ecological restoration handbooks, papers on NBS types and their assessment (e.g. Xing et al. 2017a) and handbooks of ES process-based models (process-based models as defined by Santos-Martin et al. 2018). The handbooks on land management techniques (Morgan, 2013;Triest et al., 2016) and types of restoration ecology interventions (Hobbs, Higgs, & Harris, 2009;van Andel & Aronson, 2012) complemented the identification of NBS that were less studied in the urban ES studies. The papers and ES process-based model handbooks were identified making use of a snow-balling approach (Badampudi et al., 2015), starting from the references of the systematic literature review. Only papers clearly stating the biophysical and social attributes influencing the supply of specific ES classes were included.

The classification of NBS types modifies the one proposed by IUCN (Cohen-Shacham et al., 2016). In fact, the classification of IUCN is also an adaptation of the three NBS types of Eggermont et al (2015). The classification also makes use of three categories (i.e. ecosystem types, dominant media and spatial levels), as summarised in Figure 4. In terms of conceptualisation, NBS is assumed as an umbrella concept for other ecological concepts, and the definitions of IUCN and EC are respected. Therefore, NBS included in the classification are actions applied to enhance living solutions or which are formed of them that protect, sustainably manage, restore or create (natural, modified or novel) ecosystems (Cohen-Sacham 2016,

European Commission 2016).

In this paper, NBS Type 1 are considered solutions that permit not only a better use, but also a better management (i.e. non-physical modifications) of existing natural or naturalistic ecosystems. Making better use of an ecosystem implies a change in its management or in the management of surrounding ecosystems (indirect change). It could also imply changes in how the resources obtained from the ecosystem are exploited. In this sense, from the authors' perspective, it is not possible to distinguish between "a better use" and "a better management" as two different NBS types. With this modification NBS Type 2 include only solutions and procedures to restore ecosystems. These are further differentiated into reclamation and restoration categories. Following the adaptation of IUCN, NBS Type 3 are maintained as solutions that involve creating novel ecosystems. These also include solutions that involve the extensive (i.e. a large percentage of area) and intensive (i.e. high degree) modifications of existing ecosystems. This would be the case of converting a highly artificialized urban green area into a highly naturalised one.

Regarding the categories, the three types of NBS refer to actions applied to ecosystems in one way or can also help to understand the dominant media (on the left in Figure 4) per each ecosystem type, which may constrain the specific NBS that can be implemented. References to (semi)natural and artificial ecosystems were included in the conceptualisation of NBS types (in the centre of Figure 4) as auxiliary elements. These ecosystems are the biophysical support on which NBS Types 1, 2 and 3 can be developed and define the initial ES supply that should be enhanced (see the y axis in Figure 4) by the implementation of NBS.

Finally, to make the classification relevant for urban planners and decision-makers, urban NBS also need to be organised according to the spatial level at which they should be implemented (on the right in Figure 4). The spatial level indicates the range of required space for each specific NBS, and consequently its adequacy for different types of urban strategies and interventions.

Integrated analysis and visualisation of the outputs

The outputs from the UC and ES literature review were analysed making use of the categories described in Section 3.1. The contextual attributes of the case studies were analysed in order to understand the similarities and differences in the UC, ES and NBS depending on the specific socioeconomic and environmental conditions of their urban contexts. First, the location of the case studies was used to georeference them. Second, data associated with the remaining contextual attributes (e.g. population, climate) were collected making use of existing databases. Data on population and population density were extracted from Angel et al. (2011), which provide an informational database for 3,646 urban agglomerations worldwide. For urban agglomerations not included in Angel et al. (2011), the data were collected from databases found one by one on specific municipal, metropolitan or regional websites. The updated Köppen-Geiger climate classification world map (Kottek et al., 2006) was used to assign regional climatic classes to each case study. The one-kilometre resolution map of the GLOBE project (Hastings & Dunbar, 1993) was applied to differentiate urban areas with low-lying elevation. Third, per each contextual attribute, qualitative classes were established to make easier the differentiation between urban contexts with similar contextual conditions. In terms of urban size, the cities were classified according to their population making use of OECD (2019) classes. Regarding inhabitants' income capacity, the GNI classes proposed by the World Bank (2019) were used.

The visualisation of the data for each UC with respect to the type of document and urban contextual conditions was done using an ordinal ranking approach supported on conditional tables. The same procedure was used for the visualisation of the most widely studied ES and NBS, UC-ES nexuses and ES-NBS nexuses (only links appearing more than three times were kept in the visualisation), but with a graphbased approach (Bastian et al., 2009). For the discernment of UC-ES plausible causal relationships, the nature of the limiting factors of the UC were analysed as well as if they could be overcome with increased natural capital (as stated in Figure 1). Similarly, for the discernment of ES-NBS plausible causal relationships, the conditions of the framework in Figure 1 were followed based on the identified social and biophysical attributes and socio-ecological processes. The names of these attributes and processes were harmonised to avoid repetitions and overlaps and the related information populated in a table. In the case of attributes and processes, the nexuses were not ranked because the objective was only to identify them and not to illustrate the most acknowledged ones. Finally, a qualitative correspondence diagram was constructed to depict potential plausible causal relationships among the most frequently mentioned UC, ES and NBS. The diagram also includes the association of UC with specific urban contextual classes and the attributes and processes linking ES and NBS.

Results & Discussion

The two-step systematic review examined 304 documents, 170 from the ES review and 126 from the UC review (further details in Figure 2). These documents included 366 case studies (i.e. several papers have more than one case study). As Figure 5 shows, the highest number of cases are located in Europe (162), America (109) and Asia (56), while only few documents investigated case studies in Africa ( 14) and Oceania (7). Among the selected documents, some also focused on the global context (17). In terms of the ES review, most of the case studies are from North America, Europe and Asia, which is consistent with the results of precedent reviews (Dobbs et al., 2019;D. Haase et al., 2014;Keeler et al., 2019;Luederitz et al., 2015). No difference emerged in the UC, ES, and NBS addressed in the studies based on average elevation and population density. In addition, due to the relatively low number of African and Oceanian case studies, these were not taken into consideration when looking for similarities and differences Based on the UC review, we identified 18 UC for urban sustainability and resilience and 58 associated sub-challenges. Table 1 shows the occurrence of UC and sub-challenges in the reviewed literature per type of document. At least 50% of all reviewed literature on UC focuses on built-environment issues, physical health, green and circular economy, and material & solid waste management. Public institution reports and urban planning documents mention water management and mobility in at least 50% of the documents. Around half of public institution reports consider energy and governance. Similarly, only urban planning documents consider social vulnerability and climate change in at least 50% of the cases.

In fact, urban planning documents is the only type of literature where a sub-challenge (vulnerability to human/natural disasters) is present in at least half of the documents.

The review of the most frequently mentioned UC and sub-challenges (i.e. those present in at least 15% of the UC literature) across urban contextual classes (e.g. medium size urban areas) helps us to understand similarities and differences among specific urban contexts (Table 2). Regarding similarities, the green and circular economy is the only UC mentioned in at least 50% of the case studies in every contextual class. Other UC, such as social vulnerability, built environment, mobility, water management, material & solid waste management and physical health, show a frequency close to 50% in most of the contextual classes. When looking for differences, some UC and sub-challenges appear to be of higher interest in urban areas sharing certain conditions. For example regarding continents, solid waste management, governance and the sub-challenge of vulnerability to human/natural disasters, are only present in around 50% of the cases of urban areas belonging to the Community of Latin America and Caribbean States (CELAC), whilst energy and the sub-challenge of employment (job) development are mainly taken into consideration in European contexts. Also, more than 50% of all cases in the review (bold numbers in Table 2) identifying wastewater management, expenditure and debt management, and the sub-challenge urban violence and insecurity come from CELAC urban areas. In terms of urban size, sociospatial equity, climate change and the sub-challenge flooding risk, are mostly studied in large metropolitan areas. Moreover, more than 50% of all cases in the review for the sub-challenges of ageing and inadequate infrastructure and urban heat island effect, sea level rise, and vulnerability to disease outbreak correspond to large metropolitan areas. Climate classes constitute a particular case, where the sub-challenge energy efficiency appears to be of interest only in urban contexts in the climatic class Cfb.

The different prioritisation of UC according to particular contextual conditions (e.g. urban size, continent) suggests that the presence of some UC might be more likely where specific social and biophysical contextual factors occur. We are not able to identify if causal relationships exist or not, and in this case only nexuses are identified. In the future, in-depth studies of UC in regard to contextual classes can help us understand whether these links are causal relationships or not. For example, the UC sociospatial equity appears to be closely associated with large metropolitan areas, and therefore it might be worth investigating whether there is a causality associated with urban size. Future studies could also inform us whether lessons learnt from other urban contexts regarding the mitigation of UC through the ES supplied by NBS are transferable or not.

Part of our results on UC across urban contextual classes (Table 2) have also been stressed in the review of Dobbs et al. (2019). Their review emphasises governance and vulnerability to human/natural disasters as UC intrinsic to CELAC countries. As implied by these authors, these UC are triggered by a combination of social, political and biophysical factors specific to CELAC countries. Consequently, in their opinion, global urban ES lessons extracted predominantly from studies in northern developed countries and applied directly to advise on urban strategies and policies in CELAC countries could generate socially, environmentally and economically mismatched decisions (Dobbs et al., 2019). Instead, win-win situations could arise if policy-makers and urban planners of municipalities with similar contextual conditions and UC collaborate and exchange knowledge about their research and experiences on urban ES or policies, strategies and interventions on NBS. Consistently, scholars argued that NBS should not be copied from one place and applied exactly as they are to others (Dorst et al., 2019;A. Haase, 2017). NBS should be sensitive both to the socio-spatial context in which they are applied, as well as to the specific UC that they aim to tackle, in order to be considered "solutions" (Dorst et al., 2019;A. Haase, 2017).

Identification of ecosystem services and nexuses with urban challenges 395

In the ES literature, the most frequently mentioned UC and sub-challenges (right-hand column of 396 Table 1) are physical health (39% of the papers), mental health (29%), climate change (29%), water 397 management (22%) and the sub-challenge urban heat island and heatwaves (15%). In contrast, several 398 UC are not explicitly mentioned in the ES articles (e.g. mobility, digital connectivity, governance), which 399 could be interpreted as a lack of direct causal relationship between ES and UC, and therefore NBS. In 400 addition, for some UC (e.g. social cohesion, green and circular economy) either no sub-challenges are 401 specified in the papers or it was difficult to distinguish them (e.g. energy sub-challenges) due to the use 402 of vague terminology. 403

Case studies per contextual class >≈50% <10%

In terms of ES, regulation services are the most frequently assessed (131 papers), followed by cultural services (73 papers) and provisioning services (41 papers). This is consistent with previous literature reviews (Ziter 2016, Luederitz et al. 2015, Haase et al. 2014). The analysis of ES studies by urban contextual classes does not present major differences and therefore this aspect is not discussed. Cause-effect relationships underpinning the identified UC-ES nexuses are in many cases evident and can be easily understood. For example, this is the case for the relationship between characteristics of living systems that enable activities promoting health (ES class belonging to the ES section of "cultural services") and the mitigation of physical health or mental health issues (see UC in Table 1). Similarly, the relationship between the above ES class and the UC socio-spatial equity can also be easily explained once it is known that several case studies describe a lack of adequate distribution of living systems (and therefore their associated characteristics promoting health) in urban areas. This is why the study of this ES class in regard to socio-spatial equity is recurrent in the ES papers reviewed. In fact, this outcome is consistent with several papers on environmental justice focused on urban inequity derived from the distribution of green areas (Anguelovski et al., 2018;B. Lin et al., 2015;Shen et al., 2017;Wu et al., 2018).

In contrast, for other UC-ES nexuses, such as characteristics of living systems that enable activities promoting health (ES) and biodiversity (UC), the direct causality is not evident. However, it might be that an indirect causal relationship exists due to synergies and trade-offs between the supply of this ES and the supply of other ES related directly to the UC biodiversity. For example, Lin et al. (2018) show that an increase in the maintenance of habitats and gene pool reserve might also increase the presence of the characteristics of living systems that enable activities promoting health and mitigate biodiversity decline. In the first group of UC-ES-NBS (upper left, Figure 7) neither direct nor indirect plausible causal relationships occur. This is the case of digital connectivity and mobility, which are both limited by human and technology limiting factors, rather than biophysical ones. The enhancement of ES would neither be able to compensate for the limitations and nor mitigate or address these UC. For example, an NBS could make a road infrastructure more pleasant, or protect it against flooding, but it cannot overcome mobility issues due to infrastructure limitations. Nevertheless, these UC would not jeopardise either the implementation of NBS or their perception by people.

In the second group (upper right Figure 7), the limiting factors of UC are of human-social nature, such as decision-making issues (governance, public participation and expenditure) or changes in socioeconomic trends (demographic dynamics), which cannot be compensated by ES. Some of these UC are relevant to several urban contexts (as shown in Table 2). In contrast to the first group, the planning and implementation of NBS in those urban contexts might need to tackle these UC or adapt to the consequences resulting from them to ensure the subsequent wide acceptance/use of the NBS implemented by society. For example, in the case of demographic dynamics, when related to an ageing population, NBS will not be able to mitigate this issue. However, if the ageing issue exists in an urban context, it might need to be considered when planning or designing NBS for mitigating other UC. This would be the case of a municipality with aging issues that implements urban forests to increase (in the long term) nature-based recreation with the aim of enhancing the physical health conditions of its citizens.

In that case, to ensure that the accessibility and usability of the urban forests is adequate for an elderly population might be more relevant than in other contexts. As another example, to mitigate UC related to decision-making issues, representatives of different political parties, levels of governance, private partnerships and citizens need to be included early on in the planning and implementation phases of NBS and consensual planning should be followed to avoid failure or non-implementation due to citizens' opposition or changes in the government representatives. In the same sense, Dorst et al. (2019) state that the multifunctionality of NBS can be hampered by governance issues such as fragmentation (or lack of consensus) in the decision-making process.

In the third group (lower left Figure 7), the UC (socio-spatial equity, social cohesion, social vulnerability, green and circular economy) are partially driven by limiting biophysical factors and/or their enhancement might contribute to the mitigation of UC. However, the mitigation of UC depends on how, where, and for whom (i.e. the interests of which social groups are considered) NBS are implemented. For example, in terms of how to move towards a green and circular economy, business and financial mechanisms need to be considered in the implementation of NBS. This will ensure the economic exploitation of their products and services (Chen & Warren, 2011;TECNALIA et al., 2018;Toxopeus & Polzin, 2017). Concerning where and for whom, in order to address socio-spatial equity factors such as location of NBS with respect to existing recreational areas (i.e. where), public accessibility, inclusion of different social groups in the plan, design, and implementation stages (i.e. for whom) and potentially governance aspects (e.g. to prevent green gentrification) need to be considered (Almohamad et al., 2018;Haase et al., 2017;Park & Kim, 2019). In this sense, as already illustrated by other scholars through empirical cases (Haase et al., 2017;Kotsila et al., 2020), NBS per se do not solve social cohesion issues. In order to tackle social and economic UC, the specific NBS implementation model chosen, including governance, finance and business mechanisms, is also relevant.

The fourth group (lower right Figure 7) occurs when the UC (or one of its sub-challenges) is driven by limiting biophysical factors or could be overcome by processes depending on biophysical attributes, which influence the ES supply. These are mainly the cases where the nexuses between UC and ES are evident or can be easily explained, as illustrated at the end of Section 4.2. For example, in cities suffering from the urban heat island effect, the solution requires the regulation of temperature and humidity through the enhancement of biophysical processes such as evapotranspiration and/or shading (see Zardo et al. 2017).

Section 4.6 describes the identified social and biophysical attributes and processes, on which the supply of the most frequently mentioned ES (of this review) depend, are described.

Classification and identification of NBS types

In the systematic and non-systematic literature reviews, solutions based on (or applied to) nature or living features were often framed under different ecological concepts: green infrastructure, urban green (and blue) spaces, services providing units, services providing elements, sustainable urban drainage systems and ecosystem-based adaptation. As already stated in Section 3.2, these ecological concepts and their specific solutions could be assimilated under the umbrella of NBS. For example, ecosystem-based adaptation refers to the protection, management and restoration of the spatial structures (Munang et al.

2013, Geneletti and Zardo 2016) corresponding to NBS Type 1 and 2. As another example, green infrastructure types, urban green (and blue) spaces, services providing units and sustainable urban drainage systems correspond to biophysical structures and in many cases are equivalent to specific MAES Ecosystem types (and NBS Type 3). Hence, following the NBS conceptualisation (Figure 4), specific solutions framed under these ecological concepts were included in the three main types of urban NBS, leading to the detailed classification of NBS types shown in Figures 8a andb. In the systematic literature, only urban case studies referring to (semi)natural ecosystems and NBS Type 3 are found. This illustrates a clear prevalence of physical solutions in urban ES studies, although the underlying reason is not entirely clear. It could be because there is still a strong need to bring back natural structures into cities before solutions focused on managing and restoring them (NBS Type 1 and 2) become relevant. It could also mean that the extensive and intensive modification of existing urban ecosystems and the creation of new ones are perceived as more effective solutions for addressing UC. Alternatively, the lack of Type 1 and 2 urban NBS in our review may be due to the difficulty of assessing them independently from the physical structures on which they are applied. For example, for the assessment of the individual contribution of NBS Type 1 to ES supply it might be necessary to compare changes in the supply of ES by the same physical structure before and after an NBS Type 1 or 2 was applied. Furthermore, urban environmental management and urban ecological restoration might not usually be framed in the research area of ES and therefore these related studies were not captured in our review.

The most frequently studied NBS Type 3 per type of media are green roofs, green walls, woodland-like structures, urban grasslands and meadows, urban scrubland and heathland, horticultural gardens, vegetated filter strips, swales, constructed wetlands, natural(ised) wetlands, natural(ised) ponds and bioretention basins (Figure 9). Similar to previous reviews, woodland-like structures appear as the most frequently studied supplier of ES (Haase et al., 2014;Keeler et al., 2019;Luederitz et al., 2015). In contrast to previous reviews, studies of ES supplied by green roofs have increased. For many other solutions, it is difficult to draw parallels with previous reviews due to a lack of common NBS classification and because not all of the previous reviews analysed solutions relating to nature (e.g. urban fabric, land use mixture and infrastructure appear as ES suppliers in Haase et al. (2014)). This outcome also illustrates the importance of an agreed-upon NBS classification, so as to allow comparison among studies and facilitate transfer of information to support urban policies, strategies and interventions.

Identification of links between ecosystem services and nature-based solutions

Green roofs, woodland-like, urban grasslands and meadows, horticultural gardens, (natural(ised)

wetlands and natural(ised) ponds have links to a higher number of ES classes than other NBS (Figure 9).

Regarding the specific ES-NBS nexuses, all the highlighted NBS except vegetated filter strips, green roofs and green walls appear to be strongly associated with characteristics of living systems that enable activities promoting health. In addition, green roofs (built structures media) are mostly linked to regulation services (i.e. regulation of temperature and humidity, filtration, sequestration, storage of air pollutants and regulation of the hydrological cycle and water). Urban woodland-like environments, urban grasslands and horticultural gardens (land media) are also often associated with the regulation of temperature and humidity, regulation of chemical condition of the atmosphere, filtration, sequestration and storage of air pollutants, regulation of hydrological cycle and water flow and cultivated terrestrial plants for nutritional purposes. Natural(ised) wetland, constructed and natural(ised) ponds (water media) prove to be relevant not only for regulation services but also for cultural ones, being mostly linked to the regulation of chemical conditions (water), regulation of hydrological cycle and water, filtration (water), sequestration and storage of water pollutants, characteristics of living systems enabling aesthetics experiences, and education and symbolic value.

As with the visualisation of the UC-ES nexuses (Figure 6), Figure 9 shows that the ES classes smell reduction, erosion regulation and bioremediation are not mentioned in connection with NBS or seem to have low momentum in urban studies. Similarly to the results outlined in Figure 6 for the UC-ES nexuses, multiple links between specific ES classes and NBS do also occur. This is unsurprising given that NBS are described as solutions capable of addressing multiple challenges and providing multiple benefits (see Section 1). To investigate the causality underpinning these nexuses, the processes and factors responsible are discussed below. However, in some cases, especially for cultural services, these processes and factors were difficult to identify. For example, in characteristic of living systems that enable education or with symbolic values or resonant in cultural values, the attributes are very subjective and it was not possible to identify the ones that were reiterated several times in the case studies and that did not rely on human agreements, such as the application of a legal protection status.

Interpretation of the links between ecosystem services and nature-based solutions

The main social and biophysical attributes and processes identified in the reviews, which influence the most frequently studied ES classes, are summarised in Table 3.

In most cases, biophysical processes are partially dependent on attributes of NBS. For example, the amount of tree canopy coverage, which influences shading, and therefore regulation of temperature and humidity, relates to a biotic component (trees) of woodland-like NBS type. In other cases, such as for the sub-challenge lack of water quality, it is important to know which pollutants are generating the issue. This helps understanding whether regulation services that could mitigate the issue (e.g. filtration, sequestration, storage and accumulation by plants, microorganisms and algae) can be provided by the NBS or not. In some cases it is difficult to state the suitability of an NBS for supplying an ES class without a detailed analysis of the UC and related limiting factors. In the case of water-related ES, the reviewed literature focused mainly on processes specific to nitrogen and phosphorus compounds (e.g. [START_REF] Adhikari | Removal of nutrients and metals by constructed and naturally created wetlands in the Las Vegas Valley, Nevada[END_REF]Adyel et al., 2016;Liquete et al., 2016;Nocco et al., 2016;Olguin et al., 2017;Sun et al., 2017) and very few papers on processes specific to other pollutants (e.g. Krzeminski et al., 2019). Several reviewed papers also emphasise that physical attributes of the human-made contexts and/or their social characteristics (e.g. people's perception) are relevant to the supply of ES by NBS, especially in the case of cultural services (e.g. Andersson-Sköld et al., 2018;Brill et al., 2017;Fry et al., 2009;Ode et al., 2008;Szücs et al., 2015).

The abovementioned findings confirm the importance of making explicit which attributes of specific NBS, and the urban context, influence the processes underpinning the supply of a particular ES. It is necessary to state whether certain ES-NBS nexuses are plausible causal relationships or not. These reflections are also stressed by Keeler et al. (2019), who state i) that decision-makers need more applied information describing the conditions (of NBS and contexts) in which specific approaches and NBS are effective for some UC; and ii) that many NBS studies overlook the influence of social, ecological and technological factors when assessing their performance. For example, the regulation of hydrological cycle and water flow depends on infiltration, with several papers in this review indicating that it is influenced by root distribution and depth, among other attributes (Kim et al. 2016;Zölch et al. 2017). However, root depth varies depending on whether we implement a horticultural garden or a woodland (two of the NBS indicated as suppliers of this ES class). It also varies inside the same NBS depending on attributes such as the plant species. Therefore, the contribution of NBS attributes should be carefully considered by policy-makers and urban planners when designing, implementing or assessing NBS with the aim of enhancing the supply of specific ES and, as a consequence, addressing a particular UC.

As another outcome, some ES share several processes and attributes, which implies that interdependencies occur in the supply of ES (Table 3), as shown by other scholars (Bennett et al., 2009;Lorilla et al., 2018). For example, as stated in several papers of this review, evapotranspiration is one of the main processes for regulation of temperature and humidity, as well as for regulation of hydrological cycle (Lundy & Wade, 2011;Nocco et al., 2016;Pappalardo et al., 2017;Reynolds et al., 2017;Skelton et al., 2011;Zardo et al., 2017). Evapotranspiration, more specifically transpiration, is also identified as an influence of several processes of bioremediation by plants (e.g. phytovolatilisation), since it affects the rate at which pollutants are taken up and expelled from plants (Pulford & Watson, 2003;Singh & Santal, 2015b).

Moreover, Table 3 also illustrates that the supply of some ES classes might be indirectly influenced by other ES, or might prevent their demand in the first place, such as in the case of erosion regulation (De Troyer et al., 2016). Erosion dynamics influence the pollutants transported from one point to another, and therefore the input of pollutants to be filtrated, sequestrated or stored. Consequently, it seems necessary to study the supply of ES by NBS in bundles, defined as a set of ES that usually appear together (Yang et al., 2015), instead of individually. The importance of studying ES in bundles is also stressed in the review of Smith et al. (2017). Such an approach may further allow accounting for ES synergies and trade-offs, and should be recommended when planning/designing, assessing and monitoring the performance of NBS. The consideration of ES in bundles should also be recommended by policies and guidelines supporting the mainstreaming of NBS or establishing rules for their adequate implementation. Based on the evidence of this review, the UC-ES-NBS network diagram obtained as the final outcome makes explicit the nexuses that are plausible causal relationships between NBS, ES and ultimately UC. The diagram confirms that NBS provide multiple benefits and adequately address multiple challenges, even if not all. In addition, nexuses between specific classes of contextual attributes and UC that were highly reiterated (i.e. in more than 50% of the case studies including a specific class of contextual attribute) are also included. However, in the latter case plausible causal relationships were not studied.

The network diagram, in the two formats (html and Gexf) provided in the Supplementary Material F, can also be used by professionals of different backgrounds as a visual user-friendly output. The file in html format (accessible at https://mimes.list.lu/articles/network) allows an easy visualisation of the whole network and the detailed first order relationships without requiring any software. A more detailed visualisation (and manipulation) can be performed using the Gexf file, which shows relations of all the orders for each UC following the instructions included in the Supplementary Material F. Therefore, readers can use one of the three alternatives included (Figures of Supplementary Material E, html, and Gexf file) to see how specific UC can be addressed by means of ES, the processes on which the latter depend, and which attributes of NBS and their surrounding urban context mediate in those processes.

In combination with the NBS classification of Figure 8, the diagram could be used to identify the specific NBS of interest for a particular situation, also taking into account the suitability of spatial levels and ecosystem types.

It also highlights the NBS that need to be further studied in order to understand their capability for addressing specific UC through the supply of ES. In summary, the framework presented here has a double utility. First, it depicts a preliminary framework, in progress, that disentangles a wide set of UC-ES-NBS relationships in a qualitative form, which could inform urban policies, strategies and interventions. Second, in the longer term, this framework could be used as a basis to move towards broad quantitative "correspondence" tables that consider causal pathways and where each NBS is scored against every UC on the basis of the ES classes supplied and their amount. The relevance of the second point is also anticipated in Elliot et al. (2019), who review the integration of urban ES modelling within urban metabolism frameworks.

The usefulness of this framework is illustrated by the municipalities of São Paulo (Brazil) and Xalapa (Mexico), two cases that appear in the UC review. These urban areas share all the contextual attributes studied in the UC review (i.e. large metropolitan urban areas, Cfa climate, CELAC countries, upper-medium income countries).

Both cities identify built environment (illustrative UC depicted in Figure 10) as one of their main UC of concern in the reviewed urban planning documents. In a first instance, the framework presents policy-makers and urban planning professionals of both cities with the bundle of ES (third column Figure 10) that they should prioritise to mitigate this UC. It also informs them about the underpinning biophysical and social processes to enhance those ES (fourth column Figure 10) and how they relate to attributes of NBS and the urban context (fifth column Figure 10). These details provide policy-makers and urban planners with supporting information to better frame policies, strategies and interventions, as well as monitoring them. Thus, policy-makers and urban planners could use the UC-ES-NBS network diagram and the NBS classification (Figure 8) in combination to better understand which NBS Type 3 could be more relevant in their specific contexts. For example, they could prioritise a list of suitable NBS based on the dominant urban ecosystems of their municipalities, and spatial levels of proposed interventions. Moreover, in Figure 8 they would be able to see non-physical solutions (NBS Type 1 and 2) applicable to selected NBS Type 3 or dominant natural ecosystems of their cities. Policy makers and urban planners might further investigate these non-physical solutions and if they find strong evidence of their value, include them as a complement in policies, strategies and interventions. This alternative might be especially relevant when the feasibility of extensive and intensive physical modifications is limited by space or budget.

Conclusion

This paper identifies and visualises the nexuses between urban challenges (UC), ecosystem services (ES) and nature-based solutions (NBS), in order to support the mainstreaming of NBS in urban policies and sustainable and resilient urban planning interventions and strategies.

The review of UC literature from science, policy and local urban planning perspectives allowed us to identify and classify 18 UC and 58 sub-challenges for sustainability and resilience. This classification, and its equivalent for NBS, could be further improved and used as a basis for future studies aimed at identifying quantitative causal relationships among UC, ES and NBS. The results show that all the above-mentioned perspectives focus similarly on some UC in particular, namely built environment issues, physical health, green and circular economy, climate change, water management, and material and solid waste. The local urban planning literature is the one reiterating more social-led challenges (e.g. social vulnerability), showing more concern about this kind of UC.

Additionally, several UC (e.g. socio-spatial equity) are stressed differently depending on the specific urban contextual classes (e.g. large metropolitan areas). In the future, further characterisation of UC with respect to their specific urban conditions can be useful for identifying which challenges arise mainly in urban areas sharing specific contextual attributes. By doing this, these cities can join forces when researching potential solutions and better understand whether certain contextual attributes drive the emergence of specific UC.

The reviews conducted in this paper have permitted us to advance in the classification of NBS types starting from the three broad types defined by Eggermont et al. (2015). More specifically, the overlap between specific NBS Types 1, 2 and 3 has been shown explicitly with the interconnections in Figure 8. Furthermore, frequently mentioned ecological concepts used in the urban ES literature (e.g. sustainable urban drainage systems, ecosystem-based approach) have first been identified and then positioned under the NBS umbrella as part of its three broad types.

From the review of urban ES literature, specific nexuses between ES classes, UC and NBS have been identified. The type of relationship between the most frequently mentioned UC, groups of ES and NBS have been discussed, depicting one group with direct plausible causal relationships (albeit qualitative). In addition, social and biophysical attributes, and processes influencing 10 ES have been made explicit, including some of their feedback relationships. These results inform which attributes (of NBS and their contexts) need to be assessed when implementing NBS for the supply of specific ES. Socio-ecological processes are shared among different ES reinforcing the importance to account for ES in bundles. As a main output, a network of relationships among UC, ES, processes, attributes and NBS has been generated and exemplified for one UC in Figure 10. The complete network of relationships can be visualised in the files included in the Supplementary Material F. This UC-ES-NBS network can be used to provide qualitative advise on urban policies, strategies and interventions that intend to make use of NBS.

Further research is needed to move from this qualitative network towards a quantification of the impact of NBS in the supply of specific ES, and the contribution of these ES to mitigate or overcome specific UC. For example, starting from this preliminary work, NBS models (e.g. mechanistic models, system dynamic models) could be developed to quantify the supply of ES classes in bundles (some examples already exist). These models could depict quantitative cause-effect relationships between those ES and NBS, and thus advise on the effectiveness of NBS to address UC. The latter outputs would be useful to further refine urban policies supporting the mainstreaming of NBS.

Additional Information

For additional information about the review, the Supplementary Material G contains the data collected in the systematic UC literature review (Spreadsheet 01) and ES literature review (Spreadsheet 02).

Mobility

To achieve an adequate and sustainable urban mobility system (vehicular and soft) that fulfils the needs of urban inhabitants. The challenge include as sub-challenges the main associated problematics that appear in the review.

Ageing & Inadequate Infrastructure To mitigate or overcome the ageing and inadequate planning and management of transport infrastructure.

Inadequate Public Transport

To mitigate or overcome the inadequate planning and management of public transport services.

Inadequate Non-Motorised Transport Systems

To enhance the planning and management of soft mobility infrastructure network in urban areas.

Physical Health

To achieve a state of complete physical health that goes beyond the absence of disease by mitigating urban environmental conditions that could jeopardise it. In this sense, the challenge focuses in the lack of air, water and soil quality affecting physical health. Lack of access to active leisure opportunities was allocated as part of mental health despite it also has implications for physical health.

Lack of Air Quality

To improve the air quality in the urban area by planning and management of its physical spaces and urban activities Lack of Soil Quality To improve the soil quality in the urban area by planning and management of its physical spaces and urban activities Lack/Deficient Sanitation Systems & Water Quality

To improve the water quality in the urban area by planning and management of its physical spaces, water infrastructures and urban activities.

Mental Health

To achieve a state of complete mental well-being in which urban inhabitants are able to fulfil their own potential, cope with normal life stress and are able to contribute to the society [START_REF] Who | Mental health: a state of well-being[END_REF]. The challenge include as sub-challenges the main associated problematics that appear in the review.

Urban Stress

To mitigate the psychological stress of inhabitants through the planning, management of physical spaces and the urban activities.

Lack of Education and Training

To mitigate access or supply of education and training along the whole life of inhabitants to permit their constant professional and personal progress ensuring also their mental health.

Lack of Culture & Leisure Opportunities

To mitigate the lack of cultural and leisure services in the city that provides relax and enjoyment of people, ensuring their mental health.

Green and Circular Economy

To achieve an economic system were products and services followed closed loops, ensuring optimal use of them , refurbishing, remanufacturing and recycling (circular), reducing environmental risks and ecological scarcities and providing increased well-being [START_REF] Kraaijenhagen | Circular business: collaborate and circulate[END_REF]Becker, P., 2014). The challenge include as sub-challenges the main problematics that appear associated in the literature review, but also typical economic problematics in urban areas such as job development.

Economic Efficiency and Competitiveness

The improvement of economic performance of the urban activities and their competitiveness against the ones of other urban areas.

Lack of Economic Diversification

Lack of variety in the main economic activities of an urban area, being dependent on few sectors, what could increase the risk of economic failure and difficult recovery when changes in the national or global economy occur. Employment (Job) Development Problematics associated to the lack of employment, insufficient increase or its quality.

Innovation and Green Entrepreneurship Problematics or lack of economic innovation and transitioning towards a green and circular economy.

Climate Change

To mitigate or adapt to the change of climate attributed directly or indirectly to human activities, which alters the composition of the global atmosphere (UNFCCC, 1994, Art.1). The challenge includes as sub-challenge the cause (anthropogenic greenhouse gas emissions), the consequence due to the associated increase of global warming, such as intensification of heatwaves and urban heat island effect, and the raise of sea level. Indirect consequences such as intensification of natural disasters are not included under this challenge, since they will still occur without climate change. Green House Gas Emissions Anthropogenic greenhouse gas emissions due to urban activities Urban Heat Island & Heatwaves Increased temperature in urban areas and short-term peaks of high temperature levels that could affect negatively people and urban activities.

Sea Level Rise & Coastal Resilience

Modification of sea levels and coastal water behaviours affecting current coastal functions in urban areas.

Water Management

To enhance the planning, development, and management of water resources in urban areas, including wastewater and the control of risks. The challenge include as subchallenges the main associated problematics that appear in the literature review.

Flooding Risk

To reduce flooding risk consequences of natural events influenced or not by human activities and improve the resilience against flooding events Freshwater Shortage

To mitigate water scarcity in urban areas for all kinds of activities due to lack of access to it or problems with the infrastructure.

Wastewater Management

To optimise of wastewater management, including its depuration/purification and re-use for adequate urban activities.

Material & Solid Waste Management

To enhance the planning, development and management of materials in urban areas, including the reduction of solid waste due to an improvement in waste management. The challenge include as sub-challenges the main associated problematics that appear in the review.

Raw Material Shortage

To mitigate the scarcity of raw materials in urban areas for all kind of activities due to lack of local production or access to sources.

Food Shortage

To mitigate the scarcity of food provision in urban areas for inhabitants due to lack of local production.

Solid Waste Management

To optimise the management of solid waste through an optimisation of the life of products and re-use of waste for new products, services or food production (only organic matter).

Energy

To optimise the generation, management and use of energy in urban areas, including excessive dependency on one type of source, ensuring redundancy of sources Energy Efficiency

To optimise the loss of energy due to inadequate infrastructure and building systems Increased Energy Demand To mitigate the increase of energy demand by users by optimising the use of energy or reducing the need of it.

Lack of Diversification of Energy sources (Renewable Energy)

To increase the variation in types of energy sources ensuring redundancy of sources.

Biodiversity

To mitigate the loss and decline of species population in urban areas and surroundings or the causes that lead to their decline. The challenge include as sub-challenges the main associated problematics that appear in the review

Loss and Degradation of Habitats

To mitigate the reduction and degradation of viable habitat in urban areas.

Lack of Ecological Integrity and Connectivity

To mitigate the loss of naturalness and complexity of habitats and ensure their feasible connectivity between urban habitat patches for the species using them.

Digital Connectivity

To enhance the access to telephone, internet and the public data generated from these networks to improve social connectivity through the digital network. The challenge include as sub-challenges the main associated problematics that appear in the review Unreliable Digital Infrastructure Lack of access or permanent connection to internet or telephone networks Insufficient Public Access to Open Data Lack of access to the data generated by the society through the use of the digital network.

Public Participation

To enhance the inclusion of citizens, private/semi-private/public associations and NGO's in the decision-making of urban areas. The challenge include as sub-challenges the main associated problematics that appear in the review Empower Communities in Decisionmaking To increase the presence of local communities and their representatives in the decisionmaking of urban areas

Promote Stakeholder Involvement

To increase the active involvement of all type of stakeholders in the decision-making of urban areas

Governance

To increase the efficiency, collaboration, and share of information of governance institutions to improve development and management of current and future urban initiatives. The challenge include as sub-challenges the main associated problematics that appear in the review

Transparency

To enhance the access to governance data in terms of initiatives, budgets, expenses, etc. by the public.

Relationship Private Stakeholders-Government

To enhance the collaboration between public and private institutions in the development and management of urban initiatives

Collaborative Multi-level governance

To reduce the silos behaviour of governance institutions and ensure the share of information between them and the collaboration among different administrative levels entities.

Empower Local Representatives

To increase the presence and value of local representatives in governance decisions that affect the areas that they represent.

Expenditure

To increase the efficiency in the management of economic resources for urban initiatives and services. The challenge include as sub-challenges the main associated problematics that appear in the review Expenditure & Debt Management

To increase the efficiency in the use of funds for urban initiatives and services.

Use of Taxes & Financial Autonomy

To enhance the use of taxes towards more sustainable performance of urban areas and ensure enough financial autonomy of cities to allow capacity to pursues local initiatives. Even if a solution based on nature was not defined as an NBS, it was recorded. Later it was checked if the solution could be considered an NBS before including it as a final record.

ES literature

Text (LaPoint et al., 2015;Mörtberg & Wallentinus, 2000;Rayfield et al., 2011) Fragmentation of habitat dimension of vegetation patches, number of vegetation patches, distances between vegetation patches, presence of barriers, suitability of surrounding patches for movement (Gurrutxaga et al., 2010;Kindlmann & Burel, 2008;LaPoint et al., 2015;Mörtberg & Wallentinus, 2000;Rayfield et al., 2011;Saura & Rubio, 2010) Cultural Characteristic of living systems enabling activities promoting health or enjoyment Accessibility (visual, physical, legal) Presence of obstacles (e.g. fences, dense vegetation), Public access allowance, Proximity (Kaczynski et al., 2008(Kaczynski et al., , 2009;;Larondelle & Haase, 2013;Mak et al., 2017;Roberts et al., 2017;Schipperijn et al., 2010Schipperijn et al., , 2013;;Szücs et al., 2015) Provision of recreational infrastructure Footpaths and cycling routes, Plant diversity adjacent to paths, Lighting along path, Sport facilities, Benches, Pleasant views, Wooded areas, Water features, Parking lots (Mak et al., 2017;Roberts et al., 2017;Schipperijn et al., 2013;Sikorska et al., 2017) Perception of safety lack of traffic, availability of walking facilities (Degenhardt et al., 2011;Roberts et al., 2017)

Social characteristics

People's age, Health, Level of education (Kaczynski et al., 2009;Schipperijn et al., 2013 (Andersson-Sköld et al., 2018;Brill et al., 2017;Fry et al., 2009;Ode et al., 2008;Szücs et al., 2015) Provision Cultivated plants

Vegetation growth

Growing season, Species composition and density, Plant health condition, Light conditions, Landscape (vegetation and soil) management, Dimension of tree, Wood density (only for Trees) (Dusza et al. 2017, Schelhaas et al. 2004;Hutyra et al. 2011;Strohbach et al. 2012;Vaccari et al. 2013;Nowak et al. 2013;Timilsina et al. 2014;Tao et al. 2015;Kim et al. 2016;Boukili et al. 2017;Manolaki and Vogiatzakis 2017;Reynolds et al. 2017)

Pollination

Abundance and distribution of pollinators, Availability of forage and nesting habitat, land management (Davies et al., 2017;Grafius et al., 2016;Lowenstein et al., 2015) Figure

Figure 1 .

 1 Figure 1. Conceptual diagram for the identification of UC-ES-NBS nexuses and plausible causal relationships, making explicit the factors, attributes and processes that define the latter; UC=Urban Challenge(s); ES=Ecosystem Service(s); NBS=Nature-based solution(s).

Figure 3 .

 3 Figure 3. Search string for the UC and ES literature review

Figure 4 .

 4 Figure 4. Conceptualisation of NBS types. Elements and their positioning in an X-Y diagram built on the framework of Eggermont et al. (2015) and IUCN (2016).

  between continents. Likewise, only four climatic classes (Tropical Savannah with dry winter (Aw); Temperate without dry season and with hot summer (Cfa); Temperate without dry season and with warm summer (Cfb) and Mediterranean hot summer (Csa)) and three GNI classes (high income, upper-medium income and lower medium income) were considered, since the number of studies for other classes was negligible.

Figure 5 .

 5 Figure 5. Location of the case studies in the reviewed documents

  : UP = Urban Planning Reports; SA-UC = Scientific Articles from UC review; PI/WG = Public Institution and Working Groups Reports; SA-ES = Scientific Articles from ES review. Papers per type of document >≈50% <10%

  Notes: Aw = Tropical Savannah with dry winter; Cfa = Temperate climate without dry season and with hot summer; Cfb = Temperate climate without dry season and with warm summer; Csa = Mediterranean hot summer; CELAC = Community of Latin American and Caribbean States; R.Am = Rest of America; Eur. = Europe; LMA = Large Metropolitan Areas (>1.5 Million inhabitants); MA = Metropolitan Areas (500.000-1.5 Million inhabitants); MC = Medium Cities (200.000-500.000 inhabitants); SC = Small Cities (200.000-50.000 inhabitants); HI = Higher income class country; UMI = Upper-medium income class country *As stated in Section 4, not all the classes of each contextual attribute are included because in some cases the number of studies was considered too low to be relevant. Case studies per contextual class >≈50% <10%

  = Tropical Savannah with dry winter; Cfa = Temperate climate without dry season and with hot summer; Cfb = Temperate climate without dry season and with warm summer; Csa = Mediterranean hot summer; CELAC = Community of Latin American and Caribbean States; R.Am = Rest of America; Eur. = Europe; LMA = Large Metropolitan Areas (>1.5 Million inhabitants); MA = Metropolitan Areas (500.000-1.5 Million inhabitants); MC = Medium Cities (200.000-500.000 inhabitants); SC = Small Cities (200.000-50.000 inhabitants); HI = Higher income class country; UMI = Upper-medium income class country *As stated in Section 4, not all the classes of each contextual attribute are included because in some cases the number of studies was considered too low to be relevant.

Figure 6 .

 6 Figure 6. Visualisation of UC-ES nexuses by edges (links weighted by number of cases) and nodes (circles weighted by the number of ES and cases per ES). Urban challenges are written in bold.

4. 3 .

 3 Limiting factors and UC-ES-NBS causal relationship groupsThe results from the above sections, together with the characterisation of the nature of limiting factors (e.g. biophysical, technological) driving specific UC provide the basis to differentiate groups of UC-ES-NBS relationships (Figure7), identifying when direct plausible causal relationship between UC-ES-NBS exist.

Figure 7 .

 7 Figure 7. Scheme of the four types of causal relations among UC, ES and NBS; UC=Urban Challenge(s); ES=Ecosystem Service(s); NBS=nature-based solution(s).

Figure 8a .

 8a Figure 8a. Built structure and water NBS Types 1, 2 and 3 (as described in Figure 4) corresponding to MAES Level 1 ecosystem types. NBS Type 2 are differentiated in actions that apply to the abiotic and the biotic factors of NBS. The spatial scales refer only to NBS Type 3.

Figure 8b .

 8b Figure 8b. Land NBS Types 1, 2 and 3 (as described in Figure 4) corresponding to MAES Level 1 ecosystem types. NBS Type 2 are differentiated in actions that apply to the abiotic and the biotic factors of NBS. The spatial scales refer only to NBS Type 3.

Figure 9 .

 9 Figure 9. NBS (Type 3)-ES links identified in the ES review. Edges (links weighted by number of case studies) and Nodes (circles weighted by number of ES (or NBS) and number of case studies per ES).

Figure 9

 9 also presents ES classes rarely assessed in the case studies, which do not appear in Figure6(i.e. drinking water, pollination, filtration (soil), decomposition (soil), weathering processes, noise reduction, cultivated crops for manufacturing purposes and cultivated crops for energy). The differences between both figures occur because 55 of the 170 papers in the ES literature review do not clarify the specific UC that could be addressed by the specific ES studied therein. Consequently, links with UC are not established for all the ES classes identified in the literature review. The review ofHaase et al. (2014) also shows the scarce consideration of many of the abovementioned ES classes in urban studies, making it less likely that this outcome is the result of an unnoticed bias in the current review. It might be that the roles of those ES classes in urban areas are not clear enough. A reason that could also explain why in this review clear connections with specific UC are not found for many of those ES classes.

638 4 .

 4 7. UC-ES-NBS FrameworkOur results were used to create a network diagram outlining all the UC-ES-NBS relationships observed.An illustrative example of this diagram is shown in Figure10for the relationships of the UC Built Environment. Further information for the remaining UC-ES-NBS relationships is included in Supplementary Materials E and F. Supplementary material F includes two files containing the UC-ES-NBS network in the form of a set of graphs. These graphs are constructed in Gephi(Bastian et al., 2009), a graph-based software allowing a more complete visualisation of the network.

Figure 10 .

 10 Figure 10. Illustrative diagram of Contextual attributes-UC-ES-Process-Attributes-NBS Network for the UC Built Environment. a) Diagram of the complete network, only relationships of the Built Environment are in colour; b) Zoom to the Built Environment Network Diagram. Arrows are non-weighted to facilitate visualisation.

  a city specific focus the population of the urban area (metropolitan region) mentioned in the study. Organised according to a modification of Apte et al. (2013). Population obtained from United of ES Section assessed in the case study following the CICES classification Text: Provisioning; Regulation; Cultural ES Classes Specific Type of ES class assessed in the study. If more than one ES is assessed all of them are recorded. Text: e.g. Hydrological and water flow regulation Type of ecological concept The type of ecological concept used in the paper to refer to solutions based or relying on natural features. Text: e.g. green Infrastructure, ecosystem based adaptation solutions, service providing units, nature-based solution, sustainable urban drainage systems, etc Type of urban NBSThe specific type of NBS evaluated as part of the study.

  axes, View-leading elements, Ratio between open spaces and forests, Existence of mature forests, Existence of fruit trees, Amount of natural vegetation, Amount of naturalised waterbodies, Diversity of natural features, Presence of landmarks or iconic elements of interest (natural and human-made), Land management Type and frequency, Number of disturbing elements, Shape diversity, Patch and edge attributes, and Seasonal changes in vegetation and water features

  Figure E2. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the UC Climate change. Only specific set of NBS that relates to all the ES classes associated with Climate Change are included.

Figure E3 .

 E3 Figure E3. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the sub-challenge Urban heat island & Heatwaves. Only specific set of NBS that relates to all the ES classes associated with Urban heat island & Heatwaves are included.

Figure E4 .

 E4 Figure E4. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the UC Energy. Only specific set of NBS that relates to all the ES classes associated with Energy are included.

Figure E5 .

 E5 Figure E5. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the UC Mental Health. All the NBS are included because there was no specific set of NBS that relates to all the ES classes associated with Mental Health.

Figure E6 .

 E6 Figure E6. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the UC Physical Health. All the NBS are included because there was no specific set of NBS that relates to all the ES classes associated with Physical Health.

Figure E7 .

 E7 Figure E7. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the sub-challenge Lack of water quality. Only specific set of NBS that relates to all the ES classes associated with Lack of water quality are included.

Figure E8 .

 E8 Figure E8. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the UC Water Management. Only specific set of NBS that relates to all the ES classes associated with Water Management are included.

Figure E9 .

 E9 Figure E9. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the sub-challenge Freshwater shortage. Only specific set of NBS that relates to all the ES classes associated with Freshwater shortage are included.

Figure E10 .

 E10 Figure E10. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the sub-challenge Wastewater Management. Only specific set of NBS that relates to all the ES classes associated with Wastewater Management are included.

Figure E11 .

 E11 Figure E11. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the sub-challenge Flooding risk. Only specific set of NBS that relates to all the ES classes associated with Flooding risk are included.

Figure E12 .

 E12 Figure E12. Illustrative UC-ES-Process-Attributes-NBS Network Diagram of the sub-challenge Lack of liveable and adaptable Public Spaces. Only specific set of NBS that relates to all the ES classes associated with Lack of liveable and adaptable Public Spaces. are included.

Table 1 .

 1 Urban Challenges and sub-challenges considered in the literature review

	Urban Challenges and sub-challenges	UP	SA-UC	PI/WG	SA-ES
	Total Number of Documents	87	29	18	178
	Socio-spatial Equity	42	10		8
	Socio-spatial Segregation	7	3		0
	Lack of Gender Equity	2	0		0
	Lack of Age Equity	1	0		0
	Lack of Racial Equity	6	0		0
	Lack of Income Class Equity	11	0		2
	Social Cohesion	29	6		3
	Lack of Inclusion of Immigrants and Refugees	3	1		0
	Lack of Community-Building	8	3		0
	Lack of Public Spaces for Social Interaction	6	1		0
	Social Vulnerability	65	5		11
	Vulnerability to Human/Natural Disasters	49	1		10
	Vulnerability to Disease Outbreak	19	0		0
	Vulnerability to Terrorism	7	0		0
	Urban Violence & Insecurity	28	1		1
	Demographic Dynamics	15	7		1
	Increasing Population	9	4		1
	Decreasing Population	1	3		0
	Population Displacement	3	0		0
	Ageing Population	5	1		0
	Built Environment	48	15	11	5
	Preservation of the Cultural Heritage and Identity	8	2		0
	Affordable Housing	17	3		0
	Lack of Liveable and Adaptable Public Spaces	16	4		4
	Urban Sprawl	6	6		0
	Informal Settlements	2	4		0
	Mobility	55	10	11	0
	Inadequate Public Transport	11	0		0
	Ageing & Inadequate Infrastructure	30	5		0
	Inadequate Non-Motorised Transport Systems	6	1		0
	Physical Health	46	19		66
	Lack of Air Quality	22	8		8
	Lack of Soil Quality	2	1		3
	Lack/Deficient Sanitation Systems & Water Quality	26	9		11
	Mental Health	33	9		50
	Urban Stress & Lack of Psychological Relaxation	4	0		3
	Lack of Education and Training	24	8		0
	Lack of Cultural & Leisure Opportunities	1	1		1

Notes: UP = Urban Planning Reports; SA-UC = Scientific Articles from UC review; PI/WG = Public Institution and Working Groups Reports; SA-ES = Scientific Articles from ES review. Papers per type of document >≈50% <10%

Table 1 (

 1 Continued). Urban Challenges and sub-challenges considered in the literature review

	Urban Challenges and sub-challenges	UP	SA	PI/WG	SA-ES
	Total Number of Documents	87	29	18	178
	Green and Circular Economy	58	15	12	2
	Economic Efficiency and Competitiveness	14	1		0
	Lack of Economic Diversification	8	1		0
	Employment (Job) Development	31	10		0
	Innovation and Green Entrepreneurship	16	0		0
	Climate Change	52	13		52
	Greenhouse Gas Emissions	4	2		1
	Urban Heat Island & Heatwaves	21	6		31
	Sea Level Rise & Coastal Resilience	16	0		1
	Water Management	59	11		45
	Flooding Risk	27	3		32
	Freshwater Shortage	33	7		11
	Wastewater Management	26	4		11
	Material & Solid Waste Management	43	16		6
	Raw Material Shortage	8	2		0
	Food Shortage	16	4		4
	Solid Waste Management	31	10		2
	Energy	31	9		10
	Energy Efficiency	9	5		3
	Increased Energy Demand	7	2		2
	Lack of Diversification of Energy sources (Renewable Energy)	12	0		0
	Biodiversity	16	9		18
	Loss and Degradation of Habitats	10	3		7
	Lack of Ecological Integrity and Connectivity	4	0		1
	Digital Connectivity	21	3		0
	Unreliable Digital Infrastructure	3	0		0
	Insufficient Public Access to Open Data	2	0		0
	Public Participation	31	4		0
	Empowerment of Communities in Decision-making	14	0		0
	Promotion of Stakeholder Involvement	16	1		0
	Governance	39	5	10	0
	Transparency	11	1		0
	Relationship Private Stakeholders -Government	7	1		0
	Collaborative Multi-level Governance	6	2		0
	Empowerment of Local Representatives

Table 2 .

 2 Urban (sub)challenges most frequently assessed per contextual class in the UC literature review. Bold numbers in a cell indicate that more than 50% of the case studies including a specific (sub)challenge come from that specific class.

	Urban Challenges and sub-challenges	No.	Climate Classes* Aw Cfa Cfb Csa R.Am Asia CELAC Eur LMA MA MC SC VSC HI Continents* Size of Urban Areas GNI Classes* UMI LMI
	Cases per Contextual Class		16	21	39	9	20	18	45	47	49	30	20	17	10	77	42	11
	Socio-spatial Equity	59	10	9	7	1	15	7	21	7	23	11	7		0	26	17	5
	Social Cohesion	47	1	6	12	4	11	4	6	18	16	10	5		1	29	6	2
	Lack of Inclusion of Immigrants and Refugees	7	0	2	1	0	3	0	0	4	3	1	0		0	4	0	0
	Social Vulnerability	75	10	15	12	4	14	10	33	6	33	16	10		2	33	27	7
	Vulnerability to Natural Disasters	53	9	11	7	4	7	7	28	3	24	14	6		0	20	22	6
	Vulnerability to Disease Outbreak	19	1	7	5	2	2	4	5	4	13	3	2		0	11	6	1
	Reduction of Urban violence & insecurity	31	7	4	2	2	6	2	20	1	14	5	5		1	9	15	4
	Demographic Dynamics	26	1	5	7	2	2	5	1	8	12	1	4		0	14	3	2
	Built Environment	85	11	11	16	6	12	13	25	25	31	16	12		2	36	27	9
	Affordable Housing	25	1	5	4	1	9	4	2	6	11	2	3		0	17	3	1
	Lack of Liveability & Adaptability of Public Spaces	35	1	6	13	2	6	2	5	20	14	6	7		0	24	6	1
	Urban Sprawl	15	0	1	2	1	0	4	4	4	7	1	0		1	2	7	2
	Mobility	83	10	11	18	5	14	12	29	18	24	18	8		10	36	28	6
	Ageing & Inadequate Infrastructure	38	3	6	4	5	11	11	4	6	20	6	2		1	19	8	5
	Physical Health	81	9	14	16	4	10	15	24	22	28	15	7		9	39	20	10
	Lack of Air Quality	42	4	6	13	3	6	5	12	17	16	5	2		9	23	10	3
	Lack/Deficient Sanitation Systems & Water Quality	44	5	6	12	2	5	9	14	13	16	7	4		9	24	11	6
	Mental Health	62	6	9	23	2	9	6	18	25	13	15	11		8	38	14	5
	Improved Education & Training	47	5	7	18	1	8	4	14	21	9	12	9		8	30	12	3
	Green and Circular Economy	97	12	11	26	6	11	12	27	33	28	17	14		10	50	23	6
	Enhancement of Economic Efficiency & Competitiveness	16	7	0	2	1	0	1	13	1	1	6	3		2	2	10	2
	Employment (Job) Development	61	8	6	21	3	8	4	14	28	15	11	10		9	37	10	4
	Innovation and Green Entrepreneurship	16	0	9	4	0	3	5	4	2	10	4	2		0	12	3	1
	Climate Change	81	7	12	22	3	15	8	18	27	34	11	7		9	49	15	6
	Urban Heat Island & Heatwaves	28	0	9	5	4	6	5	4	9	19	3	1		0	19	6	2
	Sea Level Rise & Coastal Resilience	17	1	7	2	1	5	3	2	3	11	2	2		0	11	2	2

Table 2 (

 2 Continued). Urban (sub)challenges most frequently assessed per contextual class in the UC literature review. Bold numbers in a cell indicates that more than 50% of the cases including a specific (sub)challenge come from that specific class.

	Urban Challenges and sub-challenges	No.	Climate classes* Aw Cfa Cfb Csa R.Am Asia CELAC Eur LMA MA MC SC VSC HI Continents* Size of Urban Areas GNI Classes* UMI LMI
	Cases per Contextual Class		16	21	39	9	20	18	45	47	49	30	20	17	10	77	42	11
	Water Management	85	10	17	13	3	12	15	29	18	35	17	13	8	1	41	27	9
	Flooding Risk	36	2	11	9	3	9	6	7	14	25	5	6	3	1	28	10	2
	Freshwater Shortage	42	5	6	5	2	5	9	15	5	20	11	3	4	0	17	13	8
	Wastewater Management	33	7	4	3	2	4	4	19	3	12	9	5	5	0	11	14	5
	Material Solid Waste	81	7	11	21	5	9	9	23	24	26	13	10	9	9	42	20	6
	Food Shortage	28	0	5	8	3	6	2	1	10	12	4	4	3	0	19	2	1
	Solid Waste Management	53	7	7	13	3	5	8	22	13	16	9	6	7	9	24	17	5
	Energy	56	1	7	18	4	8	8	7	22	20	7	3	5	10	34	8	1
	Enhance Energy Efficiency	23	0	2	12	1	4	3	1	12	6	3	1	2	8	16	2	1
	Biodiversity	42	2	4	18	5	5	7	5	21	11	5	5	5	10	29	5	3
	Digital Connectivity	36	5	4	12	3	5	5	8	15	7	5	5	5	9	20	6	4
	Public Participation	45	5	7	15	4	6	7	13	16	14	10	5	4	9	27	11	3
	Empower Communities in Decision-making	14	4	3	2	2	2	4	6	1	4	5	1	3	1	5	7	1
	Promote Stakeholder Involvement	18	0	3	3	4	4	4	4	4	9	4	2	1	1	11	4	1
	Governance	59	10	5	8	3	8	4	25	13	13	14	9	11	0	23	17	6
	Expenditure	27	7	5	2	0	3	1	20	1	3	11	4	4	1	7	14	2
	Expenditure & Debt
	Management

  In addition, several ES classes of CICES v5.1 are not identified (e.g. visual screening and smell reduction) or appear very rarely (e.g. noise attenuation, weathering processes). A few of these ES classes, such as smell reduction, are not identified as independent classes either in CICES before version 5.1 or in other classifications (e.g. TEEB), which could explain the lack of related case studies. Furthermore, the assessment of some ES classes (e.g. bioremediation) is quite specific (i.e. pollutant by pollutant) and technically complex (e.g. analyses that require several years of experimentation in the field and lab testing). The lack of identification of these ES classes might also indicate that they have minor or no relevance to address UC or that urban NBS do not have the capacity to supply them. For the remaining ES, the review suggests multiple links occurring with UC for each ES, as visualised in Figure6. The above results might help policy-makers to frame their urban

agendas, prioritising the supply of those ES classes for which at least a nexus with a UC has been identified reiteratively by academic papers.

The visualisation of the UC-ES nexuses shows that physical health and mental health are not only the most frequently mentioned UC, as shown in Table

1

, but also are the ones with the highest number of links to different ES, including classes from all ES sections. The UC climate change and the sub-challenges urban heat island effect and heatwaves are the only ones for which nexuses are mostly concentrated in few regulating ES. Overall, the most frequently mentioned UC show numerous links with the regulation of chemical condition of the atmosphere, regulation of temperature and humidity, hydrological cycle and water flow regulation and filtration/sequestration/storage of pollutant (air or water). Additionally, physical and mental health, biodiversity, built environment, lack of adaptable and green spaces, social vulnerability, vulnerability to disasters and socio-spatial equity appear several times in relation to the characteristics of living systems that enable activities promoting health.

Table 3 .

 3 Processes and factors per ES class identified in the systematic and non-systematic literature review. Blue cells identify processes already described in an upper ES or an ES that influences the ES class analysed (references per process included in a simplified table in Supplementary Material D)

	ES Section	ES Classes	Related Processes (conditions and other ES)	Related Attributes and Associated Processes

Vegetation growth, Soil respiration, Dimension of trees, Species wood density, Carbon fraction of dry biomass (species specific), Landscape (vegetation and soil) management.

dry deposition of PM, Precipitation

  Words in italics are for processes, factors and references identified through the non-systematic review

			Vegetation growth, Leaf area, Soil moisture, Growing season, Light condition,
		Pollutants' plant uptake	Health of trees, Stomatal resistance, Aerodynamic resistance, Quasi-laminar boundary layer resistance, Canopy resistance (stomatal resistance, mesophyll
			resistance, cuticular resistance and soil resistance), Concentration of CO2
		Biological emission of	Vegetation growth, Total leaf biomass, Plant diversity, Plant composition,
		particulates (including pollen)	Type of pollination, Duration of pollen season, Height of plant
		Vegetation growth	-
	Filtration,	Dry deposition (on water)	Deposition velocity
	sequestration,	Settling (sedimentation)	Roughness of land surface
	storage,	Adsorption by sediments	Not identified
	accumulation by	Re-suspension (from water)	Not identified
	microorg., algae, plants (water) / Regulation of the chemical condition	Nitrification Denitrification	Temperature, Soil moisture Temperature, Dissolved oxygen, Water saturation, Dissolved and particulate organic carbon, Soil depth, Soil permeability, pH, Soil cover
	of freshwaters by	Ammonification	Temperature, pH, C/N ratio, Available nutrients, Soil texture, Soil permeability
	living processes	Mineralisation	Temperature, Soil moisture
	Note:

Table 3 .

 3 Processes and factors per ES class identified in the systematic and non-systematic literature review. Blue cells identify processes already described in an upper ES or an ES that influences the ES class analysed (references per process included in a simplified table in Supplementary Material D)

	ES Section	ES Classes	Related Processes (Conditions and other ES)	Related Attributes
			Plant & microbial uptake (and	Vegetation growth, Plant capacity to absorb inorganic nitrogen (N), Rate of
		Filtration, sequestration,	immobilisation) Organic matter decomposition	conversion of N into biomass, Bioavailability of N, Temperature, Soil moisture -
		storage, accumulation by	Leaching	Plant & microbial uptake, Adsorption, Soil sorption, Infiltration, Soil depth, Soil nutrients
		microorganisms,	Volatilisation	Not identified
		algae, plants (water)	Soil sorption	Amount of organic matter, Degradability of the pollutant, Temperature, Precipitation
		/ Regulation of	Vegetation growth	-
		the chemical	Erosion regulation (ES)	-
		condition of	Water run-off	-
		freshwaters by	Groundwater lateral flow	-
		living processes	Infiltration	-
			Percolation (deep infiltration)	-
				Vegetation growth, Soil storage, Hyperaccumulation capacity of plant, Plant
	Regulation	Bioremediation	Phytoextraction (plant & microbial uptake) Phytodegradation	biomass, Planting density, Cropping period, Compartmentalisation of pollutants in biomass, Rooting depth, Plant's resistance trait to the specific pollutant, Landscape management, Soil aeration, Presence of macronutrients, Soil microbiological activity, Soil pH Not identified
		by	Rhizofiltration	Vegetation growth, Microbiological activity, Rooting depth, Exhudation
		microorganisms,	Phytostabilisation	Microbiological activity
		algae, plants and	Phytovolatilisation	Vegetation growth, Plant uptake, Evapotranspiration
		animals	Evapotranspiration	-
			Leaching	Phytodegradation, Rhizofiltration, Phytostabilisation, Phytovolatilisation
			Adsorption by soil	-
			Soil sorption	-
			Infiltration	-
			Vegetation growth	-
		Maintaining	Conservation of habitat	Vegetation patches: abundance, richness, distribution& area (single patches)
		nursery		Dimension of vegetation patches, Number of vegetation patches, Distances
		populations and	Movement of species	between vegetation patches, Presence of barriers, Suitability of surrounding
		habitats		patches for movement
	Cultural	Characteristics of living systems enabling activities promoting health or enjoyment Characteristics of living systems enabling aesthetic experiences	Accessibility (visual, physical, legal) Provision of recreational infrastructure Perception of safety Social characteristics Aesthetics (ES) Sensorial perception	Presence of barriers (e.g. fences, dense vegetation), Public access allowance, Proximity Footpaths and cycling routes, Plant diversity adjacent to paths, Lighting along paths, Sport facilities, Benches, Pleasant views, Existence of forests, Water features, Parking lots Lack of traffic, Availability of footpaths People's age, Health, Level of education -Number of view axes, Presence & view of landmarks, Ratio between open spaces and forests, Existence of (mature) forests, Existence of fruit trees, Amount of natural vegetation, Amount of naturalised waterbodies, Diversity of natural features, Landscape management, Number of disturbing elements, Shape diversity, Patch and edge attributes and Seasonal changes in vegetation & water features
	Provision	Plants cultivated for food purposes	Vegetation growth Pollination (ES)	-Abundance and distribution of pollinators, Availability of forage and nesting habitat, Landscape management
	Note: Words in italics are for processes, factors and references identified through the non-systematic review

  : e.g. urban forest, green walls,Pulford and Watson 2003; Gawronski et al. 2011; Singh and Santal 2015) Pulford and Watson 2003, Singh and Santal 2015) EvapotranspirationLand cover type, Tree canopy coverage, Leaf area index, Root depth and distribution, Type of soil substrate, Soil moisture(Pulford and Watson 2003; Singh and Santal 2015)

			Phytodegradation Rhizofiltration	Not identified green roofs, swales, etc Microbiological activity, Rooting depth, Rate of exhudation	(Pulford & Watson, 2003) (Pulford & Watson, 2003)
			Phytostabilisation	Microbiological activity	(Pulford & Watson, 2003)
	Spatial level of the case	The area of intervention considered as part of the Phytovolatilisation (Leaching Text: Urban Region; City/Metropolitan; Plant uptake Not identified (Pulford and Watson 2003; Singh and Santal 2015)
	study	Maintaining nursery	assessment in the case study. abundance of vegetation patches, richness of vegetation patches, distribution and area of Neighbourhood; Building level; N/A Reduction of habitat vegetation patches
		populations and
	Social and biophysical processes habitats	described in regard to the ES assessed are recorded The social and biophysical processes evaluated or	Text: e.g. infiltration
	Social and biophysical attributes	The social and biophysical attributes identified as recorded relevant for processes influencing ES supply assessed are	Text: e.g. leaf area index

(

The first, third, fourth and fifth authors would like to acknowledge funding from the European Union's Horizon 2020 research and innovation programme under the Grant 494 No: 730468 (www.nature4cities.eu). The second author would like to acknowledge funding from the National Research Fund (FNR) of Luxembourg (CORE project "ESTIMUM" -C16/SR/11311935; wwwlistlu/en/project/estimum/) one sustainable plan (Amsterdam) and one climate change plan (Sao Paulo)). Only documents available in the websites of these initiatives and published in English, Italian, Spanish, French or Portuguese were included. If several documents for the same city exist, documents from i) and ii) were prioritised. Regarding

Interamerican Development Bank Action Plans, only the challenges selected as a priority were included in the analysis, since all the documents start from a long predefined list of challenges, but only develop strategies for the ones selected as a priority by each city.

Report Year City Country

Supplementary Material B -Brief description of urban challenges and sub-challenges

Urban challenges and sub-challenges Description

Socio-spatial Equity

To achieve a social equitable distribution of urban services, taking into account different space units and the needs of different economic, racial and political groups [START_REF] Yanhua | Spatial equity measure on urban ecological space layout based on accessibility of socially vulnerable groups-A case study of Changting[END_REF]. The challenge include as sub-challenges the main associated problematics that appear in the review.

Socio-spatial Segregation

To reduce the spatial segregation of people that affect their access to any type of urban services.

Lack of Gender Equity

To reduce the gender inequity regarding access to any type of urban services.

Lack of Age Equity

To reduce the age inequity regarding access to any type of urban services.

Lack of Racial Equity

To reduce the racial inequity regarding access to any type of urban services.

Lack of Income Class Equity

To reduce the inequity associated to the level of income power of inhabitants with respect to their access to any type of urban services.

Social Cohesion

To achieve an urban community that creates sense of belonging, interested in the wellbeing of all its members, avoiding marginalisation (OECD, 2012). The challenge include as sub-challenges the main associated problematics that appear in the review.

Lack of Inclusion of immigrants and refugees

To enhance the inclusion of immigrants of any type in local urban communities, to ensure social cohesion in urban areas.

Lack of Community building

To increase community building activities and their access to any person to enhance social cohesion in urban areas.

Lack of public spaces for social interaction

To increase the provision of public spaces in urban areas to promote the gathering of people to enhance social interactions leading to the enhancement of community building.

Social Vulnerability

To mitigate the vulnerability of society and their economic activities against temporal or permanent risks consequence of direct or indirect human actions as well as natural hazards. The challenge include as sub-challenges the main associated problematics that appear in the review.

Vulnerability to Human/Natural Disasters

To mitigate the vulnerability of people and continuity of their economic activities due to natural disasters or inadequate planning and management of these risks.

Vulnerability to Disease Outbreak

To mitigate the vulnerability of people against disease outbreaks, by improvement of their prevention and management of these events.

Vulnerability to terrorism

To mitigate the vulnerability of people against terrorist attacks, by improvement of their prevention and management of these events.

Urban violence & insecurity

To mitigate urban violence and sense of insecurity associated with it by improvement of the planning and management of urban areas.

Demographic Dynamics

To mitigate the consequences of local changes in the demographic dynamics that could led to unsustainability in the urban services, maintenance of their supply or affect the quality of life in urban areas. The challenge include as sub-challenges the main associated problematics that appear in the review.

Increasing population

To control the increase of population to ensure the adequate provision of services and quality of life in urban areas.

Decreasing population

To control the decrease of population to ensure the adequate provision of services and quality of life in urban areas.

Population displacement

To control the fluxes of population to ensure the adequate provision of services and quality of life in urban areas.

Ageing Population

To mitigate and adapt to the ageing of population to ensure the adequate provision of services and quality of life in urban areas.

Built Environment

To enhance the planning, development and management of built physical structures (excluding mobility) and their combination in the form of urban spaces to ensure good socio-economic conditions of their inhabitants and minimise negative environmental impacts. The challenge include as sub-challenges the main associated problematics that appear in the review.

Preservation of the Cultural Heritage and Identity

To preserve the built cultural heritage in the city and the elements (tangible or not) that form part of the cultural identity of the inhabitants in urban areas.

Affordable Housing

To ensure enough affordable housing for the inhabitants in urban areas.

Lack of Liveable and Adaptable Public Spaces

To mitigate the lack of multifunctional and active public spaces in urban areas that could jeopardise socio-economic activities and the use of public spaces by inhabitants.

Urban Sprawl

To mitigate the excessive and inadequate expansion of urban areas.

Informal Settlements

To overcome the expansion of urban areas as informal settlements with inadequate access to urban services and poor living conditions. Vegetation growth rate, Growing season, Dimension (Volume of tree), Wood density, Species composition and density, Plant health condition, Light conditions, Landscape (vegetation and soil) management, Carbon fraction of dry biomass (species specific) (Boukili et al., 2017;Hutyra et al., 2011;Kim et al., 2016;Manolaki & Vogiatzakis, 2017;Nowak et al., 2013;Reynolds et al., 2017;Schelhaas et al., 2004;Strohbach et al., 2012;Tao et al., 2015;Timilsina et al., 2014;Vaccari et al., 2013) Organic matter decomposition Degradation rates of organic matter types, Land management, Percentage of clay in the soil, percentage of humus, microbiological activity, temperature, soil moisture (Coleman & Jenkinson, 2014b;Farina et al., 2013;Reynolds et al., 2017) Soil respiration soil texture, soil humidity, soil management, microbiota, organic matter inflow (Coleman & Jenkinson, 2014b;Farina et al., 2013;Reynolds et al., 2017) Filtration, sequestration, storage, accumulation by microorganisms, algae, plants (air)

Dry deposition

Tree canopy coverage, Tree Density, Canopy height, Leaf area index, Shape of leaves, Roughness of leaves (due to hair, exhudates), Phenology of Plant, Plant Diversity, Concentration of air pollutants, Deposition velocity, Distance of plants from emission source (Bottalico et al., 2017;Fusaro et al., 2017;Jim & Chen, 2008;Kim et al., 2016;Ortolani & Vitale, 2016;Weber et al., 2014) Re-suspension Wind speed, accumulated PM amount in the leaf surface (Bottalico et al., 2017;Jim & Chen, 2008;Ortolani & Vitale, 2016;Staelens et al., 2017) Washing-off Accumulated PM amount in the leaf surface, deposition of PM, and storage of PM (Jim & Chen, 2008;Staelens et al., 2017) Plant uptake

Leaf area, Soil moisture, Growing season, Photoperiod, Health of trees, stomatal resistance, Aerodynamic resistance, Quasi-laminar boundary layer resistance, Canopy resistance (stomatal resistance, mesophyll resistance, cuticular resistance and soil resistance), Concentration of CO2 (Depietri et al., 2013;Fusaro et al., 2017;Hirabayashi, 2013;Jim & Chen, 2008;Kim et al., 2016;F Manes et al., 2016;Fausto Manes et al., 2012;Sanderson et al., 2007;Selmi et al., 2016) Emission of particulates (including pollen)

Total leaf biomass, Plant species composition and diversity, Type of pollination, Duration of pollen season, Height of Plant (Cariñanos et al., 2017;Jim & Chen, 2008;Reynolds et al., 2017) Filtration, sequestration, storage, accumulation by microorganisms, algae, plants (water) / Regulation of the chemical condition of freshwaters by living processes

Atmospheric deposition

Velocity of deposition (Chavan & Dennett, 2008;de Sosa et al., 2018;Hoang et al., 2017;Kazezyilmaz-Alhan et al., 2007;Lee et al., 2002;Neitsch et al., 2011) Settling (sedimentation/detention) Roughness of land surface [START_REF] Adhikari | Removal of nutrients and metals by constructed and naturally created wetlands in the Las Vegas Valley, Nevada[END_REF]Adyel et al., 2016;Nocco et al., 2016) Adsorption by sediments Not identified [START_REF] Adhikari | Removal of nutrients and metals by constructed and naturally created wetlands in the Las Vegas Valley, Nevada[END_REF]Adyel et al., 2016) Re-suspension - (Adyel et al., 2016;Staelens et al., 2017) Nitrification Temperature, Soil moisture [START_REF] Adhikari | Removal of nutrients and metals by constructed and naturally created wetlands in the Las Vegas Valley, Nevada[END_REF]Adyel et al., 2016;Chavan & Dennett, 2008;Kazezyilmaz-Alhan et al., 2007;Lee et al., 2002;Olguín et al., 2017) Denitrification Temperature, Oxygen concentration, Water saturation, Dissolved and particulate organic carbon, Soil depth, Soil permeability, pH, Vegetation cover, [START_REF] Adhikari | Removal of nutrients and metals by constructed and naturally created wetlands in the Las Vegas Valley, Nevada[END_REF]Adyel et al., 2016;Kazezyilmaz-Alhan et al., 2007;Lee et al., 2002;Olguín et al., 2017) Ammonification Temperature, Ph, C/N ratio, available nutrients, soil texture, soil structure (Adyel et al., 2016;Herath & Vithanage, 2015) Mineralisation Temperature, soil moisture (Adyel et al., 2016;Chavan & Dennett, 2008;Kazezyilmaz-Alhan et al., 2007;Lee et al., 2002) Plant, microbial uptake (and immobilisation)

Plants's capacity of absorption of inorganic nitrogen, Rate of conversion of N into plant biomass, Bioavailability of nitrogen, Temperature, Soil moisture, Growth rate of plants [START_REF] Adhikari | Removal of nutrients and metals by constructed and naturally created wetlands in the Las Vegas Valley, Nevada[END_REF]Kazezyilmaz-Alhan et al., 2007;Lee et al., 2002;Olguín et al., Cabral et al., 2016;De Troyer et al., 2016;Renard et al., 1991) Water run-off - (Hoang et al., 2017;Kazezyilmaz-Alhan et al., 2007;X. Sun et al., 2017) Groundwater lateral flow - (Hoang et al., 2017;Kazezyilmaz-Alhan et al., 2007

Supplementary Material E. Visualisation of UC-ES-NBS Network per urban challenge (UC)

This supplementary material include UC-ES-Process-Attributes Network diagram for each challenge and subchallenges for which direct causal relationships with ES are found. Built Environment is not included since it is illustrated in the manuscript. For access to the original Gephi files, download Supplementary Material G.

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