Mapping the Research Landscape of Nature-Based Solutions in Urbanism

: Nature-based solutions (NBS) are an innovative concept that mimics the processes of natural ecosystems, popularized principally in the European Union. With a substantial body of literature amassed since the term’s inception in 2015, there is a need to systematically review existing literature to identify overarching gaps and trends, according to disciplinary focus, geographic scope, and key themes, and direct future research inquiry and policy recommendations. This review consists of bibliometric analysis and thematic analysis for NBS studies in urbanism. NBS studies were found to relate strongly with other concepts of ‘Ecosystem Services’, ‘Green Infrastructure’, ‘Climate Change’, and ‘Risk management and Resilience’, which align with four major thematic goals set by the European Commission. Within NBS scholarship, various sub-themes have emerged, namely, ‘Greening’, ‘Urban Development’, ‘Water’, ‘Wellbeing’, and ‘Governance’. Furthermore, we illustrate that the amount and thematic focus of NBS research have been unevenly distributed worldwide. Analysis of emerging trends shows a recent increase in topics, such as adaptive governance of NBS, and the incorporation of social justice in sustainability transitions. Based on an assessment of extant NBS literature, we offer some recommendations for the future direction of the research ﬁelds.


Introduction
Rapid urbanization and climate change are two global trends that are predicted to shape society and the biosphere in the coming decades [1]. Planning and designing cities to address climate change risks (e.g., floods and heatwaves) and social challenges (e.g., public health stress, poverty, and inequity), has become increasingly important. In response, scholars and practitioners have proposed that protecting and restoring ecosystems and enhancing ecosystem services provision could be an effective way to combat such challenges. Consequently, ecosystem-based management has become increasingly important in climate change policy and has contributed to the recent emergence of nature-based solutions (NBS). Since the concepts of sustainability, biodiversity, ecosystem services, and climate change were popular scientific research areas, the NBS concept was proposed with the European Union Horizon 2020 call in 2015. Although it had been presented in academic literature from the mid-2000s, as early as in 2015, the Paris Agreement on Climate Change realized that the 'NBS' concept was a vital tool for meeting global goals on Climate Change and Sustainable Development. The Paris Agreement signatories further revised the Nationally green infrastructure was dominant in the research and implementation. Examples of green roofs and urban green parks reduce urban heat stress; vegetation and rain gardens intercept stormwater; city lagoons and wetlands store permeable surface water. Under the framework of NBS, some actions in cities were researched, such as the preservation of soil [14], and watersheds [15][16][17], the protection of biodiversity [18,19], the improvement of air quality [20,21], the reduction of greenhouse gas [22], and the reclamation of land for increasing green space [23,24]. NBS are intended to make cities locally adapted, resourceefficient, and systemic intervened by bringing more natural features and processes. This is perceived as the overarching perspective of governing cities and environments adaptively for a sustainable future.
To synthesize existing knowledge on NBS and their environmental, social and economic impacts, extensive literature reviews have been carried out. Seventeen reviews specifically focused on NBS have been published since 2017, covering themes of 'public health' [25,26], 'urban redevelopment and regeneration' [27], 'urban agriculture' [28], 'built environment' [29], 'urban greening' [23], 'urban resilience' [30], 'water systems' (e.g., hydro-meteorological risk and urban pluvial flood risk) [31][32][33], and 'tourisms' [34]. Escobedo et al. (2019) [35] reviewed the relationships among relevant NBS concepts of urban forests, ecosystem services, and green infrastructure. Besides these, governance on NBS, such as 'key enablers and barriers to the implementation of NBS' [36], 'stakeholder engagement' [37], has started to gain more interest. However, there is a need for a comprehensive and systematic review that combines both network and thematic analysis and encompasses the full breadth of NBS literature.
In this review, we assessed the NBS literature in urbanism more comprehensively and systematically to open a wider discussion on how cities can effectively mainstream NBS to achieve diverse thematic goals. We utilized a systematic review approach, including bibliometric analysis, and followed with thematic analysis to contribute to the understanding of NBS in urban planning and identifying research gaps. The bibliometric analysis based on network analysis was used because it is an efficient technique to map knowledge visually, identify research gaps and explore new areas. Network analysis was originated based on the seminal works of Garfield, who proposed academic literature citation indexing explicitly and formally [38], and applied in various fields since then. For example, social studies [39][40][41], biology [42], and medicine [43][44][45] have most commonly utilized this approach. It has become increasingly important to analyze environmental issues, such as ecological networks [46,47], strategic environment assessment [48], environmental governance [49], climate change mitigation and adaptation [50] (e.g., greenhouse gas mitigation [51]), and more especially, sponge city [52], and urban green infrastructure [53]. These elements can be considered under the umbrella of the NBS concept in recent years.
The existing challenges are interacting with the NBS concept, and its application in urbanism include the inadequate synergies in urban land use and planning to align with environmental, social, and economic goals [1], the vague understating to NBS with relevant concepts [35], the unequal research and distribution of practice in different nations and disciplines [35,54], and the insufficient research on the suitable governance approaches for NBS [55]. Therefore, we aim to map and evaluate the existing NBS literature to address these challenges and identify future research directions and policy implications. We pursue the following three specific research objectives: (1) Understand the associations between NBS and related concepts, themes, and their implications for concept evolution and sector cooperation; (2) Identify the up-to-date research foci, trends, and gaps of NBS studies in urbanism by analyzing its disciplinary focus, geographic scope, study methods, and funding sources; (3) Discuss the governance approaches of NBS and provide recommendations for decisionmakers and researchers. In this paper, we first illustrate the methodology, including bibliometric and thematic analysis in Section 2 to achieve Objectives 1 and 2. The corresponding results of these two analyses are presented in Sections 3.1 and 3.2. Next, we discuss NBS as an umbrella and boundary concept for related terms to answer Objective 1 in Section 4.1. The implications among geographic scope, funding support, and thematic goals to answer Objective 2 in Section 4.2. We then deduce that the governance of NBS was one of the emerging themes from the results in Sections 3.1 and 3.2. Therefore, the way forward to improving the governance approaches of NBS in urbanism governance is discussed in Section 4.3 to answer Objective 3. Finally, the conclusion of this review, along with four recommendations for future research and policy insights, are proposed in Section 5.

Methods
The research approach consisted of four procedures: (1) Paper searching, screening, and selection; (2) bibliometric analysis of included papers; (3) thematic analysis of included papers; and (4) recommendations for future research. The following three sub-sections elaborated each step. Figure 1. Schematic diagram of this was presented explicitly in Figure 1 to show the connections between research methodology, objectives, and findings in the corresponding subsequent sections.
(2) Identify the up-to-date research foci, trends, and gaps of NBS studies in urbanism by analyzing its disciplinary focus, geographic scope, study methods, and funding sources; (3) Discuss the governance approaches of NBS and provide recommendations for decision-makers and researchers.
In this paper, we first illustrate the methodology, including bibliometric and thematic analysis in Section 2 to achieve Objectives 1 and 2. The corresponding results of these two analyses are presented in Sections 3.1 and 3.2. Next, we discuss NBS as an umbrella and boundary concept for related terms to answer Objective 1 in Section 4.1. The implications among geographic scope, funding support, and thematic goals to answer Objective 2 in Section 4.2. We then deduce that the governance of NBS was one of the emerging themes from the results in Sections 3.1 and 3.2. Therefore, the way forward to improving the governance approaches of NBS in urbanism governance is discussed in Section 4.3 to answer Objective 3. Finally, the conclusion of this review, along with four recommendations for future research and policy insights, are proposed in Section 5.

Methods
The research approach consisted of four procedures: (1) Paper searching, screening, and selection; (2) bibliometric analysis of included papers; (3) thematic analysis of included papers; and (4) recommendations for future research. The following three subsections elaborated each step. Figure 1. Schematic diagram of this was presented explicitly in Figure 1 to show the connections between research methodology, objectives, and findings in the corresponding subsequent sections.

Search Strategy and Selection Criteria of Literature
In this section, we described how we conducted our literature search and the procedure we used to select the reviewed literature.

Search Strategy and Selection Criteria of Literature
In this section, we described how we conducted our literature search and the procedure we used to select the reviewed literature.

Data Collection and Identification
This study employed an extensive search strategy to locate relevant published studies by a computer search. The Web of Science (WOS) was used because it is the largest and inclusive academic database with more than 8700 core academic journals and seven databases [56,57]. WOS provides a platform to share comprehensive, formats uniformed, and academic references. WOS was supplemented by relevant reports by organizations involved with implementing NBS, such as World Bank, IUCN, and European Commission. The process of literature searching and analysis was depicted in the PRISMA flowchart [58], shown in Figure 2. The first step of this systematic analysis was to develop a robust search string to ensure all related papers were captured.

Quantitative Analysis: Bibliometric Analysis of Publications
The initial citation report, including the data of title, authors, sources title, publication year, total and average citations, and per year citations, were obtained from the WOS. The results of records including publication years, document types, research areas, funding agencies, organizations-enhanced, source titles, countries/regions were analyzed using Excel showing results in Section 3.1.
Bibliometric information was compiled, including cooccurrence of keywords and cocitation analysis to identify highly cited publications and frequently occurred keywords and the clusters by the extent of similarity between documents [59] based on network theory. The network analysis has been demonstrated quite useful regarding concepts with strong interdisciplinary characters [60,61]. The keywords acted as the extraction of research concepts, methods, and topics that could reflect the patterns and trends of various NBS fields by applying cooccurrence of keywords based on network analysis [62]. The cocitation analysis of papers could uncover the most popular works and their relations easily. As a result, it was used to develop a comparative study between the six most cited papers with the cocitation frequency to articulate the evolution of NBS and frame better the NBS concept with other related concepts, as well as reflect NBS governance challenges.
Bibliometric analysis was visualized via the software VOSviewer. It is a software tool in a Java programming language that generates visualizations of network data [63]. A minimum of two items of variables (e.g., keywords) could be set as a threshold of interrelations, either cooccurrence or cocitation. The weight attributes depend on the number First, a keyword search based on the topic of 'Nature-Based Solutions*' and then with the combination with other related keywords, 'Nature-Based Solutions* AND (urban OR city*)' in advanced research, which means the studies of NBS in urbanism, hereby aligning our search profile that merged with the major study aim. Second, a keyword search of title included 'Nature-Based Solutions*' in the (urban OR city*) context in advanced research was conducted. The years in which specific keywords were searched for papers were not limited. The findings revealed the timeframe from the start of 2015 to the end of October 2020, which indicated the first five-year timeframe of the NBS research publications.
The reference lists were be checked using the iterative process (snowball method) consisting of the identification of synonymous terms. We repeated this iterative cycle of searching, reviewing, and selecting relevant papers several times from October 2019 to October 2020, given that the topic is dynamic and new. During each iteration, we enriched our review database with new identified papers.

Screening and Eligibility
The second step was considered 'screening', which included screening the papers according to designed inclusion and exclusion criteria. The inclusion criteria were (1) NBS was the main focus of investigation rather than only referred the term in the introduction Sustainability 2021, 13, 3876 6 of 41 or discussion part; (2) the paper was written in English; (3) papers with peer-review published between 2015 and 2020. The exclusion criteria were applied to (1) the publications containing correct terms, but discussing unrelated topics were excluded, such as mathematics, neurosciences, and nanoscience; (2) duplicate papers; (3) publications (i.e., proceed paper, book chapter, editorial material, early access), because most of them lack keywords to conduct network analysis; (4) papers without access. The authors cross-checked the included papers for data validity and verification. At the end of the review and screening process, 308 articles published by topic search and 89 articles by title search were identified and included in our database.

Quantitative Analysis: Bibliometric Analysis of Publications
The initial citation report, including the data of title, authors, sources title, publication year, total and average citations, and per year citations, were obtained from the WOS. The results of records including publication years, document types, research areas, funding agencies, organizations-enhanced, source titles, countries/regions were analyzed using Excel showing results in Section 3.1.
Bibliometric information was compiled, including cooccurrence of keywords and cocitation analysis to identify highly cited publications and frequently occurred keywords and the clusters by the extent of similarity between documents [59] based on network theory. The network analysis has been demonstrated quite useful regarding concepts with strong interdisciplinary characters [60,61]. The keywords acted as the extraction of research concepts, methods, and topics that could reflect the patterns and trends of various NBS fields by applying cooccurrence of keywords based on network analysis [62]. The cocitation analysis of papers could uncover the most popular works and their relations easily. As a result, it was used to develop a comparative study between the six most cited papers with the cocitation frequency to articulate the evolution of NBS and frame better the NBS concept with other related concepts, as well as reflect NBS governance challenges.
Bibliometric analysis was visualized via the software VOSviewer. It is a software tool in a Java programming language that generates visualizations of network data [63]. A minimum of two items of variables (e.g., keywords) could be set as a threshold of interrelations, either cooccurrence or cocitation. The weight attributes depend on the number of links and the total link strength [63]. For each item meeting the threshold, the total strength of the bibliographic coupling links with other items (e.g., cooccurrence links) was calculated. In all cases, the size of nodes indicates the relative importance of the items. The analysis approach of 'LinLog method and modularity clustering technique' was selected to show the network map [64,65] with same-color keywords grouped into one cluster, prior to finding fast-growing topics and areas of collaboration.

Thematic Analysis
Thematic analysis was developed to further investigate NBS in urbanism specifically, as well as urban NBS governance, because it was aroused as one of the most important themes. There were three main steps of thematic analysis. Firstly, we classified all included papers according to their thematic goals, specific themes, study methods, and geographic scope. Secondly, we mapped their inter-relations and discussed the implications. Next, we discussed the papers under each thematic goal.
There were four thematic goals with corresponding research and innovation actions, in the report "Towards an EU research and innovation policy agenda for NBS and Re-Naturing cities" published as the first official article for the subsequent NBS research [3]. Each included a paper that mentions corresponding goals were labeled, namely, "Enhancing sustainable urbanization by human well-being (sic) enhancement and urban regeneration; Developing climate mitigation and adaptation; Restoring degraded ecosystem; Improving risk management and resilience". Then we discussed included NBS research under each thematic goal in terms of its evolution, contribution and trend. For each paper, besides the corresponding thematic goals (either single or integrated goal) and their specific themes we assigned, such as greening, urban regeneration, wellbeing, governance, water, soil, and air, we have also identified study methods (e.g., review, empirical, conceptual/reflection, and modeling), geographical location of the study and scale of application (e.g., global and Europe). The results of relation flows of NBS studies were shown in a Sankey diagram visually. A four-dimensional typology of NBS publications was created indicating the relationships among study continents, thematic goals, specific themes, and study methods by analyzing the abstract of each paper and the full-text papers.
The specific theme 'NBS governance' was identified with increasingly high interest, but are yet to be fully explored according to results of bibliometric analysis and content analysis in the review process. The inclusion criteria were using 'governance' by topic search within 89 included NBS papers, such as the context about institutionalization, stakeholders, planning, education, policy, and finance. Specifically, these selected papers, including both the general conceptualization and case studies, were summarized (in Appendix A: Tables A2 and A3) and discussed in terms of their thematic goals, specific themes, funding resources, and main contributions.
On the other hand, through this meta-analyses and review, we further discussed the governance approaches (i.e., top-down, bottom-up, hybrid or adaptive). We provided some valuable recommendations to draw the key lessons from case studies to transfer or scale up to foster new NBS strategies. The gaps and trends under this theme were argued, and the recommendations were suggested based on the current pattern of NBS governance.

Results
The results of analyses were presented below, beginning with the bibliometric analysis of general statistics of publications and network analysis, including the most commonly cooccurring keywords and cocitation relations (Section 3.1), and following in-depth thematic analysis of NBS studies, including thematic goals, specific themes, study methods, and geographic scope (Section 3.2).

General Statistics of Publications
When we searched keywords of NBS in the topic (English language), there were 541 papers of NBS studies (470 research articles and 71 reviews) in total (until the end of October 2020), accounting for 89% of the total 611 of all document types. Among these, 308 NBS papers (266 research articles and 42 reviews) in urbanism were identified, accounting for 57% of total NBS research in general. Specifically, for NBS studies within urbanism context searching by title, there were 89 papers left explicitly focusing on NBS research with 72 articles and 17 reviews. Research articles accounted for the majority of the total number of publications (over 75%), as opposed to review, proceed paper, book chapter, editorial material, and early access.
The published year of NBS research and review papers started in 2015, and then the number of published works significantly increased since 2016 (when the term 'NBS' was coined). While in 2017-2018 there was a slight fluctuation, the general trends of publication numbers were increasing till the highest number in 2020, indicating a growing interest in this field. The sum of times cited was increasing dramatically in 2018-2019 and 2019-2020, while the annual citation number was the highest in 2017 with 564 for NBS in urbanism (title search) (see Figure 3).
The published year of NBS research and review papers started in 2015, and then the number of published works significantly increased since 2016 (when the term 'NBS' was coined). While in 2017-2018 there was a slight fluctuation, the general trends of publication numbers were increasing till the highest number in 2020, indicating a growing interest in this field. The sum of times cited was increasing dramatically in 2018-2019 and 2019-2020, while the annual citation number was the highest in 2017 with 564 for NBS in urbanism (title search) (see Figure 3). It was notable that most journals were in environmental fields. The top research area was 'Environmental Sciences', with 63%, 72%, and 77%, respectively, for NBS related studies in general, NBS in urbanism by topic search and specifically by title search. It was notable that most journals were in environmental fields. The most popular journal published NBS articles were Sustainability, followed by Environmental Research, Science of the Total Environment, and Environmental Science  Policy, which were included in all three searching results. The reasons could be these journals have a broader scope of NBS studies and the openness to publications in the area of innovative concepts and technologies.
The specific priority research areas on NBS can be divide into land, water, greening, and policy. The land and policy were the most popular within NBS in urbanism by a title search, while the topic search in urbanism focused more on greening and water. Land-  It was notable that most journals were in environmental fields. The top research area was 'Environmental Sciences', with 63%, 72%, and 77%, respectively, for NBS related studies in general, NBS in urbanism by topic search and specifically by title search. It was notable that most journals were in environmental fields. The most popular journal published NBS articles were Sustainability, followed by Environmental Research, Science of the Total Environment, and Environmental Science & Policy, which were included in all three searching results. The reasons could be these journals have a broader scope of NBS studies and the openness to publications in the area of innovative concepts and technologies.
The specific priority research areas on NBS can be divide into land, water, greening, and policy. The land and policy were the most popular within NBS in urbanism by a title search, while the topic search in urbanism focused more on greening and water. Landrelated journals, such as Land, Land Use Policy, and Landscape and Urban Planning, ranked higher specifically in the NBS publications in urbanism than the NBS publications in general (see Table 1). This may indicate the research on land was one of the key areas of NBS studies in urbanism that urban planning and policy were being important to involve NBS. Using a Sustainability 2021, 13, 3876 9 of 41 topic search, the research on Urban Forestry & Urban Greening for NBS studies in urbanism was more popular (5.38%) compared to NBS studies in general (3.36%), which indicated the 'greening' was a hot topic in urbanism. Water research was also important with the high-percentage publications using a topic search that might disclose the solutions on water issues using NBS-related approaches.
The most frequent studied countries in NBS research were within the EU, which was not very surprising given that NBS was the major EU-driven policy and the European Commission and European Research Council to provide financial support in various levels (i.e., academic research funding; funding for enhancing NBS projects and practices delivery, funding for the maintenance of the (NBS) measures and structures). In particular, for the Western European countries (e.g., Netherlands, Italy, and Germany) and the United Kingdom (UK), with the largest publications that played a major role in the world's ability to lead NBS research. Italy was also identified as the earliest research case study on NBS, focusing on isolated environmental problems, such as water pollution [66], air quality [21], and soil sealing [14].
For the new EU member states, such as Eastern EU, Poland focused more NBS cases than Czech Republic, Estonia, Romania, Slovenia, and Bulgaria. Outside the EU, there were countries being popular in NBS studies, including China and Australia. Popular NBS case cities identified included Melbourne, Rome, Barcelona, and New York City. In Asia, more cases were in Chinese mega-cities, such as Shanghai and Chongqing, while others were in South Korea, Thailand, India, and Japan.
Geographically, NBS was a popular and widely cited concept among academics of the EU. The rest countries located in Africa, South America, and Asia published only between one to three papers focusing on NBS. The top 10 countries ranked by the number of publications from high to low were the UK (31% in total and the majority from England with 22%), the Netherlands (20%), Italy (19%), Germany (16%), Australia (11%), Sweden (11%), China (9%), Portugal (9%), Spain (8%), and the United States (8%) (Figure 4). They played a dominant role in NBS research and discourse, due to their coauthorships shared with other countries and the number of publications, which can be considered as the 'leading forces'. More links of coauthorship within one country indicated more knowledge and research of NBS could be transferred. Thereby, the EU has a good foundation to share frameworks and cases within the EU member states.
European Commission was the largest organization for NBS research (15%), and the European Union was the biggest funding agency for NBS research (18%), followed by EU Horizon 2020 Research and Innovation Program, Swedish Research Council Formas (Sweden), Federal Ministry of Education Research BMBF (Germany), National Natural Science Foundation of China NSFC (China), and Portuguese Foundation for Science and Technology (Portugal) according to the existing literature search of NBS studies in urbanism. Accordingly, the top organizations-enhanced of NBS research in urbanism were within the EU mostly, especially in Sweden, the Netherlands, Italy, and Germany, such as Swedish University of Agricultural Sciences, Wageningen University research, Humboldt University of Berlin, Universita Degli Studi Di Bari Aldo Moro, Erasmus University Rotterdam, Helmholtz Association, and Technical University of Munich. As a result, funding support and strong organizations-enhanced contributed to higher research productivity in these corresponding countries.

Network Analysis of Publications: Indicators of Research Integration Cooccurrence Analysis of Keywords to Identify Closely Related Concepts, Terms, and Topics
The resulted network graph of the keywords (both author keywords and keywords plus) and connections between those keywords are displayed in Figure 5. The frequency of occurrence of each keyword and the cooccurrence of pairs of keywords were used to indicate the foci of publications. The thickness of the connections represented the frequency of cooccurrence that three keywords occur together, and the size of each sphere (i.e., label and nodes) illustrated the occurrence frequency, and the colors indicated different clusters. ogy (Portugal) according to the existing literature search of NBS studies in urbanism. Ac-cordingly, the top organizations-enhanced of NBS research in urbanism were within the EU mostly, especially in Sweden, the Netherlands, Italy, and Germany, such as Swedish University of Agricultural Sciences, Wageningen University research, Humboldt University of Berlin, Universita Degli Studi Di Bari Aldo Moro, Erasmus University Rotterdam, Helmholtz Association, and Technical University of Munich. As a result, funding support and strong organizations-enhanced contributed to higher research productivity in these corresponding countries.

Network Analysis of Publications: Indicators of Research Integration
Cooccurrence Analysis of Keywords to Identify Closely Related Concepts, Terms, and Topics The resulted network graph of the keywords (both author keywords and keywords plus) and connections between those keywords are displayed in Figure 5. The frequency of occurrence of each keyword and the cooccurrence of pairs of keywords were used to indicate the foci of publications. The thickness of the connections represented the frequency of cooccurrence that three keywords occur together, and the size of each sphere (i.e., label and nodes) illustrated the occurrence frequency, and the colors indicated different clusters.
A total of 80 stemmed and synonyms of keywords were replaced (e.g., nature-based solution was replaced by nature-based solutions). When putting bibliographical data of 89 papers into the VOSviewer, there were 70 keywords of the 638 keywords that met the threshold requirement of minimum three occurrences. For each of 70 keywords, the total A total of 80 stemmed and synonyms of keywords were replaced (e.g., nature-based solution was replaced by nature-based solutions). When putting bibliographical data of 89 papers into the VOSviewer, there were 70 keywords of the 638 keywords that met the threshold requirement of minimum three occurrences. For each of 70 keywords, the total strength of the cooccurrence links with other keywords was calculated based on the frequency of times that cooccurred with other keywords.
NBS research in urbanism presented strong relations with terms of ecosystem services (41 occurrences, 66 links, and 265 total link strength) and green infrastructure (with 31 occurrences, 62 links, and 200 total link strength), and climate change (19 occurrences, 47 links, and 109 total link strength). In the second order of importance, NBS was strongly connected with the terms of resilience (14 occurrences, 44 links, and 103 total link strength), and sustainability (15 occurrences, 46 links, and 106 total link strength), and adaptation, including both climate-change adaptation and ecosystem-based adaption. The 'management', 'governance', 'benefits', 'framework', 'challenges', and 'lessons' were the popular keywords identified in NBS publications with the strength of a total link from high to low, respectively.
In addition, it reflected fewer terms involved with social objectives and results (e.g., 'local', 'assess' and 'cohesion'), with 'environmental justice' and 'people' mentioned relatively more frequently. Economic terms linked to cost as expected, with less frequency than environmental and social terms, respectively, showing a stronger tendency of focusing on the environmental and ecological characteristics of NBS.
Keywords among each other with strong association strength were grouped as a cluster, highlighted by the same color in the network. In this context, keywords within a cluster (or community in the broad network analysis) are associated closely with each other and have more common research connections [63]. The relations of two keywords were determined by calculated link strength (presented by the thickness of the connectors rather than proximity of items (as the position in the map was optimized) [63], but the peripheral elements in the map usually have fewer occurrences. strength of the cooccurrence links with other keywords was calculated based on quency of times that cooccurred with other keywords. NBS research in urbanism presented strong relations with terms of ecosyst vices (41 occurrences, 66 links, and 265 total link strength) and green infrastructur 31 occurrences, 62 links, and 200 total link strength), and climate change (19 occur 47 links, and 109 total link strength). In the second order of importance, NBS was s connected with the terms of resilience (14 occurrences, 44 links, and 103 to strength), and sustainability (15 occurrences, 46 links, and 106 total link strengt adaptation, including both climate-change adaptation and ecosystem-based ad The 'management', 'governance', 'benefits', 'framework', 'challenges', and 'lesson the popular keywords identified in NBS publications with the strength of a total lin high to low, respectively. In addition, it reflected fewer terms involved with social objectives and resul 'local', 'assess' and 'cohesion'), with 'environmental justice' and 'people' mention tively more frequently. Economic terms linked to cost as expected, with less fre than environmental and social terms, respectively, showing a stronger tendency o ing on the environmental and ecological characteristics of NBS. In fact, we can see that there were 5 clusters in Figure 5. Cluster 1 to 5 accounted for about 29% (red color), 27% (green color), 21% (blue color), 16% (yellow color) and 7% (purple color) of all keywords, respectively. Cluster 1 focused on 'ecosystem services' (with the terms 'ecosystem', 'ecology', 'ecological restoration'), and 'green infrastructure' (related to terms of green space). Cluster 2 focused 'benefits' of 'NBS' on 'public heath' and 'water'-related' topic, such as 'low impact development' and 'flood'. Cluster 3 was formed by 'governance'-related studies, including 'collaborative governance', 'knowledge', 'lessons', 'participation', and 'trade-offs'. Cluster 4 addressed the assessment of NBS with the keywords of 'performance', 'life cycle assessment', 'model', and 'system'. Cluster 5 researched the conceptualization of NBS, such as 'challenges', 'cobenefits', 'management', and 'framework'. It was notable to mention that the link strength between the term of 'governance' and 'NBS' was as same as 'NBS' with 'GI', 'climate change', and 'management', which indicated the importance of governance topic on NBS.

Cocitation Analysis to Identity Pioneers of NBS Papers
Cocitation refers to the frequency that two items are simultaneously cited in the literature [67]. Such patterns observed over these years could provide clues to understanding the mechanism of NBS development and insights to the domain of knowledge, which is more reliable than citation-only analysis [68].
There were 5518 cited references, 28 meeting the threshold with 10 as the minimum number of citations of a cited reference ( Figure 6). The most cited articles were usually regarded as landmarks because of their ground-breaking contributions [68]. In Table 2, we showed the top six of most cited papers with cocitation frequency. We summarized the content and relations of these papers to facilitate the framing of the NBS concept by illustrating the connections between the NBS concept and other related concepts, also the hotspot of state of the arts in current NBS research and the governance challenges they revealed.   Due to the high cocitations of these papers, they were more influential in the discourse and metaphor for the NBS insights. These six papers all framed NBS in a theoretical level rather than focused on a specific theme or practice. We developed a comparative study to summarize the main targets and the common ground of these papers as follows: (1) Framing NBS several stages from identification, selection, designing, implementation, engaging/communication, transferring/upscaling, to monitoring and evaluation; (2) producing stronger evidence on NBS to address current (and future) social, economic, and environmental challenges (frequently mentioned sustainable urbanization, climate change adaptation, and mitigation, ecosystem restoration, or disaster risk reduction) based on its performance or cobenefit assessment; (3) classifying and justifying NBS as an innovative and cost-effective opportunity from classic/traditional strategies and green-related concepts, including green infrastructure and ecosystem services; (4) reflecting knowledge gaps, potential barriers and opportunities on the implications for science, policy, and practice of NBS with a foci on the European context for its sustainable development.
The cocitations, as well as coauthorship demonstrated the NBS studies, were limited within EU research expert groups as the pioneer researchers. For example, as one of the authors with the highest cocitations, Kabisch was the coauthor with Raymond on the highly-cited NBS framework paper [70]. They identified ten societal challenges in urban areas globally and proposed a seven-stage process for evaluating and employing cobenefits of NBS in policy implementation. The assessment framework of this paper to assess the effectiveness of NBS was developed by EKLIPSE Expert Working Groups funding by the EU and was the most accepted evaluation framework for NBS. The innovation of this framework was that synergies and trade-offs among the challenges reflect a significant aspect of the assessment process [36]. This framework [70] as a basis was adopted and refined by some researchers in later studies.
For instance, Frantzeskaki (2019) [71] proposed seven lessons for planning NBS in cities and expanding proof-of-concept and demonstration stages of NBS to organize lessons learned for further studying the issue of sustainable urbanization with NBS. Lafortezza and Sanesi (2019) [72] also adopted this framework [70] as a basis of the DPSIR (Driving force-Pressure-State-Impact-Response) model to explore their impacts on the dynamics of urban areas, in conformity with European Commission standards. Nesshöver et al. (2017) [11] further divided the third step on the designing project of implementing NBS into five dynamic steps, which are ensuring the use of adequate knowledge, concepts and methods, dealing with uncertainty, complexity, ambiguity, and conflicts for trade-offs, guaranteeing the involvement of numerous stakeholders and a wider public, developing a mutual understanding of multiple-functional solutions, evaluating, and monitoring for cross-scale learning. Governance of NBS, such as stakeholder engagement, gradually got more attention for ensuring the success of NBS implementation.
The other three pioneer papers specifically focused on EU context and closely linked with European Commission's four thematic goals [4,7,12], providing insights on multiple benefits of NBS, opportunities, and challenges. Maes and Jacobs [12] in 2017 interestingly highlighted socioeconomic benefits of NBS (i.e., employment opportunities, and low-carbon technology innovations), which were rarely researched compared to environmental benefits. NBS has the potential to provide a transition path with realistic and gradual steps to a sustainable economy as expected by the EU Horizon 2020 vision. The main contribution was proposed criteria to guide and assess the implementation of NBS.
The social-economic impacts and trade-offs between social and ecological developments in cities, such as issues of ecosystem disservices, green/ eco-gentrification were still lacked attention at the early stages of NBS research, though some of them mentioned, for example, NBS is encouraged to explore "win-win" situations, besides trade-offs and uncertainties [4]. It was possible to interpret that this aspect was still in its infancy, and most studies tried to understand the benefits of NBS, in order to push forward it driven by EU-funded NBS projects that four of which are supported by FP7. This justified the goal of our review to synthesize various studies and offer possible future research directions, especially to solve governance challenges with policy, funding, and management on social-economic aspects.

Thematic Analysis
Regarding the thematic analysis, firstly, we analyzed the content of reviewed papers according to four priority areas that EC has identified [3]. Four thematic goals were extracted based on this [3], and then were used to categorize included NBS papers. The results indicated 66% of NBS publications included over one integrated goal, and 30% of NBS publications covered all four thematic goals. It was notable that 81% of included papers mentioned thematic goal 1-'Enhancing Sustainable Urbanization', while 65% were combined with another three goals.
We analyzed NBS included publications comprehensively and divided them into various narrowed themes, revealing more focus on 'water' (i.e., stormwater, flood, river and lake, sewers, and waterscapes), 'wellbeing' (including public health), 'greening' (including the specific topic of GI, green space, green land, green city, urban forest, urban gardens, etc.), 'urban development' (i.e., urban agriculture, urban regeneration, urban resilience, urban sustainability, built environment) (see more details in Appendix A: Table A1). It can be seen that under the NBS context, integrated research has received more attention than isolated topics, such as soil or air (see Figure 7). However, the 'climate change adaptation' by NBS in urbanism has lacked priority [73]. Linkages between NBS and human wellbeing were increasingly understood, but the possibility to create and strengthen social cohesion based on NBS was rather rarely noticed. The least recognized was the impact of NBS on the potential economic development and its governance, especially outside the EU region, which needs more investigation and evidence. actors) to deliver an upscale urban NBS globally is a key opportunity both in developed and developing countries [81]. The types included market-driven, stakeholder-driven, and citizen-driven. These new governance arrangements (i.e., adaptive governance, hybrid governance, and polycentric governance) have also been proposed as a locally unique approach in implementing the NBS practice, evidently provided from examples on climate change adaptation [82,83], urban biodiversity [84], urban water management [85][86][87], communal urban gardens [24] in EU region, and urban forestry [88] mostly in Melbourne Australia. However, hybrid governance with various choices faced risks of either improvements and deterioration of distributional, recognition, or procedural justice.
Interestingly, we found that was notable for the Asian studies on NBS research currently absent on governance theme and conceptual methods. A total of 35 papers focused on the topic of NBS governance in urbanism, including conceptual studies (e.g., analytical framework to assess cobenefits or policy cycle, transition path, education, NBS thinking, stakeholder engagement, institutionalization) and specific empirical case studies. And we selected 17 papers which were the most typical and influential ones from the included 89 papers to compare and learn lessons on governance approaches (see Appendices A: Table  A2 and A3).  The study methods of included publications that were classified into four types: Review, empirical, conceptual/reflection, and modeling [74]. In this manuscript, we found that the empirical method was the most popular study method (67%, including integration with other methods). The integration of empirical and modeling was the most frequent among all types of integration (20%). The empirical method was mainly conducted as case studies using qualitative methods (interviews, workshops, focus groups, and surveys) or experimental approaches. Most papers presented a single case study, with 23% were cross-case studies, while 42% of cross-case studies were still within the EU. Modeling (21%) included economic modeling (e.g., general equilibrium models, regression models, and cost-benefit analysis), evaluation and optimization modeling to understand cobenefits, trade-offs and identify synergies, simulation models (e.g., microclimatic, air dispersion, hydraulic and hydrological models), life cycle assessment, multi-criteria criteria assessment, and participatory analysis. The conceptual method (21%) reflected frameworks and typologies by identifying and assessing NBS characteristics, principles, indicators, benefits, enablers and barriers, opportunities and risks, pathways, synergies, and trade-offs. The review method accounted for 25% of all included papers.
To provide insights into the geographic scope of NBS research, the papers were classified into the continents based on the main subject of their work: Europe, Asia, North America, South America, Africa, and Oceania. Some global studies covered countries with more than one continent. In some cases, the international level studies applied in the global in general; thus, such cases studies were labeled as 'Global' [51] (see Figure 7). Global and EU-focused research about NBS in urbanism were widely popular. Moreover, integrated research accounted for half of global scope research. The most frequently studied countries were within the EU region, followed by Asia. The case studies on Africa were absent and rare in South America. The research in Oceania and South America both included the thematic goal 3-'restoring degraded ecosystems'. Except for Oceania, all other continents that covered the 'Water' research theme mainly focused on flood risks and water pollution management-47% research of this theme was from EU studies.
While popular advocated for, NBS has so far not been implemented systematically, especially in developing countries. Despite their enormous potential, the application of NBS remains highly uneven, marginal, and fragmented within and between countries and cities. We also found that most case studies and authors were actually from the developed countries, especially among European countries. It is anticipated that more applicative studies and researchers from developing countries could contribute to NBS studies. Explicitly we identified the research gaps in developing countries. There was growing research on thematic 1 in terms of urban regeneration (e.g., Asia, especially China [75]). However, research on enhancing sustainable urbanization by human wellbeing was still rare in developing countries. It is crucial to shift the scientific foci of NBS contribution to sustainable urbanization from developed countries to low and middle-income developing countries as they face rapid urbanization pressures, such as inadequate green spaces and GI, population growth, and social stress. The NBS research in developing countries in urbanism regarding thematic goals 2 and 3 were usually integrated with thematic goal 4 for risk reduction and resilience, such as water security in India [76], floods in Brazil [77], integrated disaster risks in Brazil [78], and the Pacific Ocean cities [30].
In addition, the role of NBS as risk solutions was explored in Asia, consisting of urban heat island effects in Thailand [79], and natural disaster vulnerability for urban regeneration in China [75]. However, the research width, by applying NBS as ecological management practices, were limited, and solutions for social challenges (e.g., urban poverty, public health, and business) were even rare due to space availability, data scarcity, immature governance mechanisms, and limited funding in developing nations. Such conflicts between the high demand of NBS and low opportunities to implement NBS in these counties require support from developed countries (e.g., EU) to share more experiences to assistant the uptake of NBS, and cooperate for more research funding opportunities. In contrast, developing nations themselves are encouraged to strive for chances to investigate, learn, corporate, and contribute local knowledge to the field of NBS in return.
There was a growing recognition that the 'governance' was one of the most important themes (accounting for 44% of all papers) for NBS effective implementation and upscaling to solve a broad range of society challenges [80]. At the same time, there was a growing interest on the political level, such as the latest EU research strategy (see EC 2015 [3]). Research on NBS governance was increasing, especially in recent two years, including the enablers, barriers and strategies for overcoming barriers, (co)benefits, contributions or multi-functionality, principles, steps, and insights for planning, indicators, barriers, knowledge gaps, and opportunities for action.
Hybrid governance as a new type of governance (identified as governed by multiactors) to deliver an upscale urban NBS globally is a key opportunity both in developed and developing countries [81]. The types included market-driven, stakeholder-driven, and citizen-driven. These new governance arrangements (i.e., adaptive governance, hybrid governance, and polycentric governance) have also been proposed as a locally unique approach in implementing the NBS practice, evidently provided from examples on climate change adaptation [82,83], urban biodiversity [84], urban water management [85][86][87], communal urban gardens [24] in EU region, and urban forestry [88] mostly in Melbourne Australia.
However, hybrid governance with various choices faced risks of either improvements and deterioration of distributional, recognition, or procedural justice.
Interestingly, we found that was notable for the Asian studies on NBS research currently absent on governance theme and conceptual methods. A total of 35 papers focused on the topic of NBS governance in urbanism, including conceptual studies (e.g., analytical framework to assess cobenefits or policy cycle, transition path, education, NBS thinking, stakeholder engagement, institutionalization) and specific empirical case studies. And we selected 17 papers which were the most typical and influential ones from the included 89 papers to compare and learn lessons on governance approaches (see Appendix A: Tables A2 and A3).
In the following part, we illustrated and discussed NBS research in urbanism under each thematic goal.

Thematic Goal 1: Enhancing Sustainable Urbanization by Human Wellbeing Enhancement and Urban Regeneration
Improving human wellbeing locally, regionally, or globally through the learning and the usage of nature to create sustainable socioecological systems is one of the core themes of NBS. It was also recommended to improve wellbeing in urban areas to promote the goal of sustainable urbanization [3]. Three reviews focusing on wellbeing mainly discussed the evidence, role functions, contributions, and benefits of NBS for public health [25,26,89]. These papers indicated a tendency to a positive effect linking specific NBS to different elements of human wellbeing, particularly urban blue-green space and human health, but conclusive evidence was hard to find and quantify.  [26] provided an overview of the existing evidence specifically on the relationship between the health of population groups of children and the elderly and NBS (urban green and blue spaces) to urbanization-associated challenges. Van den Bosch and Ode Sang (2017) [59] showed strong evidence for an improved effect on heat reduction from green space and then cardiovascular disease (CVD)-related mortality by exposure to NBS (urban natural environments, including blue-green, etc.). The research articles proved the NBS, mainly by the exposure to the green space, could enhance human wellbeing through the cases of depletion of cognitive resources (i.e., less ego depletion) in hot summers [90], improve mental health (recuperation from depression, stress, and anxiety) [91], and minimize the adverse impacts on public health (pollen emissions) [92].
Most studies in this field were published in 2016, and the cases were conducted mostly in developed countries, mainly the western EU and the US, while there is growing research on urban regeneration in developing countries facing rapid urbanization pressures, such as China [75,93]. Urban regeneration can considerably contribute to inclusive urban development, and NBS can act as a catalyst for neighborhood life and promote the creation of new social ties, and simultaneously devote to a healthier urban environment and a more inclusive society [94]. Xiang et al. (2017) [63] demonstrated the contribution and effective measures by using the advanced concept of NBS in Chongqing China to culturally diverse, healthier, and greener regenerative cities (e.g., deprived districts and neglected or abandoned areas), improving living conditions, improved air and water quality, increased GI and biodiversity, enhanced wellbeing, as well as the awareness of re-naturing cities. Furthermore, Xiang et al. [64], lately proposed the concept of inclusive urban regeneration combining NBS with society-based solutions by paying more attention to social aspects.
In summary, achieving sustainable urban development and greening are the more favorable dimensions compared to concerns of previous early years on human adaptation and disaster risk reduction. Evidence assessed almost exclusively from the case studies in developed countries except for a few ones, such as cases in Serbia with numerous acute and chronic stressors and accumulated trauma after the general disproportion of urban development and the socioeconomic crisis [91]. The role of NBS within GI in urban development, especially 'resilience and regeneration', is well recognized in developed countries, such as the awareness of cobenefits of GI for public health. Considered the urbanization pressures, including inadequate green spaces and GI, population growth, and social stress, are predicted to take place in low and middle-income developing countries in Asia and Africa, taking the example of China with the enormous need for urban regeneration [75,93], it is necessary to shift the scientific focus of NBS contribution on sustainable urbanization by improving urban regeneration and wellbeing to these regions.

Thematic Goal 2: Developing Climate Mitigation and Adaptation
The discussion of climate change and NBS comes into public more straight and popular after the Paris Agreement was signed in 2016, with the earliest paper related to NBS published by Kabisch et al. and Fink in 2016 [69,95], and increased significantly since 2017. Climate change is frequently mentioned in the NBS studies with over 62% of publications, and it is one of the closest keywords to the NBS concept according to the keywords cooccurrence analysis.
There were about 55% of NBS studies mentioned climate change adaptations and mitigation. Reviews, including both climate and NBS in titles, were interpreted from the perspectives of indicators, barriers, opportunities, and knowledge gaps for action in urban areas [69], and connecting science, policy, and communities for evidence-based decisionmaking [82]. NBS has the potential to provide climate adaptation and mitigation measures and simultaneously offer climate-resilient and sustainable planning, especially in urban areas. Plural research outputs have highlighted the valuable role of nature and NBS in addressing climate change in urbanism and the importance of NBS as taking the first step during the integration process between adaptation and mitigation strategies, furthermore implicated its role for broader sustainability [95,96].
NBS is not a panacea for all climate-related urban challenges. The evidence base is still inadequate and needs to be researched considering challenges of the robustness, efficacy, and performance of NBS in distributing numerous benefits to deal with climate mitigation and adaptation in cities. Frantzeskaki et al. (2019) [82] proposed three suggested ways forward, including collaborative research, evaluating schemes and big data, new investment models, and innovative financing mechanisms to implement NBS. Technologybased solutions, cultural-based solutions, and behavior-based solutions should complement the work of NBS. The pioneering article focused on the effectiveness of NBS and contributed to transfer applications and knowledge to the outside the EU [69]. Therefore, an increasingly urgent area is combining multiple solutions and exploration more evidence to address climate change (via adaptations and mitigation); and integrate these aspects by considering socio-environmental justice and cohesion in the governance practice.

Thematic Goal 3: Restoring Degraded Ecosystems
Restoring degraded ecosystems using NBS to improve ecosystems and meet societal challenges is one of the thematic goals of NBS [3]. It was found over half of the NBS publications mentioned restoration and conservation, and almost all publications mentioned keywords related to eco-based (i.e., ecosystem services, ecosystem-based adaptation, eco urbanism, ecological infrastructure, eco-friendly technologies). NBS has fostered novel interdisciplinary knowledge and practices in urban ecosystems [8]. The keyword of 'Ecosystem Services' has a strong link to the NBS concept according to the keywords cooccurrence analysis.
Maes and Jacobs (2017) [12] published one of the pioneer articles on NBS, highlighting the relations with ecosystem services and policies in the EU. This relation with ecosystem services was illustrated by the definition of NBS promoted by several researchers.  [35] further reviewed a series of related metaphors consisting ecosystem services, GI, urban forest with NBS since concepts fostering interdisciplinary knowledge on urban ecosystems have been evolved in the last few decades.
NBS can restore modified ecosystems. In the literature, a large proportion of restoration of ecosystem services within NBS studies discussed the restoration of the water cycle within an urban catchment [97], in line with the recommendations of multifunctional nature-based watershed management and ecosystem restoration by the European Commision (2015) [3]. Water-related NBS concepts are strongly related to the well-known concepts of 'Sustainable Urban Drainage' (SUDS), 'Water Sensitive Urban Design' (WSUD), 'Low Impact Development' (LID), and 'Sponge City Program' (SCP) [98,99].
Some case studies, inspired by NBS to recover and restore lakes [16] and rivers [100] to achieve multiple benefits, take certain measures-mainly include bio-manipulation, small doses of chemicals (to active phosphorus), green linking corridor, and a variety of GI (constructed wetland). The stages of an urban ecosystem developed from conventional, green, sustainable, and restorative to regenerative thinking, to shift from technical systems development to living systems development, and from degenerating to regenerative design to connect natural and human into the landscape pattern in a coevolving relationship [100].
Recently, a consensus has reached that the NBS concept embraces human and ecological benefits beyond fundamental objectives in ecosystem conservation and restoration [36]. In real cases, multiple solutions are conducted, such as the combined grey and blue-green infrastructure. The widespread use of NBS requires a systematic shift from separately relying on several technologies to learning to allow nature to care for them in an integrated way, such as restoring water balance close to the local nature environment and further boost up the natural functions.

Thematic Goal 4: Improving Risk Management and Resilience
Using NBS to improve resilience and risk management could contribute to larger benefits than traditional methods and offer solutions in decreasing various risks of disasters [3]. Over 67% of NBS publications have mentioned 'resilience' in NBS studies, including resilient landscapes and cities [101,102], resilient human and wellbeing [90], flood and ecosystem resilience [103][104][105]. It was noticeable a majority of research discussed flood risks and resilience. There were 80% of NBS publications that referred to 'reduction' of risk factors associated with urbanization, such as air pollution, heat, noise, flood, and food security issues [99,106,107], also for the mental stress [91], urban poverty [108], and economic inequalities [109]. These factors have been reflected in substantial literature proving the role of NBS for risk management and resilient cities and landscapes.
The case studies of NBS for disaster risk reduction have researched flood risks [17,34,84], hydro-metrological risks [104], public health risks [110,111] related to soil pollution. Most disaster risks are related to climate change that focused on urban heat islands or flooding, and the prominent studies for reducing risks of such hazards using BGI (e.g., green space, wetlands, trees, and parks) as adaptation and mitigation strategies [106].
In summary, NBS emerged as innovative solutions with the potential to solve environmental, social, and economic issues that cities are facing today, contributing to protect, manage, and restore natural or modified ecosystems sustainably in urbanism, simultaneously offering multiple benefits, including climate change adaption and mitigation, risk reduction and resilience, and human wellbeing.

Discussions
This section discussed urban NBS and other related concepts, existing research and its implications, urban NBS governance in the following three sub-sections to answer three research objectives, respectively, with the main findings summarized in Table 3 below. Table 3. Results summary from the systematic review in this study.

Results Types for Research Objectives Results Details Research Implications
The number of publications-to answer research Objective 2 (current research foci and trends) The NBS studies have increased in the past five years (2015-2020) and were expected to continue to grow, with 57% of NBS related publications have focused on urbanism, which accounts for publications in the topic search that were three times greater than in title search.
Implied the trend of NBS in urbanism and relevant studies were productive and increasing. The broad content of NBS with other concepts has enriched NBS topic literature. The frequent and popular keywords, terms, and themes-to answer research Objective 1 (the connection between urban NBS and other related concepts); Objective 2 (research foci, specific themes, and trends); Objective 3 (urban NBS governance) NBS studies presented strong relations with terms of green infrastructure and ecosystem services. Enhancing Sustainable Urban Development and Greening were the lately favorable dimensions. The case studies on greening had the most attention, followed by urban development (e.g., urban agriculture and regeneration), wellbeing (mainly public health), and water than isolated soil or air research. There was a growing number of NBS papers on governance since the last year. Implied NBS research does not sit on its own as an 'island', but other relevant concepts have been imported. Implement NBS in practice could take advantage of green infrastructure and ecosystem services in order to achieve broader sustainability goals. Governance of NBS research could be explored in three ways forward, i.e., hybrid governance, finance incentives, and participatory process with multi-stakeholders integrating local culture and knowledge.
Numerous academic studies on climate change and ecosystem services through systematic review have drawn some assumptions that they are closely linked to the concept of NBS [4,35,70,112]. The earliest relevant paper about NBS was found in 2015. It can be explained by the fact that the term NBS firstly entered the mainstream of scientific literature in the early 21st century.
This review has found that over 57% of current NBS literature focused on urbanism and governance themes accounted for 47% of NBS in urbanism. The field of NBS in general and in urbanism has grown rapidly, particularly with sharp increasing literature since 2018, which was consistent with previous research. For example, according to Hanson et al. (2020) [74], around 57% of NBS publications concern the urban contexts. In the urban context of NBS, parks, private and urban gardens, peri-urban forests within the greening topic were mentioned frequently. NBS can address societal challenges across a wide-ranging of spatial scales and temporal scales; multiple current NBS research articles addressed city scale.
More evidence has emerged globally that the benefits and effectiveness of NBS in addressing environmental, economic, and societal challenges, particularly in highly urbanized areas, from the diverse and inter-related studies. And the thematic analysis of our review also demonstrated the emphasis of NBS studies on greening and sustainable urban development in recent research. A large number of publications were coupling NBS and climate change together through the functions of NBS to improve risk management and resilience in urban areas [56]. More recently, the focus has been shifted to greening in urban areas, such as urban forests than disaster risk reduction.

NBS-As an Umbrella and Boundary Concept for Related Terms
From the analysis of bibliometric analysis, it was found that ecosystem services, GI and climate change has been paid the closest attention to NBS research. Sustainable urbanization, risk management and resilience, ecosystem services, and climate change were key themes related to NBS in the extensive literature since the European Commission set these four thematic goals in 2015. The ecosystem services have been adopted by different authors, disciplines, and regions, and now it is starting to merge and morph into NBS and GI [35]. Some publications integrating provisioning or regulating ecosystem services into GI assessment [108,113], and now considered an essential function of NBS [35], and involved cultural ecosystem services for public health [91,114]. We have found that common concerns, such as climate change, sustainability, resilience, conservation, and restoration, are closely connected to NBS and frequently researched using such metaphors (e.g., ecosystem services, GI, and NBS).
As a result, NBS research does not sit on its own as an 'island', but that other relevant concepts have been imported. The advantages of the NBS concept are its integrative and transdisciplinary approach [11,70,115]. The results showed the integrated theme of NBS publications accounted for the largest number than isolate topics. NBS might be a good example of a cross-cutting research paradigm as a 'boundary object' that helps break down disciplinary barriers for sustainability.
Although ecosystem services and GI have strong connections with NBS, the NBS literature did not cover them under the overall concept. In practice, what was being implemented and studied were mainly particular components of NBS, such as GI, rather than the umbrella concept of NBS [61]. These terms, metaphors, concepts, or words used were changing in linguistics and discipline knowledge, which were imperative and important for the communication and the transformation of NBS core idea-'Living and using with Natural and Semi-Natural Ecosystems' into practice in cities to strengthen the wellbeing and sustainable urban development.
Concerning the connections between the NBS concept with other related ones, it might still be controversial if NBS is an umbrella concept or a complementary concept in a multi-conceptual framework explaining the connections between humans and nature in socioecological systems. Pauleit et al. (2017) [116] compared four shades of green concepts (i.e., NBS, EbA, GI, and ESS) and concluded that as a powerful umbrella concept. In fact, NBS still relying on further refined definition and deployment of ESS and UGI on the ground systematically in the urban context. Based on this, Tzoulas et al. (2020) [117], who recognized the lacking of consensus in terms of conceptual links between these concepts (either too vague, broad, inconsistent, or missing), further compared NBS with other eight concepts based on six papers [4,7,11,35,116,118] to present the interdependence of human and nature and set a conceptual model of the social-ecological system of NBS in urban environments, which benefited the knowledge transfer at different scales and nations.
Specifically in the field of landscape planning education. Galan (2020) [119] argued the understanding and application of these concepts differed significantly in academic groups because of their heterogeneous origins. As a result, the synergistic and coordinated usage in this still faced semantic, grammatical, and operational challenges. To create clearer syntaxes between nature concepts to uptake the theory and optimize it in the practice of integrative landscape and urban planning that was suggested.

The Implications Among Geographic Scope, Funding Support, and Thematic Goals
Interestingly, we found that the funding support provided by a variety of organizations through research incentives is, in fact, the driving force that contributes towards the development of numerous subjects within the NBS concept. NBS metaphor has been rapidly adopted by European policymakers and research-funding sources since 2015 [4,70]. Most research findings on NBS governance covered all four EU thematic goals that were applied for global scale by reflecting a framework or inspiring a wide discussion and further upscaling, though they were commonly funded by EU organizations. Changes in funding opportunities, particularly in the EU and the US, and the potential for increased "cite-ability" using important terms were able to interpret the frequent usage of these specific terms [4]. However, to finish projects by using applied funding sources in a certain period may lead to publication growth, but the quality and long-term research and monitoring of NBS projects could be difficult to guarantee.
Another advantage is that the funding opportunities on NBS might attract various umbrella research, such as ecosystem services, green infrastructure, and urban forest. However, they are not specifically focusing on NBS or combined with grey infrastructure as hybrid solutions. We argued by seeing an increasing number of publications on NBS topic, which should give credits to the preferential funding of research projects from the US, Australia, and Countries of the EU (the UK included) [35], and subsequent publications addressed these topics (e.g., European Commission's Horizon 2020 research and innovation program, European Union FP7 program, US National Science Foundation, National Natural Science Foundation of China). A multi-case study transdisciplinary research in 11 European cities was conducted by the Resilient Europe project funded by EU that set up 15 NBS experiments to transfer learnt knowledge on lessons from experiments to urban planning, policy, and governance [82].
European Commission as the key enabler, the proposer, and the primary sponsor for this growth, according to priorities in EU set by Cohesion Policy with 11 thematic objectives supporting growth for the period 2014-2020, Objectives 1-4 were the main priorities for investment by European Regional Development Fund (ERDF).
Among these four objectives, 1 (strengthening research, technological development, and innovation) and 4 (supporting the shift towards a low-carbon economy) are more aligned with the mainstreams of NBS goals, which interpreted and implied that NBS has potential opportunities to promote within the EU. The Objectives 4 and 5 (promoting climate change adaptation, risk prevention, and management), 6 (preserving and protecting the environment and promoting resource efficiency) supported by Cohesion Fund (concerns eastern EU countries) were closely related to NBS, which might support other EU countries with weaker economy to reduce regional disparities. The according to funding mostly are not available in the continents outside the EU.
Other major funding agencies following EU were mainly independent country-level scholarships, such as Swedish Research Council Forms, National Natural Science Foundation of China, Portuguese Foundation for Science and Technology, National Science Foundation NSF (US), National Key Research, and Development Program of China.
In this review, we also found the significant gap for the Global North-South divide regarding scientific productivity and funding for NBS research. Even within the EU region, the western EU and eastern EU have big differences in terms of funding opportunities and research publications. It is evident that NBS case studies outside the western EU lack funding sources, for example, in the countries, such as Brazil [77,78], Costa Rica [120], Sri Lanka [121], Poland [16], and South Korea [122]. We found that the EU-funding instruments were mostly available for cities, but the application processes were competitive, requiring additional administrative staff and time and experts. More importantly, cofinancing obstructed numerous cities, especially in developing countries, without mature market mechanisms and investment to afford such projects [112].
China and Australia were the countries with more published documents and author relationships outside the EU. In this review, we have found currently, the NBS field has relatively weak connections between the EU and other continents. Case studies and publications were mainly contributed by developed countries. More attention and support should be given to other parts of regions around the world, especially developing countries, such as those located in Asia, Africa, and South America, as these regions (e.g., Bangladesh and Brazil) are more vulnerable to climate change and increasing urbanizations (i.e., growing populations) and extreme weathers (i.e., urban floods and heatwaves).
In fact, NBS might not be specific enough to accommodate any difference in other countries outside the EU, due to geography, population, social-cultural-political conditions, and there was a sharp learning curve to match such new concept with urban planners and city practicalities. Local place-based knowledge and culture were suggested to cocreate NBS strategies by several research [4,24,30,81]. More novel ideas of cities were closely related to NBS, such as 'climate-proof city', 'resilient city', 'green city', 'eco-city' and 'Sponge City'these terms were used in different locations, which could help us to rethink the concepts of urban design and planning. It was not about choosing nature-based or "green" approaches instead of "grey" infrastructure (e.g., canals and pipes). That was a "false dichotomy", according to the UN report. The UN proposing was just adding "green" solutions to the mix more often, an approach "that works with natural systems rather than against them" [123]. By combining existing infrastructure with flexible, decentralized innovations and merging conventional engineering with NBS, infrastructure can be integrated with urban systems for energy, waste, transport, housing, and food supply [9].
Considering the NBS research subjects, most publications were from environmental fields from the broad topics in 'Sustainability' and 'Environmental Sciences' to narrow subject areas, such as green sustainable science technology, ecology, public health, water resources, and forestry. Articles with the NBS concepts indicated that transdisciplinary was increasingly important for NBS studies [74]. Mostly the NBS articles currently were citing and cited in a wide range of disciplines. The keywords and clusters of NBS also came from extensive areas, including "Ecology", "Environment", "Engineering", and "Public Health". Economic terms were less frequently mentioned than environmental and social terms, indicating a strong tendency on the environmental and ecological characteristics of NBS. It was also noted that 'solution', 'approach', 'assessment', 'framework', 'challenges', 'benefits', and 'impacts' were hot research topics, indicating the research foci and directions towards more diverse covering the research categories. These findings unraveled that interand transdisciplinary approaches were increasingly important in providing a common platform for research and promoting innovative and sustainable solutions for urban areas.

The Way Forward: Improving the Governance Approaches of NBS in Urbanism
For the governance mechanisms, there was no evidence of either top-down or bottomup initiatives being more effective than the other. The case studies related to greening especially urban community gardens [24], were notably grass-roots force-led and selfgovernance [55]. The bottom-up of volunteer information applied on NBS in the UK mainly for urban water management [85] highlighted "the most important value of Volunteered Geographic Information may lie in what it can tell about local activities in various geographic locations that go unnoticed by the world's media, and about life at a local level". The initiatives and governance approached largely depended on socio-political context-for example, the Chinese "Sponge Cities Program" via NBS was a more top-down driven and involved multi-stakeholders, however, the competing priorities of stakeholders and their reluctance to make trade-offs hindered investment, due to the current political status and hierarchy (traditional) governance structure in China [87].
In fact, there was no single right solution or one-size-fits-all solution to achieve successful governance of NBS. Main enablers involving governance innovation and applying in cases across different planning and social-political-cultural contexts were identified, namely, participatory process (multi-stakeholder engagement), and financial incentives [124]. Common recommendations from the literature of participatory process by multi-stakeholder engagement, such as the collaboration between scientific experts with local communities [81], government institutions with NGOs [125]. Understanding the perspectives of multi-stakeholders to utilize more resources and forms of dialogue to integrate local values and cultures into science-based interventions for the most suitable NBS interventions was important. Usually, the larger the number of services and stakeholder groups were targeted, the lower the ability to maximize the delivery of each services (other than addressing urban stormwater issues) and fulfill the specific needs of all stakeholder groups at the same time, thus trade-off and cost-benefit assessment was further needed [4].
A large number of studies focused on NBS cobenefits and multi-functionality, while there were lacking actions to support citizens to use it. It is gradually realized that transforming experiences or policies from one place to another was difficult, due to the differences of geographical and social-political conditions. Authorities remained doubtable about how they can make NBS being mainstream in their practices. Upscale the implementation of NBS should embed them into the local urban agenda. An assessment of urban policy documents to identify gaps and potentials for NBS inclusion could promote to organize and harmonize different sectors [73]. The environmental characteristics of NBS were well researched by substantial literature, and quantitative methods attracted more attention by decision-makers [61]. The policy should safeguard democratic control and justice, which is socially accepted by diverse stakeholder groups and public. The importance of incorporating policy development into participatory processes that weave together multiple knowledge systems across and within institutions and governance processes was highlighted [70].
Adaptive governance that integrated place-based knowledge and local culture was suggested to cocreate NBS strategies by many researchers for social justice, cohesion, and resilience [4,24,30,81]. The application of NBS in the EU may not be directly replicable for ocean-focused regions, such as cities located in the Pacific Rim, with a considerably important and relevant traditional knowledge and emerging NBS experience [30]. The NBS studies in different regions faced various types of challenges. For example, Southern Europe was at the forefront of climate change [37], and their solutions combined with local knowledge can inform global understanding to address coupling risks of urbanization and climate change, but showed less attention among the South-Eastern and Eastern EU member states [37].
Inadequate financial resources also considerably hindered the NBS implementation. Previous studies identified the availability and limited funding as a barrier for NBS, as well as space availability and data scarcity in the rapidly urbanizing setups in developing nations [120]. Similarly, the ecosystem-based solutions within NBS indicated the lessons that policy mainstreaming was inadequate: Funding instruments should be integrated with the policy framework to facilitate the implementation and adaption. Multiple funding sources across policy streams should demonstrate cobenefits, therefore, ensure stakeholders' support [22].
Whilst, the extent of multiple benefits of NBS (such as urban forests) was largely dependent on the availability of funding, urban forest management objectives, the primary delivery agents of urban forests, and the understanding of ecosystem service concepts within local governments [126]. For nature-based stormwater treatment systems (e.g., constructed wetlands, ponds/basins, and rain gardens), the cases studies, such as Water Sensitive Urban Design (WSUD) in Melbourne, Australia [127], and Sponge cities in China, revealed a similar issue that the budgeted funding, in reality, was not reliable enough to predict systems' usage, and thus, policymakers prefer to plan expenditures based on installation costs. And the willingness to pay from the public was not as high as expected to cover the long-term costs, including maintenance fees [128].
Furthermore, potential funders, investors, and decision-makers were unconfident that NBS initiatives they support were effective and scalable, and considered potential externalities. In order to fill the gap, IUCN (2020) establish a governance structure of NBS global standard to analyze and evaluate NBS projects. It depicted that the international standard committee (oversight and safeguarding, revision of standard) need the input and feedback loop from the science and knowledge committee (scientific oversight of standard research priorities and evidence base for standard revisions), national/regional hubs (technical expertise, capacity-building, standard adaptation and assurance, user and learning community), and user group (community of practice). Therefore, to upscale the implementation of NBS and improve its effectiveness, thus, to increases NBS investment confidence, interested stakeholders were encouraged to pilot the global standard in current and future applications, and engage with such governance structure.

Conclusions
This paper explored how current studies of NBS were researched and interpreted in scientific peer-reviewed publications and different thematic fields, how NBS were implemented in practices connecting with related concepts. This paper contributed by identifying future research gaps and broadening the understanding of NBS works to sustainable urban development.
In this study, we found a large number of environmental issues, such as climate change adaptation and mitigation, disaster risk reduction and resilience management, ecosystem conservation and restoration, and sustainable urbanization. These issues have been gradually embedded into science and policy agendas via the instrument of NBS; for example, in a specific way of GI in urban context-though it has still not reached a consensus on the conceptual links between NBS and other concepts in the social-ecological system.
With the increasing studies of NBS, more sound evidence of the NBS multiple functions have added more weights to include NBS in urban planning strategies, while such practices still remained relatively niche, especially in developing countries, because of persistent myths about their feasibility and funding limitation. We demonstrated some vibrant pathways to inform researchers and decision-makers that on the future research foci, gaps, and pathways via our detailed meta-analysis from four major research themes, specific themes, and the governance discussion in this review.
In fact, the EU has taken the opportunity to strengthen its leading global role in urban sustainability with extensive case studies, social practices, research, and policy. We concluded by highlighting the applications and knowledge transfer potential of NBS to other regions of the world especially developing nations, as well as adapting for challenges in terms of governance, finance, and justice in a socioecological system when implementing NBS, which provided some 'Global Best Practice/Champion of Practice' aspects already and started influencing non-EU countries, such as China, as stated in this paper.
Although there was no 'one-size-fits-all' solution to achieve successful governance of NBS, main gaps involving governance and applying in cases across different planning and social-political-cultural contexts were identified, with corresponding recommendations, namely, adaptive governance, participatory process (multi-stakeholder engagement), and financial incentives.
The recent NBS paradigm was constructed most recently as a leeway from the previous knowledge, decisively entering into a domain with more transdisciplinary knowledgeexchange. This was in line with offering more pragmatic solutions rather than unification [35]. In this way, future literature mining could be explored with more detailed and comprehensive searching strategies and analysis tools (e.g., CiteSpace and Gephi for visual analysis as each software have drawbacks and advantages, and more thematic and comparative analysis for evaluation in details), for example, the application of specific techniques of NBS (e.g., different types of GI) in different nations, disciplines, and dimensions could be explored in-depth, as well as their relations and knowledge transfer.
Furthermore, selected thematic fields (as per European Commission) were mainly based on EU viewpoints at the early stage of NBS (proposed in 2015) and focused more on environmental issues. With the dynamic development of NBS in both theory and practice, the perspectives and frames aroused in developing countries are encouraged to be investigated and combined with EU pioneer insights in future research to maximize the knowledge transfer and experience sharing.
After this systematic review, we would like to offer some key recommendations for future research and policy insights as follow: (1) The linkages of NBS and GI as its solutions in practice, and how they link to four types of ecosystem services; the application of NBS with other methods (i.e., green-grey measures) for maximized benefits in the real cases of urban planning and management, gradually shifting to more NBS strategies in urban agenda. That could be contributed from multiple stakeholders through a participatory process of learning-by-doing experiences, linking scientific research and policy with local techniques, knowledge, and culture; (2) The assessment and research at a different scale from micro-scale to macro-scale and crossing different habitats to provide evidence that is not limited to cities and urban context; the wide-scale, upscale, and refinement of NBS linking air-soil-water metrics and various institutions for more rational decision-making; (3) The adjustment of the governance to be more adaptive of local context and promote the integration of NBS into urban planning agenda; incorporation both environmental aspects with social justice goals and financial opportunities; highlighting best practices and promoting NBS to bridge communications and strengthen the evidence of multiple benefits of NBS, therefore allocate adequate budget and explore innovative financial mechanisms for the implementation, monitoring, and maintenance of NBS in a long-term legacy; (4) The evaluation of NBS from comprehensive and transdisciplinary aspects, considering the cost-effectiveness, cost-benefits, multiple criteria analysis, life-cycle assessment, and socio-environmental justice; the development of multi-lateral dialogues and collaboration mechanisms for proactive investment, multiple assessments, selfassessment, and knowledge transfer in planning procedures between policy, practice, and science.

Conflicts of Interest:
The authors declare no conflict of interest.
Appendix A  (1) Proposed a framework for assessing NBS cobenefits across various challenge areas considering relevant indicators and methods; (2) translated the framework from theoretical support to practical importance by presenting a seven-stage process to guide thinking and identify the multiple values of NBS.
(1) The selection and assessment of NBS-related actions require the participation of a wide range of stakeholders, multi-disciplinary teams, and policy and decision-makers; (