More than half of the world’s population currently lives in urban areas, and this number is projected to reach 70% by 2070 [1]. Cities consume over 78% of total human resources and are responsible for 70% of greenhouse gas emissions, yet they occupy less than 2% of the Earth’s surface [2,3]. Furthermore, environmental crises such as climate change exacerbate drivers of urban inequality, threatening the future of cities [3]. While cities are centers of economic, social, and cultural innovation, they face significant challenges due to population growth outpacing adaptive capacity [4].

These challenges include biodiversity loss, climate change impacts, health issues, and social injustices. Cities are highly susceptible to inequalities, leading to higher crime rates, social exclusion, and income disparities [5]. Additionally, policy proposals that offer singular approaches to urban problems have been found to have unintended negative consequences, such as green gentrification [6]. This causes the displacement of low-income residents because of environmentally conscious urban development projects [7]. In many urban areas, wealthy neighborhoods tend to have better living conditions, while less affluent areas often experience poorer conditions, leading to negative health outcomes [8]. This means that social inequality can cause groups of people in urban areas to face greater health risks due to factors such as air pollution and heat stress [9,10,11].

This will increasingly be an issue because urban areas are warming at double the rate as their rural counterparts [12,13]. As cities become denser and expand, the amount of green area in and around them decreases, leading to a significant contribution to the urban heat island effect [14,15,16]. In the future, climate change consequences will pose an increasing danger to urban citizens, including more frequent and severe heat waves, storms, and droughts, and rising sea levels [12].

The other environmental challenge cities are facing is their negative impact on biodiversity. The homogenization of species, fragmentation, and loss of open, green space are all contributing factors to the decline in biodiversity in urban areas [17]. Moreover, the level of green area in a city is a crucial determinant of its biodiversity, with a drop in biodiversity occurring when the percentage of green area drops below ten percent [18]. As cities continue to expand, striking a balance between urban development and green areas becomes increasingly difficult [16].

This range of urban challenges can effectively be addressed by nature-based solutions (NBS). NBS draw inspiration and power from nature and have proven to be effective in tackling societal challenges, promoting biodiversity, and providing multiple benefits [19]. NBS are projects that deliberately work with ecosystems to ensure additional benefits for people and nature in comparison to grey infrastructure [19]. These projects can vary in their implementation, from using to restoring ecosystems or to even creating new ecosystems [20]. The multitude of benefits provided by NBS at a relatively low cost makes them suited in the response to many urban challenges [21,22,23]. NBS are also seen as a way to promote sustainability and equity in cities [23]. A comprehensive approach that incorporates NBS as part of the solution can address the aforementioned challenges in many ways including but not limited to the following.

NBS can contribute to the easing of social issues in cities, e.g., the presence of parks and urban green spaces reduces crime and can promote social cohesion [24,25]. By facilitating social cohesion and physical activity and reducing environmental stressors and pollutants these urban green areas also influence residents’ health [26,27]. Furthermore, people living near urban green have been found to feel healthier and happier [9,27,28,29]. In the face of climate change and increasing urban heat island effects, parks tend to be one degree Celsius cooler than the surrounding urban areas and this cooling effect extends beyond their borders [30,31]. Finally, NBS have emerged offering a way to create green infrastructure and enhance urban biodiversity [32]. Cities can be hotspots for threatened species; thus, by creating habitats urban biodiversity can increase, even in tiny patches [33,34].

The focus of this article is on NBS implemented in cities in Flanders, a region in Belgium that has experienced significant urbanization since the 1950s [35]. With one-third of its territory occupied by settlements, Flanders has become Europe’s most urbanized region [36,37]. Impervious surfaces cover 14.93% of Flanders, with 60% of this surface not occupied by buildings [38]. Compared to other wealthy and densely populated European regions of similar size, Flanders has the highest share of urban and built-up areas and the lowest share of natural ecosystems [36,37]. Consequently, Flanders is confronted with the urbanization challenges mentioned previously.

Flanders is experiencing a growing demand for urban green spaces, which are diminishing in size and unable to keep up with the increasing population dependent on them [38,39,40]. A governmental report published in 2012 has outlined various social benefits associated with urban green in Flanders, for instance, they function as a catalyst for social cohesion and integration, as well as foster a sense of community and belonging among residents in their respective neighborhoods [41].

Among the challenges that Flanders is currently facing in urban areas, one of them is related to threats to the health of its citizens. Air quality is a major concern in Flanders, with WHO standards being exceeded everywhere, resulting in 4200 premature deaths due to particulate matter in 2021 [42]. A recent literature review by the Belgian health department found direct links between the presence of urban green spaces and health benefits in Flanders [43]. Another major concern is heat stress in urban areas. During recent summers, there was a significant amount of excess mortality during summer heatwaves [38,44].

The impacts of climate change are increasingly visible in Flanders, where heavy rainfall days have been on the rise [45]. Heat waves are expected to become more frequent, longer, and hotter in the coming years [44,46]. Flanders has already experienced a warming of 2.46 °C since the 19th century, which is faster than the global and European averages [46]. This increased warming trend is largely attributed to land cover changes, particularly from urbanization [12,14,47]. Flanders’ 2050 climate strategy includes a section on climate adaptation, which highlights the use of NBS as a means of adapting to the effects of climate change in urban areas. The strategy recommends the use of NBS to address the aforementioned urban challenges [47].

This would be beneficial for biodiversity in Flanders as well. The most recent nature report from the Flemish Research Institute for Nature and Forests highlights that increased urbanization is fragmenting natural areas in Flanders, which is a significant pressure on biodiversity. The report notes that 89% of the natural areas are smaller than 1 ha, resulting in edge effects and limited dispersal opportunities. In urban areas, less than half of the species found in undisturbed scenarios are present. To address this issue, the report suggests increasing the presence of green–blue infrastructure in urban areas, which could improve the ecological quality and connectivity of the urban environment [37].

Assessing urban nature involves many variables, as green spaces play a vital role in offering diverse functions to local inhabitants and the cityscape. As a result, research in this field requires different disciplinary perspectives, such as biodiversity science, and social and economic sciences. However, this study aims to provide an overview of the diversity of functionalities and values that urban nature provides. A plural valuation framework is applied to achieve a broad perspective [48,49,50,51,52].

The projects originated from the three main online accessible repositories on these projects in the Flanders region. These three repositories together offer the widest range of NBS-type projects in Flanders. All repositories, although slightly varying in focus and embedded in various parts of the Flanders administration, have the common goal of inspiring professionals and the general public (Table 1).

(2) They were located in a Flemish city classified as a “main city” (“centrumstad”, Figure 1), which entails having a population of at least 80,000 people, a significant social and economic impact, and meeting various geographical criteria [53,54]. As a result, projects are highly diverse regarding their socio-economic and physical characteristics. The final sample of 108 descriptions of 106 projects can be regarded as representative of the diversity of successful green space projects in the main urban areas in Flanders.

This code tree is based on the specific values framework of IPBES [51]. The structure of the code classification was adopted from the URBAN Gaia study regarding indicators of urban green space [48]. The dimensions of the code tree can be aligned with the IPBES values.

“Nature” or intrinsic values: Worth of nature independent of any reference to humans as valuers and are worth protecting for their inherent value.

“Nature contributions to people” (NCP) or instrumental values: Nature’s benefits to humans, associated with nature as an asset or resource.

“People” or relational values: Importance of interactions between individuals and nature, as well as between individuals through nature, such as a connection to place, spiritual beliefs, acts of caring, and mutual exchange.

Throughout the research process, additional adjustments were made to the code tree. These modifications encompassed new aspects and categories and served to enhance the assessment of urban green initiatives (see results Section 3.1 for the final code tree). Adaptations to the code tree were made in discussion between authors, based upon issues uncovered during the coding process.

After the selection and text extraction, each sentence or statement was coded using Dedoose software following the adapted code tree [55]. Statements were coded using the most specific code available in the code tree. Only statements explicitly mentioning a certain value, e.g., “this measure was taken to improve local biodiversity,” were coded to reduce personal interpretation bias, improve traceability, and ensure repeatability. In case of doubt or disagreement on coding of certain statements, final codes were assigned after deliberation between the authors. This resulted in 567 coded statements representing 977 communicated values.

In parallel with the elicitation of values, physical, ecological, and social descriptors of each project and surrounding context were gathered. Correlations between these socio-economic and environmental variables and the elicited values were explored. To achieve this objective, data pertaining to the socio-economic parameters of the surrounding neighborhoods were sourced from a public database [56]. Pearson correlation analyses were conducted to explore relationships among parameters related to ethnicity, wealth, and spatial context. Strongly correlated parameter groups were identified. To streamline the analysis and avoid redundancy, an indicator was selected by prioritizing interpretability (Table 2). The chosen parameter demonstrated strong correlation within its group and offered straightforward interpretation in the study context, e.g., green area was chosen to be more informative than non-build up area.

Furthermore, an indicator was developed to assess the degree of technical and natural measures implemented in every project. A nature gradient of the Flanders nature report was utilized for this purpose [57]. This concerns a 5-step scale ranging from completely technical (e.g. parking lot renewal with water permeable paving) to completely natural measures (e.g. creation of novel wetland as a water buffering area), with intermediate steps including dominant technical with a natural presence (e.g. square renovation with trees and grass as cooling elements), equal measures (e. g. green roof constructions), and dominant natural measures with a technical presence (e.g. green swale connected by tubes) (Table 3) in order to verify if a pattern emerges.

A “size descriptor” for each project was also developed, which was based roughly on the Flemish green space standards. [58]:

Local: street or small neighborhood green areas.

Small: park-sized projects.

Large: neighborhood-sized projects, housing development.

Subsequently, the participation policy ladder developed by Edelenbos in 2000 was used to indicate the participation level in each project [59].

Informing: This involves one-way communication from the project organizer to the local public.

Consultation: This level allows the local public to voice their opinions without any commitment from the organizer to take them into account.

Advising: At this level, the local public plays an active role and provides feedback that the organizer takes into consideration. With substantial justification, it remains possible to reject input from the public.

Co-producing: This level involves a strong commitment from the local public from start to finish, resulting in a clear impact of their involvement, only limited by predefined conditions.

Co-deciding: At the highest level, the local public takes the initiative for the project and leads it from start to finish, with an advisory role for policymakers.

For analysis purposes, the participation levels were consolidated into two categories: “passive” (which involves informing and consultation) and “active” (which involves advising, co-producing, and co-deciding). This categorization aligns is based upon Arnstein (1969) [60].

Additionally, an investigation was conducted to determine whether any actions were taken to enhance inclusion in the participation process. A parameter was then established to reflect the presence or absence of inclusion measures, and another parameter to specify the type of measure taken, such as providing multiple participation opportunities, establishing a comprehensive participation project, or targeting specific audiences such as youth or the elderly. The results of these descriptors can be found in Table A1 and Table A2 in Appendix A.

In order to reach a conclusive analysis, a numerical indicator was constructed to encompass the diversity of dimensions, categories, and subcategories into a single index. This index is higher when subcategories or categories from different dimensions are co-occurring. This multifunctionality index (see also Hölting et al. [61]) is calculated in the following way:

DP: Dimensions present at project p

CP: Categories present at project p

SCP: Subcategories present at project p

3: Dimensional factor

2: Category factor

1: Subcategory factor

Example below:

Victoria Regia Park in Ghent:

A co-occurrence analysis of the subcategories was conducted using the “cooccur” R package [62]. Only subcategories that appeared more than fifteen times were chosen to ensure clarity of the analysis. The “Eulerr” R package was utilized to visualize the co-occurrence of the value dimensions [63].

While values associated with urban green projects are highly diverse, the main finding is that the distribution of dimensions remains remarkably constant across different repositories, contexts, and project types. The dimension related to “People” consistently ends up between 50–60%, with “NCP” at 20–30%, and “Nature” at 10–20% (Figure 2). It is important to note that these values are explicitly mentioned in the projects’ description in order to be coded as such and that they are the values that the project managers seek to highlight.

One hypothesis is that this distribution is driven by the number of subcategories per dimension (17/32~53%, 10/32~30%, and 5/32~15%). However, the occurrence of the single subcategories clearly shows that a small number of subcategories strongly influence the dimension distribution, rather than an equal distribution over subcategories per dimension (Figure 2 and Figure 4). Moreover, when disregarding a substantial part of the data and selecting the same number of subcategories (five most abundant) for each dimension, relational values are still the most prevalent dimension (43% “People”, 34% “NCP” and 23% “Nature”). So, regardless of the potential influence of the coding framework, relational aspects seem to be consistently the most abundant—and thus important—values associated with urban green infrastructure in Flanders.

These relational values are a prominent feature in the IPBES values assessment and similar literature. While research has been focused on instrumental and intrinsic values, a growing call has been emerging to give equal attention to relational values which resonate broadly and differently [51,64,65]. The results show that relational values are abundantly present. The main relational values highlighted are the emotions or experiences that people will have when they visit a particular site. The three most frequently occurring values in this regard are “Experiences”, “Identity, sense of place”, and “Social relations” and they often occur together (Figure 4 and Figure 5). Additionally, there is a significant emphasis on the intrinsic value of nature and its beauty, evident in the values of “Nature itself”, “City attractiveness”, and “Biodiversity”. However, purely economic values are noticeably absent from the descriptions, suggesting they are less important here.

Prior research has emphasized the importance of relational values in personal preferences. For instance, a study in Australia investigated people’s preferences for parks and determined that the most desired values were associated with health and safety [66]. Similarly, Arias-Arévalo et al. (2017) conducted a study on individuals’ preferences for a river system and discovered that relational values were ubiquitous in people’s preferences [67]. Drawing from these value distributions, it is evident that relational values are assuming a pivotal position in people’s perception of these NBS projects. Thus, this research confirms that there might be a mismatch between the values most often the focus of research, and the values regarded as important in practice [51].

Previous research has found that personal variables (e.g., age and gender) can influence preferences for urban green spaces [68]. Salm et al. (2023) found that income and the amount of green in the neighborhood can also influence urban green preferences, indicating that social or environmental variables may have an influence as well [69]. This study did not focus on differences in personal preferences. However, the high variability in subcategories might point to the impact of local context but average neighborhood income and greenness did not impact the values communicated. It is thus possible that the communicated values in the public repositories do not fully reflect the diversity of preferences between social groups.

Examining the co-occurrence of the subcategories (Figure 5), a clear group emerges. There is a group of subcategories that are explicitly mentioned in the context of water management (“Extreme events”, “Freshwater quality”, “Soils”). The recent literature has confirmed the effectiveness of NBS for urban water management [70,71]. Oral et al. (2020) highlight the benefits of restoring water to the natural hydrological cycle, which is the focus of the desealing projects in the data sample [70]. Huang et al. (2020) clearly state that this helps greatly with water quality and run-off management [71]. This is confirmed by these results as being an important focus of urban NBS in Flanders through which project managers want to illustrate the novel, nature-based approach to responding to increasing heavy rainfall.

Participation is often considered crucial for taking into account the diverse needs and interests of citizens in the context of NBS [19,23,72]. Studies have shown that participatory efforts are strongly associated with positive social sustainability outcomes, such as social learning, a sense of belonging, and environmental stewardship [72]. This research also examined the correlation between reported participation and the distribution of the values. Notably, this study found a substantial number of reported citizen-powered participatory efforts, which is not frequently found in the literature [73]. While no clear correlation was found between participation and the distribution of values, it should be noted that almost half of the analyzed projects did not report on their participatory efforts, leaving room for uncertainty. It is possible that these projects did involve participants and therefore reported the values preferred by the community, or conversely, that they were not inclusive and did not align with local needs and desires. Thus, this remains a blind spot, and further research is needed to draw any definitive conclusions.

In this study, a multifunctionality index was adopted to quantify the diversity of values reported. This analysis revealed a high diversity of values and a significant correlation observed between project size and multifunctionality. This is consistent with the expectation that larger projects have more resources, surface, and flexibility to pursue a variety of approaches to promote their goals (Figure 6).

By examining the distribution of dimensions (Figure 3) and the multifunctionality index (Figure 6), it becomes evident that there is a strong tendency to incorporate multiple values into project representation. Nature-based solutions are not just a nature reserve, not just a social space or water regulation system, they are often all these combined.

However, a point to be made is that highly diverse functions are not necessary for every project. Hölting et al. (2019), argue that a mono-functional project can still be valuable by providing a unique value to the neighborhood, called beta-multifunctionality [61]. In these cases, a project can be especially valuable if it offers services that are missing in that area. For example, a small open grass field in the city may not offer diverse benefits, but it can still be highly valued because it might be the only open space in the neighborhood.

This research is the first to explore values in urban nature projects and emphasizes the importance of relational values, often overshadowed by biodiversity and economic indicators [51]. Relational values need to be considered by researchers when looking at the value of urban nature. These values might be hard to quantify, monetize, or visualize but they clearly are relevant to research into urban NBS projects.