Introduction
Urbanisation is associated with many adverse environmental impacts which can form the basis of public health issues in cities (WHO
2010). There is currently a burgeoning body of research demonstrating how increasing the proportional cover of greenspace (trees, green roofs, green walls) can ameliorate these problems. The benefits which greenspace provides for humans are known as ecosystem services (ES) (Nyelele et al.
2019). These ES include removal of air pollution via passive filtration (Beckett et al.
1998), providing local cooling through shading and evapotranspiration (Speak et al.
2020), and interception of rainfall runoff (Livesley et al.
2014). Increasing the quantity and quality of urban greenspace brings with it a multitude of benefits which work to improve the livability of cities and enhance urban resilience to threats such as climate change (Kim and Lim
2016).
The widespread implementation of successful urban greening initiatives has several obstacles to overcome. Most importantly, there is a lack of cheap, available land in cities and trees often require maintenance in order to survive in an environment which presents challenges (Salmond et al.
2016). Next, different urban tree species do not provide the same ES at the same magnitude, and the ES do not benefit all urban residents equally. There are often trade-offs to consider when selecting tree species, for example a species such as
Gleditsia triacanthos has a pleasing aesthetic due to its attractive foliage and flowers, however it does not provide much shade (McPherson
1984). Trees also have a number of ecosystem disservices (EDiss) to factor in, which are controversial negative aspects arising from the complexity of ecosystems in an urban setting (Lyytimäki
2015; Von Döhren and Haase
2015). These include allergy-inducing pollen production, and the emission of Biological Volatile Organic Compounds (BVOC) which can be the precursor to ground level ozone pollution in cities with consequent negative respiratory effects (Calfapietra et al.
2013).
There is thus a need for highly thought-out public green spaces which consider the trade-offs, disservices, and co-benefits with planning issues (Salmond et al.
2016). It follows that there is a need for research which considers the impacts of different species choices and demonstrates how a little experimentation in the species used within tree planting strategies can reap diverse benefits. Single issue modelling studies need to be incorporated into work that considers the bigger picture, and investment in green infrastructure needs to assimilate evidence-based research on the multiple biophysical and social processes whilst remaining relevant to human decision making.
A further issue to consider is the current low level of involvement of the public and stakeholders in urban tree planning and management processes (Alam and Lovett
2019). As the main users and beneficiaries of these spaces, they are the actors who have vested interests in the design, use and protection of public green spaces. Scientific lists (tree species and ES provided) and planning frameworks should not be the only sources consulted during the planning phase, and the voices and perspectives of communities should be integrated into the process (Salmond et al.
2016). This is especially important in the light of concerns around environmental justice and inequalities in access to urban green space, which raise questions around for whom urban greening is carried out (Sister et al.
2009).
Participatory urban green planning as well as transdisciplinary research for sustainable cities has gained momentum in the past decades (Pauleit et al.
2019; Pogačar et al.
2020). These approaches comprise an integration of urban residents into the identification of current research questions, involving them in the research process, and deriving recommendations for the practice of urban planning together with actors and stakeholders. Participatory and transdisciplinary tools and methodologies have been developed for these particular purposes by the social sciences (e.g. Parker et al.
2020). The governance of urban green spaces can be improved by participatory approaches (Alam and Lovett
2019), and this can be a two-way process which educates citizens and encourages the participation of actors who previously felt excluded from the process. The public are generally unaware of the full range of benefits that greenspace provide, with education being key to the acknowledgement of tree services (Fernandes et al.
2019). Citizens frequently cite shade and cooling as a benefit but neglect to mention spiritual or biodiversity aspects or water retention capacity (Lohr et al.
2004; Ordóñez et al.
2016). Opening people to the full range of benefits through education will increase participation and protection of green space (Pauleit et al.
2019).
At the municipal level, data are often not available to practitioners, and all indicators of ES and EDiss are rarely covered in city greening programs. Cities tend to make pragmatic decisions based on limited indicator sets (Carmen et al.
2020) with feasibility of planting schemes related to costs and local contexts being key. Maintenance costs of urban trees rather out prioritise their value for the urban ecosystem and human wellbeing (Kronenberg
2015). Methods are needed therefore to demonstrate the multiple benefits of different combinations of tree species, in real-world settings whilst also considering the EDiss which may be linked to certain decisions. Traditional urban forestry tends to follow conservative guidelines, with little room for experimentation, resulting in for example, monospecific plantings of low maintenance tree species.
There are a number of tools available to researchers and urban forestry practitioners which facilitate the quantification of ES, often translating these benefits into a financial metric useful to greenspace practitioners restricted by budgets. In particular, the i-Tree suite of applications (
www.itreetools.org), developed by the USDA forest service are well known (Nowak and Crane
2000). They use tree inventory data to quantify an array of ES and EDiss and the statistical models are based on peer-reviewed literature and empirical data from reference cities in different climate zones across the United States, with support added recently for worldwide regions. i-Tree has been used for a variety of research outputs such as assessing the benefits of vacant land (Kim
2016), modelling air pollution reduction in different planting scenarios (Jayasooriya et al.
2017), and estimating the carbon sequestration potential of urban forests (Boukili et al.
2017; Russo et al.
2014). It is particularly suited for modelling the projected future impacts of tree-planting schemes and quantifying how the ES increase as the trees approach their mature size (Nyelele et al.
2019; Parsa et al.
2019). It has been used by municipalities worldwide to communicate the benefits of existing urban forests such as in Campbelltown, Australia (Seed Consulting Services
2018), and London, United Kingdom (Rogers et al.
2015). In fact, the very process of using i-Tree has the added benefit of educating urban forest managers about the multiple benefits they provide (Raum et al.
2019).
In the present study, part of the greenCITIES project, we consider a large number of different tree planting scenarios in three piazzas of the City of Bolzano in Northern Italy. The scenarios are designed as realistically as possible and are based on themes, such as choosing all native species, trees that provide food, or optimizing ES provision. A public participatory approach is first used to gather data upon which to base several scenarios. This novel methodology consists of public consultations and stakeholder workshops, and a purpose-built smartphone application to record the opinions and tree species preferences of city residents. The aim of this inter- and transdisciplinary research is to compare different tree planting scenarios in terms of the relative provision of ES and EDiss using the i-Tree software. Optimising species mixes within the coordinated implementation of urban greening programs may be of interest to municipalities hoping to maximize the benefits provided by public green space. The inclusion of data from public participation allows discussion on the usefulness of these approaches for knowledge exchange.
Discussion
The use of a smartphone app to collect data on citizen attitudes to urban forestry generated some interesting results despite the relatively low number of respondents. It is a cheap and rapid way to get both an overview of the level of knowledge of citizens regarding tree services and disservices, and an indication of the importance attached to them by considering the response frequencies. For example, as in other studies, tree shade was revealed to be the most reported reason for visiting public green spaces (Lohr et al.
2004), and shade was second in importance in determining a favourite species. Unsurprisingly, when asked to name a favourite species, aesthetics was the most reported reason for the choice, as visual interaction has frequently been seen as the primary method of engagement with nature and defines initial relational experiences (Ordóñez et al.
2016). The more abstract, less visible, services, such as rainfall retention and air pollution reduction, would not factor into a person’s choice of favourite tree. People frequently named one tree in particular as their favourite, such as a large, monumental
Platanus acerifolia which is the first thing one sees upon leaving Bolzano train station, and had connotations of ‘home’ for some respondents. This exemplifies the strong personal bond which people can feel for trees.
Tree disservices mentioned were mostly related to the lack of provision of shade by small trees, and physical impacts such as damage to infrastructure, branch-fall and the removal or over-pruning of damaged or overgrown trees. With regards the last point, some respondents specifically lamented the lack of communication from the municipality with regards to removal of older trees, which citizens often feel emotionally attached to. Suggestions for improvement of the urban forest mostly included a call to plant more trees, and to leave older trees. Some specific suggestions were made about species choices, such as avoid planting female Ginkgo biloba trees to prevent the foul-smelling fruit, and plant more tall conifers in parks.
The app responses also revealed that urban heat islands are a less well-known phenomenon than climate change. This reinforces the idea that education of the general public is advisable (Fernandes et al.
2019) on the specific benefits of urban trees such as cooling urban microclimates. Most respondents recognized the importance of trees in cities but the results hint at the possibility that knowledge of the multiple reasons why may be limited. These knowledge gaps can be addressed by environmental education in cities. Besides school and university education for sustainability, community-based stewardship (Tidball and Krasny
2010) or civic ecology practices, such as focal practice of urban ecosystem restoration (Higgs
2003) and community gardening can add to a better understanding of environmental patterns and processes by the public.
The mapping exercise with schoolchildren and the workshop with stakeholders and citizens revealed that the inclusion of trees as a means of improving the public spaces was very important. It was felt that the piazzas should become meeting places, with facilities such as cafes, benches and playgrounds to facilitate the use of the spaces by as many different groups of people. The planting of trees and increasing green space was integral to making the piazzas lived spaces and revitalizing local businesses. The feedback from the workshop and stakeholder interviews revealed criticism of the past behaviour of the City Council’s Buildings Commission because they rejected any proposals made by the public. The piazzas Vittoria and Domenicani, in their current forms, were seen as suffering from neglect and degradation. Stakeholders were often playful and provocative with their proposals for the spaces, which reflects a frustration with the inability of the city to break free from its past and incorporate the views of citizens. (Solly
in press).
Dissemination of the results of the modelling work presented in this study, for instance via local newspapers or leaflets available in public buildings, might be one way to demonstrate the multiplicity of ES and EDiss associated with public green space. The models experimented with a wide range of scenarios with different themes and species mixes and showed how this can produce variability in the provision of ES and EDiss. It can be used to show urban forestry practitioners the potential impacts of changing or augmenting existing public spaces. Existing dense park areas with large trees (Piazza Vittoria) proved hard to beat in terms of ES provision. This underlines the importance of protecting the valuable resource of old trees in cities (Lindenmayer et al.
2014). When the trees in the baseline scenario are small, such as with the
Magnolia trees in Piazza Domenicani and the
Gymnocladus dioicus in Piazza Mazzini, improvements in ES provision are easily achieved in the model scenarios with huge increases in carbon storage, and tree canopy cover (Table
2) for example. It is worth noting that in Piazza Domenicani, for scenarios
4 (shade) and
6 (ornamental), the new planting scenario only improved on the baseline scenario when the optimal mix of species was modelled. This shows that if a goal of tree planting is to maximize ES provision, then careful consideration should be given to the species and quantities. On the flipside of this, the EDiss also increased greatly in Piazza Domenicani with respect to the baseline situation, reaffirming the need for judgements regarding trade-offs. BVOCs become an issue when combined with hot, summer weather and car exhaust fumes, so perhaps planting high BVOC emitters in pedestrian zones would lower the potential impact locally, notwithstanding the fact that BVOCs can be transported by winds. The selection of trees that maximize ES and minimize EDiss (scenario
5) generally worked in all three piazzas to improve on the baseline situation relatively. The stakeholder plan for Piazza Vittoria scored very low in ES due to the sparse planting pattern and inclusion of
Wisteria sinensis which does not score well in various ES categories given the low average dimensions used for modelling, despite being a very attractive tree due to its growth form and flowers.
It is fortuitous that the popular species mix of scenario
2 was generally the best scenario, therefore, maximizing ES provision whilst increasing the frequency of the favourite trees of citizens. This is mostly because the popular trees tended to be the potentially larger species (
Pinus, Platanus, Quercus, Tilia) and the provision of ES (and EDiss) is tightly bound to high leaf biomass (pollution capture, rainfall retention, shade) and tall trunks (carbon storage). We are not advocating handing over the control of urban tree planting programs to the general public, and basing tree inventories on favourite species, but more suggesting a system in which the public are free to become more involved in the process after receiving state-of-the-art information on the pros and cons of available species.
The challenge now is how to incorporate environmental decision-making tools such as i-Tree into urban tree management practices alongside participatory and transdisciplinary approaches. i-Tree outputs are often mentioned in internal reports and external forums but rarely influence funding increases or changing tree policies (Raum et al.
2019). However, there is a strong practitioner desire for prescriptive templates in decision making, that consider financial aspects (Salmond et al.
2016) as municipal budgets are often limited so maximising ES and minimising costs is the current model.
Improvements and suggestions for future research
The use of smartphone apps for gathering data is a quick, economic and creative method of gathering data, however it limits the pool of respondents not only to those with smartphones but to those that are inclined to take part in volunteered research, which tend to be young and female, and most likely to have an extravert personality type (Stachl et al.
2017). One option for increasing participation is to use QR codes, printed on signs and flyers, which take the user to an online survey thus saving time in downloading and installing an app. We only received 57 completed surveys which unfortunately lowers the robustness of the results. The collection of demographic data may aid in comparing the sample population to the population of the city or region under survey.
i-Tree is a good tool, allowing the multiple ES and EDiss to be modelled, but it can ignore the subtleties of influencing factors such as sensitivity to light exposure and species classification (Pace et al.
2018). For this reason, it is not advised to use it for research which relies on a high level of precision and accuracy of outputs. As a tool for a holistic appraisal of the benefits of urban forests it is unsurpassed and future research could elaborate on its and its use as an educational tool for practitioners. In the present study, we modelled the piazzas with mature trees based on average empirical dimensions. An improvement on this would be to gather information on the size of nursery trees which are used for new plantings and model the increase in ES and EDiss over time until the trees reach maturity.