Urban green space (further called UGS) in this article is referred to all types of green space - like parks, squares, green corridors, playgrounds, etc. – that have permeable ground surface and vegetation or potential for the growth of plants.

Ecosystems provide services or benefits for humans. Ecosystem services (ES) are produced by the biodiversity of a landscape (Niemelä, 2010). These are classified as supporting, regulating, provisioning and cultural. Regulating ES in UGS include pollination, absorption of harmful gases, dust collection, climate and microclimate regulation at the street and city levels, carbon sequestration and storage, reduction of noise pollution, rainwater absorption (Niemelä, 2010). These ES are vital for the existence of cities, even though they are not usually the reasons why citizens appreciate UGS the most. People mostly value UGS for their cultural ES, like recreation, psycho-physical and social health benefits, education (Niemelä, 2010). Provisioning ES are not always dominant and include timber production, food (fruits, berries, mushrooms), fresh water, medicinal herbs, fish. Supporting ES are the ones that allow the other three types of ES to exist. These include soil formation, habitat provision, biogeochemical cycles, photosynthesis (Niemelä, 2010).

The role of maintenance of UGS is to manage and support the existence of UGS that provide so many positive functions in urban environment. According to some research maintenance accounts for 20% of the environmental impacts in the life cycle of UGS (Li, Liu, 2019). Moreover, often maintenance needs also to cover gaps or miscalculations in planning and choice of plants (Nilsson, 2020).

Maintenance among the most frequent activities includes irrigation, fertilization, pest and weed control, trimming, pruning, cleaning of the areas from leaves and other plant residues, ploughing. Maintenance is usually associated with intense labour and use of machines, like trucks, lawn mowers, hedge trimmers, chainsaws, etc. (Li, Liu, 2019). Machinery is usually powered by fossil fuels and emits harmful gases, that largely consist of greenhouse gases. Even though UGS indeed functions as carbon sink, it accounts only for a small part of annual CO2 emissions of a city (Niemelä, 2010). Moreover, management and use of a park transforms it into a carbon source, as it produces more CO2 that it is capable of sequestrating (Niemelä, 2010).

Greening of urban spaces is usually a sustainable solution for a variety of challenges within urban environment. However, regular intensive maintenance of UGS may compromise sustainable development of the areas by contradicting the principles of minimization of artificial disturbance and maximization of self-renewal capacity (Li, Liu, 2019). So, minimization of maintenance is a way towards sustainable development of UGS (Li, Liu, 2019).

    1.3 Objectives of the article
While UGS brings a lot of environmental and social benefits, it also usually needs a lot of resources to maintain it (Fig 1). This compromises the sustainability value UGS. The purpose of this article is to describe this issue and discuss a direction towards a sustainable alternative.

2. Methods and materials
Challenges described and discussed in this paper have first become obvious to me, when I was studying at a gardening school Hvilan Utbildning in Åkarp for a couple of months in 2017. I observed that the existing approach to design and maintain UGS requires a lot of input in the form of energy (fuel and electricity), work, fertilisers, and water for irrigation. This, I believe is an interesting aspect of UGS that is believed to be like a panacea to various environmental issues in a city. I started to be curious, if UGS could be designed in a more self-sustained way. I brought my curiosity to the subject into my everyday life and continued my observations of the green spaces along the streets to gather more insights into the subject. I have done a literature review to investigate whether this topic is covered by researchers and practitioners. Unexpectedly for me, while there are numerous resources about the undeniable positive effects of UGS, there were fewer resources than expected discussing the possibility and necessity to create self-sustained green spaces to maximise their value for sustainable cities. I arranged an interview with Madeleine Nilsson, a teacher at the gardening school Hvilan Utbildning, to better understand challenges and possibilities of creating and maintaining sustainable alternatives of UGS.

Nature is usually the primary form and medium in landscape architecture (Dee, 2012). Most often the existing natural environment is preserved to some extent (to support natural ecosystems and preserve habitats) while the rest is modified and is kept in the designed form with the help of a regular maintenance. Every landscape design employs nature in either form of function (Dee, 2012). Sometimes only the natural forms are copied, but the functions are not considered; or the natural functions are mimicked without the corresponding natural forms (Dee, 2012). The sustainable design of UGS suggests applying equal attention to both the natural forms and functions.

The possibility to introduce more sustainable options for UGS is discussed increasingly more among research community and municipalities (Nilsson, 2020). The reasons for the changes in a discourse about the need and possibilities to make UGS less maintenance dependent is largely formed by the increased awareness of environmental issues and popularity of sustainability values (Nilsson, 2020). One of the examples of low-maintenance UGS is more frequent use of meadow plantings instead of grass lawns which can benefit urban wildlife and reduce maintenance. For instance, there is a roundabout in Lund (Trollebergsrondellen) that has been designed by a former professor at SLU, Peter Gaunitz (Fig.2). The ambition is that this area needs no maintenance at all after the first years when the plants would have established themselves (a descriptive sign near the roundabout, 2020). There are more examples of alternatives to high-maintenance UGS appearing every year. However, there are no concreate policies and practical guidelines that I could find for integrating sustainable UGS into city development in a more systematic way.

3.2 Forest garden – an example of a sustainable UGS
Except for savannahs, wetlands and flood plains, most land that has been inhabited by people strives to naturally transform into a woodland. (Reynolds, 2016). The strongest factor that stops them from doing so is maintenance. Mary Reynolds – an Irish landscape designer – says in her book “The garden awakening”: “Most of our gardening energy is spent trying to stop our gardens from becoming what they want to become. We call it ‘maintenance’”. She proposes an alternative to the conventional gardening methods – forest gardens for both private and public spaces. UGS that mimics natural woodland in both the form and function is an example of a sustainable UGS in the cities. One study showed that it is recommended to introduce more native woodlands into city environment to increase tolerance and adaptability of UGS (Li, Liu, 2019). It was also proven that self-maintenance in green space optimizes the utilisation of energy and resources, reduces environmental pollution (Li, Liu, 2019).

A forest garden consists of seven layers: upper-canopy trees, sub-canopy trees, shrubs, herbaceous plants, ground cover, underground plants, climbers, and vines (Reynolds, 2016). All these layers imitate the natural woodland structure. The main idea of forest gardening is to accelerate the process of natural revitalisation of a piece of land after it experiences any kind of disturbance. This is done by mimicking the form of a natural ecosystem. However, the most effective way is to sequence the introduction of new forms (like the layers of a forest described before) in the order they are introduced in an ecological succession (Reynolds, 2016). Every stage of an ecological succession is a necessary preparation of the soil and the environment above the ground for the next stages of the succession. So, natural functions are shaped by the proper sequence and placements of forms in a landscape.

Mary Reynolds argues that the quality of soil is fundamental to the “health” of a garden (Reynolds, 2016). She suggests that “no-till” methods are the best for establishing the proper condition of soil to support the forest garden throughout its whole lifetime without maintenance (except for the first years). Eventually there is less risk for erosion, there is no need for pesticides, fertilizers, and intensive irrigation. Moreover, no-till systems are better at storing carbon, which improves its regulating ecosystem service so important in urban environment (Reynolds, 2016).

It is advised to implement continuous mulching (an activity inspired by nature) which helps to retain moisture, improves aeration, drainage, suppresses competition from neighbouring plants and creates suitable habitats for small animals and microorganisms (Reynolds, 2016). Mulching is especially useful during the first years while plants are establishing. Also, it is important to facilitate the development of a groundcover of soil-building plants to improve soil quality (Reynolds, 2016).

Plants become dependent on the various maintenance activities, including irrigation. Therefore, that is advisable to integrate swales into the planning of a landscape. That will help to collect rainwater and minimize irrigation requirements. Moreover, less artificial watering encourages the development of stronger and deeper roots, so plants eventually will require no additional watering at all.

Natural ecosystems are characterised by higher biodiversity of species than designed landscapes. Many species and their interactions can perform a variety of maintenance activities (Reynolds, 2016). So, creating more biodiversity is clearly the way forward towards low-maintenance UGS. Moreover, a different choice of low-maintenance plants with similar appearance as the high-maintenance alternatives is advisable.

The role of weeds in a garden should be reconsidered, as they are also important in establishing a balanced ecosystem (Reynolds, 2016). These plants disappear by themselves with time. Mulching can be applied to inhibit them in places where they might suppress the growth and establishment of other significant species that are supposed to remain in the garden for a longer time (Reynolds, 2016).

Usually it takes more planning in the beginning to create forest garden, but when the ecosystem establishes itself, it requires low or no maintenance at all (Reynolds, 2016). The main principle for all the planning and maintenance of a forest garden is to create an ecosystem that can survive on its own without transforming into something completely different than was designed in the beginning. Principles of forest gardening, I believe, can be applied in the design of various sustainable UGS.

4. Challenges for implementing sustainable design of UGS
My interview with Madeleine Nilsson, teacher in gardening at Hvilan Utbildning, has brough more understanding of the challenges that we could face when transforming UGS towards low maintenance. First, there are special traditions and customs associated with particular type of UGS, like churchyards, that dictate how the garden should look like (Nilsson, 2020). Moreover, there is always a particular understanding in society of what is beautiful. This is affected by principles that we could research with the help of environmental psychology, but also by trends in gardening, city planning, society and even art.

The earlier mentioned example of a no-maintenance area, Trollebergsrondellen in Lund, has spitted opinions among citizens. Many people enjoy the wild, colourful look of the place, the shifting shapes, and textures of the place throughout the year. However, there are people who believe that the place lacks aesthetic value. I believe that people who are aware of the environmental benefits and low cost of maintenance of such place have more tendency to perceive the place as beautiful. Probably the perception of beauty in urban environment may be affected in the future by an increased environmental awareness and the decrease in the amount of time people spend in the wilder types of environment.

Experts from World Health Organisation (2017) advice that a “regular maintenance is necessary so that end users perceive UGS as safe, clean and cared for”. There might be a challenge to make no-maintenance UGS safe providing enough visibility for the users. So, probably the majority of the sustainable UGS is going to require some maintenance to keep changes in vegetation compatible with urban life (Nilsson, 2020).

Cities all over the world are only starting to experiment with designing low-maintenance UGS. This has not been extensively done before probably because of a trend in design focused on transforming nature rather than recreating it, and less attention to environmental and sustainability challenges in cities. Finally, more research is needed to know how exactly to implement sustainable UGS. This should be done, I believe, from the perspective of ecology, landscape architecture, urban development, and economics.

5. Is it worth trying to make UGS more sustainable?
    5.1 Benefits of sustainable UGS
Maintenance might be limiting the amount and extent of the positive impacts UGS can provide. As discussed previously, UGS that mimic forms and functions of natural ecosystems are supposed to require less or no maintenance at all. This will reduce or eliminate the environmental impacts of UGS throughout their lifecycle. Sustainable UGS are supposed to require substantially less economical costs for their management.

Ecosystem services are produced by biodiversity. Sustainable UGS is associated with more abundant biodiversity of species (both flora and fauna) and this leads to a wider variety and strength of ecosystem services provided by the areas. This includes more habitats for large and small animals, birds, insects, microorganisms, mushrooms. This is partially facilitated by no-till gardening methods that promote good soil properties. Sustainable UGS are promising to become more attractive for pollinators. The denser vegetation may provide better cooling effect and air purification capacity. The minimisation of intensive irrigation substituted with maximisation of rainwater utilisation may lead to better rainwater management.

Sustainable UGS may provide more experience of wilderness (managed to a certain extent) within even quite dense cities. This might improve the biophilic characteristics of the areas, improving the daily human contact with nature (Beatley, 2011).

    5.2 Notions of beauty
Undoubtedly there are magnificent parks that have been created back in history and are part of the cultural heritage. However, they were planned primarily as pieces of art rather than a necessary part of urban environment benefiting the urban ecosystem and human well-being.

Parks that have been created as pieces of art have to be maintained to preserve their value. Their maintenance is a part of the art as well, I believe. However, the maintenance is a necessity, not a part of the art process when we talk about the UGS that serve specific functions in the urban environment. Therefore, there is no loss of value of these places if the areas are designed to be more self-maintained.

People are usually attracted to natural forms and what is considered as beautiful usually reminds of nature in some way. While we appreciate parks that belong to cultural heritage, we also enjoy experiencing more wild landscapes with low or no maintenance. UGS, particularly small parks, squares, greenery along the streets or other UGS are usually designed to be functional, simple, and safe. The aesthetic value alone of the type of UGS is never the priority. The transformation of the type of UGS into the natural self-evolving ecosystems that require almost no maintenance may also add aesthetic value. The areas become the representation of wilderness, therefore increasing the presence of biophilic characteristics of the place.

    5.3 As a conclusion
I believe that the way forward is to have a combination of places that require a varied amount of maintenance. However, the amount of maintenance should be adequate to the functions the place is performing. Places that are supposed to function as air purifiers and habitats for fauna and flora among other important ecosystem services could potentially become more self-sustained. Places that are created mostly for artistic purposes may require more maintenance.

To conclude, I would like to make a recommendation for future research about the viability of creating sustainable UGS. The research should involve urban living labs to test planning and establishing low-maintenance UGS. This will give us more concreate practical guidelines for transitioning towards sustainable UGS.

References
Beatley, T. (2014). Biophilic Cities. In: Wheeler, S.M. & Beatley, T. (Eds.). The Sustainable Urban Development Reader. 3 ed. London-New York: Routledge, pp. 181- 183
Dee, C. (2012). To Design Landscape – Art, Nature & Utility. London-New York: Routledge. (chapter. 3, 5, 6, 14)
Li, B. N., Liu, Y. (2019) Sustainable design in UGS. https://www.intechopen.com/online-first/sustainable-design-in-urban-green-space
Niemelä, J., Saarela S., Söderman, T., Kooperoinen, L., Yli-Pelkonen, V., Väre, S., Kotze D.J. (2010) Using the ecosystem service approach for better planning and conservation of urban green spaces: a Finland case study. Biodiversity and Conservation, 19, 3225-3243.
Nilsson, M. (2020) Interview with Madeleine Nilsson, 20 October.
Reynolds, M. (2016) The garden awakening. Designs to nurture our land and ourselves.
World Health Organisation (2017) Urban green spaces: a brief for action. https://www.euro.who.int/__data/assets/pdf_file/0010/342289/Urban-Green-Spaces_EN_WHO_web3.pdf%3Fua=1