Greening cities is a transversal solution to numerous urban challenges, not only to support the transition towards climate neutrality, but also to improve biodiversity, social justice and resilience. Greening cities can include many diverse solutions, among them tree planting and the development of parks and natural urban green areas.
Tree planting and urban forest
Planting street trees, single line trees or group of trees, as well as trees in urban parks or urban forests trees represents an effective solution to maximize carbon capture and sequestration. It also contributes to regulating microclimate in dense urban areas, supporting water infiltration, and providing aesthetic, health and well-being benefits to citizens. Planting a diverse mix of tree species provides more diverse habitats for more animal species. Diversity also ensures that the urban forest is better prepared for pests or diseases which could eradicate an entire species of trees and severely affect other species in the ecosystem (see for example urban forest solutions in ClearingHouse).
Parks and natural Urban Green Areas (UGA)
While trees are an important component of parks and natural urban green areas, these areas also include other features to boost cultural and active recreation activities, such as sport playgrounds, benches and rest areas, and thematic and therapeutic gardens (see for example the solutions in the group of re-naturing urbanization in URBAN GreenUP).
Urban green area, Izmir, Turkey. URBAN GreenUp. Source: https://www.urbangreenup.eu/#lg=1&slide=3
While the World Health Organisation recommends all cities to guarantee access to UGA within 300 metres from residents’ houses, this is still far from being reached in many EU cities [1]. The benefits related to the re-design and improvement of existing parks and UGA or to the development of new UGA are related to all the spheres of human health and wellbeing ranging from metabolic disease to better cognitive development to increased psychological wellbeing [2].
Parks and UGA should be designed as multifunctional areas and this may call for participatory processes to co-design spaces and features to better respond to the needs of diverse target groups, including vulnerable groups, living in the city (see for example stakeholder and citizen engagement approach in UNaLab).
Co-creation of UGA and NBS in Eindhoven, Netherlands. UNaLab. Source: https://unalab.eu/system/files/2020-02/d22-co-creation-workshop-report2020-02-17.pdf.
MATURITY:
While trees and parks can be considered largely available on the market and mature to be implemented everywhere, it is crucial to reflect how to properly plan and design these kind of solutions, tailoring them to various contexts. Specific expertise is needed in park design, participatory processes, and in deciding which trees to plant, including in relation to future scenarios of climate conditions, for example. Public acceptance and care of UGA are crucial for effective implementation.
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Co-benefits vary depending on the type of project implemented (from single street trees, group of trees, or new urban forests or urban green areas), the extent of the solution, the planning phase, project design, and the involvement of communities in the co-design and co-implementation of the solutions [3].
Reduce risk to natural and climate hazards – extreme heat and flash/surface floods, Reduce hot spots/urban heat islands in the city: The cooling properties of trees contribute to reducing extreme heat and the water infiltration and water retention capacity of soil and tree roots can reduce the risk of flash/surface floods [4]. For example, in a simulated experiment run in four cities in different climates (Phoenix, Singapore, Melbourne, Zurich), considering diverse conditions of air and surface temperature, the results show that evapotranspiration of well-watered trees can decrease local temperature at maximum by 3.1– 5.8 °C in the four climates during summer [5].
Reduced energy poverty: The cooling capacities of trees can reduce the need for cooling houses, thus reducing energy needs and related energy poverty [6].
Improved air quality: PM10 filtration is one of the capacities of trees and shrubs, but with limited impact on air pollution, therefore other solutions should be considered to drastically decrease air pollution (e.g. mobility services, heating and cooling, etc.).
Aesthetics, Mental wellbeing/quality of life: Trees and urban green areas (UGA) are a crucial part of the urban landscape and contribute to developing a sense of place in citizens, also deepening their connections to nature, finding stress release and improving mental health and wellbeing [2].
Social equality and justice, Social cohesion: With effective spatial distribution, trees and UGA can support social equity and climate justice in cities, offering benefits to all, including the more vulnerable groups of citizens [7].
Physical health, Healthier and more attractive lifestyles: Properly planned and designed UGA can help in supporting more active and healthy lifestyles, attracting people to walk, run, play and meet, supporting their health and wellbeing [8].
Smart-soils and phytoremediation: Phytoremediation of urban brown fields; Smart soil production in climate-smart urban farming precinct
Pollinator verges and spaces
Shading structures
Circular food cities
Urban green areas management (wood pruning)
Urban form and layout:
Many cities around Europe are supporting sustainable land use through promoting compact city development, reducing urban sprawl, and minimising land-take, but also finding spaces for nature based solutions such as trees and new urban green areas (UGA). Tree planting and UGA are enabled by availability of land for implementation of new UGA, possibility of de-sealing of abandoned areas, and re-use of abandoned spaces [16].
Policy and regulatory/legal framework:
Diverse policy and regulatory frameworks at different levels can enable implementation of tree planting and UGA.
At EU level, within the overall framework of the EU Green Deal, for example:
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The European Climate Pact and the related ‘Green Areas’ initiatives [20].
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The European Biodiversity strategies: cities with a population of more than 20,000 are called upon ‘to develop Urban Greening Plans, including measures to create biodiverse and green urban forests, parks and gardens; urban farms; green roofs and walls; tree-lined streets; urban meadows; and urban hedges.’ To facilitate this work, the Commission set up an EU Urban Greening Platform with cities and mayors in 2021 under the Green City Accord [21].
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The Environmental Strategic Assessment procedures and the inclusion of criteria to compensate, mitigate or enhance Ecosystem Services through the implementation of tree planting and/or UGA [11].
At local level, cities may refer to diverse strategies/norms and laws, for example:
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Climate adaptation and mitigation plans (see for example Brussels) [22];
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Biodiversity and Green Infrastructure strategies (See for example Barcelona) [17];
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Building codes/standards can incentivise more greening in buildings and common spaces.
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Parking policies and management can reduce the number of parking spaces, which may increase space available for greening.
Funding and financing:
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Funding for urban reforestation may be available within National Recovery and Resilience Plans for actions contributing to climate neutrality and ecological transition of urban areas.
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Public-private partnerships may be explored to support co-financing of urban forest or new UGA. For example, the project Green4C collected case studies from EU and beyond about innovative financial model, while a report by the Commission for Architecture and the Built Environment (UK) presents eight models for funding urban green spaces [18].
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Considering EU regional funds, many Rural Development Plans include reforestation activities in peri-urban areas. There are also relevant calls for proposals in other EU funding instruments (Horizon Europe, LIFE+).
Economic and social context:
Citizen awareness and education activities can support the governance model of shared care of trees and UGA. See for example the ‘City of trees’ teaching material created by Clearing House. Awareness campaigns can also be used to explain to citizens the importance of tree planting and UGA implementation and care.
Project governance and implementation modalities:
Participatory governance models are increasingly implemented in urban areas and participation has become compulsory by law in many European regions and cities (see for example Emilia Romagna planning law [14]). Inclusive and participatory planning and design of UGA is crucial not just to inform citizens about the benefits of implementing such areas, but also for designing and planning areas able to benefit specific target groups that could be otherwise excluded from these areas (e.g., elderly, migrants and refugees, women, children). For example, UNaLab uses a wide range of co-creation tools and methods to explore, design, implement and evaluate nature-based solutions (see the toolkit for preparing co-creative sessions with stakeholders). Also, workshops, consultations, and citizen advisory boards or “tree boards” are a direct tool for citizens to engage in urban forestry through advice on design, development, and maintenancehttps://euc-word-edit.officeapps.live.com/we/wordeditorframe.aspx?ui=en-GB&rs=en-IE&wopisrc=https://eceuropaeu.sharepoint.com/teams/GRP-CNC/_vti_bin/wopi.ashx/files/9b0522bfe21047c5a50bb3e3dcf2f19d&wdenableroaming=1&mscc=1&hid=834279A0-6025-5000-69EA-EEAEDB30A37C&wdorigin=ItemsView&wdhostclicktime=1668628044218&jsapi=1&jsapiver=v1&newsession=1&corrid=834d9e9f-3b48-45e4-87f9-96bb38e1d5c0&usid=834d9e9f-3b48-45e4-87f9-96bb38e1d5c0&sftc=1&cac=1&mtf=1&sfp=1&instantedit=1&wopicomplete=1&wdredirectionreason=Unified_SingleFlush&rct=Medium&ctp=LeastProtected#_ftn1 [23]. The NeighbourhoodWatch project focuses on urban forestry and testing various methods of participation tools in cities across Europe [24]. Citizen science is also a method to measure the well-being and impact of trees on community. That specifically entails scientific research conducted with participation from the public [25].
Climate and geography:
When considering planting new trees or developing a new urban forest, the focus should be on native species, which need minimal resources and have high survival rates. Greening plans should consider adaptability of diverse tree species to the changing climatic conditions of urban areas. For example, in Barcelona, the Green Infrastructure and Biodiversity Strategy suggests specific plants and vegetation that require small amount of water to grow [17].
Technical aspects/infrastructure:
Geospatial information may support to better understand where to place new UGA or improve existing one. In the case of Bologna, for instance, a UGA accessibility analysis allowed to identify areas of the city most in need [12].
Funding and financing:
The costs of maintenance of trees and UGA can be high, as they require regular watering, pruning, lighting and maintaining features included in UGA. This can be addressed by adopting a public-private partnership model. For example, GAIA developed a public-private partnership model for urban forestation through the adoption of the ‘green areas inner-city agreement’. It promotes co-responsibility by introducing an innovative management tool that fosters tree planting in urban areas to compensate for the construction of infrastructure.
Policy and regulatory/legal framework:
Land use conflict due to different interest of diverse stakeholders in the city (see for example [13]). Increasing housing demand in some cities competes with greening efforts.
Economic and social context:
Resistance to change from citizens when there is limited available space to build parks, e.g. when land is privately-owned or when used differently (e.g. for parking).
Project governance and implementation modalities:
Lack of citizen engagement knowledge or training in local government or disinterest from citizens to participate in engagement initiatives. Hesitation from other stakeholders to engage in consultations also poses a barrier.
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Allergenic effects of pollen on the population [15].
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Gentrification effects that may be caused by increased real estate price due to green regeneration projects (see GreenLULUs).
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Projects
UNaLab contributes to the development of smarter, more inclusive, more resilient and more sustainable urban communities through the implementation of nature-based solutions, which are co-created with and for local stakeholders and citizens. The three front-runner cities – Eindhoven, Tampere and Genova – establish Urban Living Labs to experiment, demonstrate and evaluate a range of different nature-based solutions addressing climate- and water-related urban challenges (H2020, 2017-2023).
Clearing House focuses on tree-based green infrastructure and urban ecosystems to address societal challenges, simultaneously providing ecosystem services for human well-being and biodiversity benefits. Together with 10 cities and urban regions in Europe and China, the project partners will develop an online application, a global benchmark tool, and guidelines that can aid in the design, governance and management of urban forests (H2020, 2019-2023).
URBAN GreenUP aims at the development, application and replication of Renaturing Urban Plans in a number of European and non-European partner cities with the aim to mitigate the effects of climate change, improve air quality and water management, as well as to increase the sustainability of cities through innovative nature-based solutions (H2020, 2017-2023).
LIFE CLIMATREE aims to contribute towards the development of a novel methodology and an innovative tool for the quantification of carbon storage in permanent tree-crops (co-financed by Life programme, 2015-2020).
GreenForCare is a three-year Knowledge Alliance project innovating and promoting nature-based health and social care through university-business alliances, that among other things, studied innovative funding mechanism for urban green (Co-funded by Erasmus+ Programme, 2020-2022).
GAIA (Green Area Inner city Agreement), developed by the municipality of Bologna, has developed an instrument by which local enterprises and firms can decrease their carbon footprint supporting trees plantation around the city and, at the same time, generate environmental and social benefits for the community (Co-funded by Life Programme, 2010-2013).
GreenLULUs (Green Locally Unwanted Land Uses) analyses the conditions under which urban greening projects in distressed neighbourhoods redistribute access of environmental amenities to historically marginalized groups. The study takes place in 40 cities in Europe, the United States, and Canada (H2020, 2016-2022).
References
[1] Pafi, M. et al. 2016. Measuring the Accessibility of Urban Green Areas: A comparison of the Green ESM with other datasets in four European cities. EUR 28068. Luxembourg (Luxembourg): Publications Office of the European Union; 2016. JRC102525 doi10.2788/279663. https://publications.jrc.ec.europa.eu/repository/handle/JRC102525.
[2] Lee A.C.K and Maheswaran R. 2011. The health benefits of urban green spaces: a review of the evidence, Journal of Public Health, Volume 33, Issue 2, June 2011, Pages 212–222, https://doi.org/10.1093/pubmed/fdq068.
[3] Raymond, C. et al. 2017. A framework for assessing and implementing the co-benefits of nature-based solutions in urban areas, Environmental Science & Policy. Volume 77, November 2017, Pages 15-24. https://doi.org/10.1016/j.envsci.2017.07.008.
[4] Szota, C. et al. 2018. Tree water-use strategies to improve stormwater retention performance of biofiltration systems, Water Research. Volume 144, 1 November 2018, Pages 285-295 https://doi.org/10.1016/j.watres.2018.07.044.
[5] Meili, N. et al. 2021. Tree effects on urban microclimate: Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects, Urban Forestry & Urban Greening. Volume 58, March 2021, https://doi.org/10.1016/j.ufug.2020.126970.
[6] Ko, Y 2018. Trees and vegetation for residential energy conservation: A critical review for evidence-based urban greening in North America, Urban Forestry & Urban Greening. Volume 34, August 2018, Pages 318-335 https://doi.org/10.1016/j.ufug.2018.07.021.
[7] Baró, F. et al. 2019. Under one canopy? Assessing the distributional environmental justice implications of street tree benefits in Barcelona, Environmental Science & Policy Volume 102, December 2019, Pages 54-64. https://doi.org/10.1016/j.envsci.2019.08.016.
[8] Sköld, Y A et al. 2018. A framework for assessing urban greenery's effects and valuing its ecosystem services. Journal of Environmental Management. Volume 205, 1 January 2018, Pages 274-285. https://doi.org/10.1016/j.jenvman.2017.09.071.
[9] Aronson, M. F. et al, 2017. Biodiversity in the city: key challenges for urban green space management. Front Ecol Environ 2017; 15(4): 189–196, https://doi.org/10.1002/fee.1480.
[10] Llodrà-Llabrés, J. and Cariñanos, P., 2022. Enhancing pollination ecosystem service in urban green areas: An opportunity for the conservation of pollinators. Urban Forestry & Urban Greening. Volume 74, August 2022, 127621. https://doi.org/10.1016/j.ufug.2022.127621.
[11] Geneletti, D. 2011. Reasons and options for integrating ecosystem services in strategic environmental assessment of spatial planning, International Journal of Biodiversity Science, Ecosystem Services & Management, 7:3, 143-149, https://doi.org/10.1080/21513732.2011.617711.
[12] De Luca et al. 2021. Accessibility to and Availability of Urban Green Spaces (UGS) to Support Health and Wellbeing during the COVID-19 Pandemic – The Case of Bologna, Sustainability 2021, 13, 11054. https://doi.org/10.3390/su131911054.
[13] Wagner, M et al. (2019). Conflicts about Urban Green Spaces in Metropolitan Areas under Conditions of Climate Change: A Multidisciplinary Analysis of Stakeholders’ Perceptions of Planning Processes, Urban Sci. 2019, 3, 15. https://doi.org/10.3390/urbansci3010015.
[14] Citizen Participation Playbook for Emilia-Romagna Region, https://www.oecd.org/gov/open-government/emilia-romagna-citizen-participation-playbook.pdf.
[15] Suanno, C. et al. 2021. Allergenic risk assessment of urban parks: Towards a standard index, Environmental Research, Volume 200,
2021, 111436, https://doi.org/10.1016/j.envres.2021.111436.
[16] Urban Agenda Partnerships on circular economy and on sustainable use of land and nature-based solutions. 2019. ‘Land use Handbook on sustainable and circular re-use of spaces and buildings’. https://futurium.ec.europa.eu/en/urban-agenda/sustainable-land-use/news/handbook-sustainable-and-circular-re-use-spaces-and-buildings-now-available.
[17] Barcelona Green Infrastructure and Biodiversity Plan 2020. 2013, available at: https://ajuntament.barcelona.cat/ecologiaurbana/sites/default/files/Barcelona%20green%20infrastructure%20and%20biodiversity%20plan%202020.pdf.
[18] The Commission for Architecture and the Built Environment. 2006. Paying for parks Eight models for funding urban green spaces. https://www.designcouncil.org.uk/fileadmin/uploads/dc/Documents/paying-for-parks.pdf.
[19] Tempesta T., 2014. Benefits and costs of urban parks, Dept. TESAF - University of Padova Council of Europe – European Landscape convention; Fifteenth Council of Europe meeting of the workshop for the implementation of the European landscape convention; Sustainable Landscapes and Economy; Urgup, Nevsheir, Turkey - 1-2 October 2014. https://rm.coe.int/16802faf1a.
[20] European Commission, EU Climate Pact, Green areas, available at: https://climate-pact.europa.eu/about/priority-topics/green-areas_en (accessed 21-9).
[21] European Commission, Green City Accord, available at: https://environment.ec.europa.eu/topics/urban-environment/green-city-accord_en (accessed 21-9).
[22] Brussels Climate Plan, available at: https://klimaat.brussel.be/en (accessed 21-9).
[23] Greenleaf, B. S., & Ries, P. D. (2020). Citizen Advisory Boards in urban forestry: Who are members and why do they serve? Urban Forestry & Urban Greening, 47, 126553. https://doi.org/10.1016/j.ufug.2019.126553
[24] Janse, G., & Konijnendijk, C. C. (2007). Communication between science, policy and citizens in public participation in urban forestry—experiences from the neighbourwoods project. Urban Forestry & Urban Greening, 6(1), 23–40. https://doi.org/10.1016/j.ufug.2006.09.005
Other useful resources
ITree: an open source software that can support in understanding and assessing tree benefits – mostly in relation with Co2 storage and sequestration and PM10 filtration. Accessible at https://www.itreetools.org/.
UNaLab Toolkit: tools for co-creation and experimentation of innovative solutions in a real-life urban environment together with the engagement of citizens and all relevant stakeholder groups in a city. Accessible at https://unalab.enoll.org/.
The main direct impact of planting trees or introducing new urban forest and parks is carbon storage and sequestration. Various research has analysed and quantified direct benefits of tree planting in relation to carbon sequestration and reduction of air pollutant PM10. While example storage capacities can be found (see for example tools developed by LIFE CLIMATREE), it is recommended to look for available data and tools at local level, as species and relative performances may vary from city to city (see for example data and information around Bologna developed by GAIA).
Awareness campaigns
Public Private Partnerships
Integrated land use and urban planning
Building codes/standards
Parking policies and management
Geospatial information
GHG scenario modelling
Co-creation workshops
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