Climate change and the progression of nature loss are global challenges that urgently need solutions. Greening and, for example, protecting trees are multi-beneficial ways to increase carbon sequestration, mitigate urban flooding and heat waves, and support the well-being of both humans and other species. Although the benefits of trees are already widely known, their position in our cities is threatened: the number of trees is decreasing, they are being felled and damaged, and there is not always enough space reserved for new trees. How do we take care of the future of urban trees?
How much tree cover there in the vicinity of my home? What affects the amount of tree cover?
The canopy cover atlas provides information on the canopy cover of the Helsinki Metropolitan Area, i.e. the percentage of the area of a city, district or block covered by trees. The atlas shows that the amount of tree cover varies greatly from one district to another in the Helsinki Metropolitan Area. At its lowest, for example in Sörnäinen and Kaartinkaupunki in Helsinki, the tree cover is less than 4%, and at its highest, for example in Nuuksio in Espoo, it is 75%.
The amount of tree cover is affected by the efficiency of construction and the space reserved for areas covered by vegetation. The structure of the blocks, the size of the plots and the proportion of paved surfaces also affect the amount of tree cover. Large plots, where, for example construction and parking are located on the edges, allow for extensive yard areas and planting of trees. In earthbound yards, trees have a better chance of success than in podium yards with underground parking facilities, which lack contact with the soil and whose structural renovations can lead to the removal of yard vegetation¹.
What climate benefits do trees offer?
Trees and their biomass, as well as soil, play a role as carbon sinks and long-term carbon storages that accumulate over time. According to research results from the Co-Carbon project, about half of a city’s carbon sinks are based on green areas of the built environment, i.e. vegetation in parks, streets and plots². This potential has not been sufficiently considered. Tree canopy cover also mitigates heat waves: green areas, and especially trees, cool temperatures by 1–3 degrees and their cooling effect can extend to a distance of 60 meters from the park³. Increasing heat waves and the cooling effect of trees must be taken into account better than at present in urban planning. Trees, when combined with vegetated surfaces, also reduce urban flooding when water is absorbed into the soil. Urban trees are the most cost-effective way to secure carbon sinks, cooling and stormwater management.
How much tree cover is needed to achieve climate benefits?
The amount of canopy cover directly affects the climate benefits provided by trees. However, as the canopy cover atlas shows, trees are not evenly distributed in the city – nor are the benefits they produce. Tree equity emphasizes the fair distribution of trees and the benefits they produce in the city⁴⁵. The international recommendation⁶ proposes 30% canopy cover for city districts, and this is justified specifically for mitigating heat waves and from the perspective of health and well-being benefits. Also based on the reviews carried out in the Co-Carbon project, it seems that city districts generally turn into carbon sinks when they exceed approximately 25% canopy cover⁷.
What other benefits do trees offer?
In addition to climate benefits, trees also offer recreational benefits. Trees support physical, mental and social well-being⁸⁹ as well as health¹⁰. Trees are also an essential part of the cityscape and the cultural history of parks and park streets. Tree canopy cover also plays a role in the biodiversity of urban nature. Trees are living organisms that provide habitat, food, shelter, nesting sites and mobility connections for other organims. The biodiversity benefits depend primarily on the age, species and growth environment of the tree. Especially old trees are important for the flora and fauna – while they are also important carbon stores and are meaningful to humans.
How should the city maintain canopy cover?
Approximately half of the trees in Helsinki and Espoo are in public and communal areas, which is why the city is a key player in securing canopy cover. More precise and functional methods need to be developed for monitoring canopy cover and its condition. It is recommended to set tree cover targets at the city and city district level. This can ensure a sufficient amount of trees in new areas and identify development targets in existing areas where canopy cover should be increased ¹¹.
There are good international examples of tree cover targets in cities: In Sweden, the Swedish Environmental Protection Agency published a 25% canopy cover target for cities in 2021¹² and the Malmö master plan presented a 30% canopy cover target for city districts¹³. Canopy cover is often monitored only at the city level, but it should be assessed and developed especially at the city district and block level, as the mitigation of heat waves and urban flooding, as well as the recreational benefits, are local. Canopy cover is needed right near home.
Preserving the canopy cover requires a good knowledge base about the city’s tree assets and guidelines for preserving them. It also requires cross-administrative cooperation and the commitment of, for example, municipal engineering operators and building contractors. If digging near trees, damage to the root system must be avoided. When taking care of the trees, it is also indispensable to prepare for the future and ensure tree renewal. At the same time, it is necessary to prepare for a changing climate and drought periods and test new tree species and their origins.
How should residents care for their neighborhood trees?
15% of Helsinki’s trees are in residential areas – around 7% in apartment buildings and 8% in single-family homes. In Espoo, the corresponding share of residential areas is 2%. Residents and property owners play an important role in caring for yard trees¹⁴. On plots, it is recommended to preserve especially older and large trees and protect them from damage. Remember that trees are also living inhabitants of your plot. The appearance of a tree does not always indicate its condition. Therefore, it is worth asking an arborist or tree expert to assess the condition of the tree and, if necessary, make maintenance measures or a monitoring plan. When building a new plot, it is worth finding out how to preserve the existing trees and soil, as they are a valuable natural resource – and at the same time increase the coziness of your plot.
It is also worth planting more trees on plots where there is little tree cover, the tree cover is of even age and old, or trees have to be removed for one reason or another. In this case, you need to choose the right tree in the right place and ensure that the seedling has room to grow to full size. At the same time, you need to ensure good growing conditions and sufficient light and moisture¹⁵. This way, you will enjoy the trees for a long time.
Three cornerstones of the manifesto
1. PRESERVE existing trees, especially old trees and areas with connected woodlands. These are of the greatest importance in producing climate benefits and also in supporting biodiversity. Depending on the tree species, it takes 40–60 years for a tree to reach its full size. A new tree will therefore not replace an old one for a long time. Try to save trees on new plots and manage the trees on old plots so that they live as long as possible.
2. INCREASE the number and species diversity of trees in areas where trees are scarce or of a single species. When planning new areas, sufficient canopy cover should be reserved. Plan parks, streets and yards so that trees can fit. Planting new trees can ensure climate benefits in the long term. Ensure the continuity of the trees in the future.
3. ENSURE good growing conditions for trees, reserve sufficient space for the root system and canopy. Care for trees after planting. Only a healthy tree will produce long-term climate benefits. Protect the tree stand and root zone from traffic, snow plowing and construction sites to prevent damage to the trunk and roots. When trees live a long time, their benefits multiply. Trees are living organisms that need to be cared for. In this way, they also take care of us.
Author: Ranja Hautamäki, Professor of Landscape Architecture, Co-Carbon project
Check out the Canopy cover atlas of the Helsinki Metropolitan Area!
Come and see the large canopy cover map at the Designs for a Cooler Planet exhibition from 5.9.- 28.10.!
References:
¹ Leppänen, P.-K., Kinnunen, A., Hautamäki, R., Järvi, L., Havu, M., Junnila, S., & Tahvonen, O. (2024). Impact of changing urban typologies on residential vegetation and its climate-effects – A case study from Helsinki, Finland. Urban Forestry & Urban Greening, 96, 128343. https://doi.org/10.1016/j.ufug.2024.128343
² Havu, M., Kulmala, L., Lee, H. S., Saranko, O., Soininen, J., Ahongshangbam, J., & Järvi, L. (2024). CO2 uptake of urban vegetation in a warming Nordic city. Urban Forestry & Urban Greening, 94, 128261-.
³ Grilo, F., Pinho, P., Aleixo, C., Catita, C., Silva, P., Lopes, N., … Branquinho, C. (2020). Using green to cool the grey: Modelling the cooling effect of green spaces with a high spatial resolution. The Science of the Total Environment, 724, 138182–138182.
⁴ Zhou, W. et al. (2021) Urban tree canopy has greater cooling effects in socially vulnerable communities in the US. One earth (Cambridge, Mass.). [Online] 4 (12), 1764–1775.
⁵ American Forests (2021). Tree Equity Score. Ensuring tree cover in cities is equitably distributed. American Forests, Washington DC. https://www.americanforests.org/tools-research-reports-and-guides/tree-equity-score/
⁶ Konijnendijk, C. C. (2023) Evidence-based guidelines for greener, healthier, more resilient neighbourhoods: Introducing the 3–30–300 rule. Journal of forestry research. [Online] 34 (3), 821–830.
⁷ Kinnunen, A (2024) In: Hautamäki, R., Ariluoma, M. (2024). Urban Greenery as a Climate Action. Finnish Architectural Review 1/2024, p. 39-45.
⁸ Marselle, M. R. et al. (2021) Pathways linking biodiversity to human health: A conceptual framework. Environment international. [Online] 150106420-.
⁹ Tyrväinen, L. et al. (2014) The influence of urban green environments on stress relief measures: A field experiment. Journal of environmental psychology. [Online] 381–9.
¹⁰ Aivelo, T. & Lehtimäki, J. (2021). Luonnon monimuotoisuus edistää kansanterveyttä. Duodecim 2021;137: 2135–41.
¹¹ Hautamäki, R., Laita, A. (2024). Latvuspeittävyys – monihyötyisen vihreän infrastruktuurin indikaattori. Toim. Mari Vaattovaara, Jussi Jännes, Mikko Posti. Kohti kestävämpiä asuinalueita: Kaupunginosien ja kortteleiden kestävyysindikaattoreita. https://helda.helsinki.fi/items/398bac8f-ab24-4f07-bd98-9389cc44070c
¹² Naturvårdsverket (2021). Uppdaterad åtgärdstabell 2021-2025 för Åtgärdsprogram för särskilt skyddsvärda träd.
¹³ Malmö stad (2023). Översiktsplan för Malmö 2023.
¹⁴ Espoon kaupunki (2024). Puu paikallaan. Miksi ja miten säästät puita kaupungissa. Ympäristönsuojelun julkaisuja 3/2024. https://admin.espoo.fi/sites/default/files/2024-07/Puu%20paikallaan%20-esite_saavutettava_VALMIS.pdf
¹⁵ Ariluoma, M., Kinnunen, A., Lampinen, J., Hautamäki, R., Ottelin, J. (2024). Optimizing the co-benefits of biodiversity and carbon sinks in urban residential yards. Front. Sustain. Cities 6. https://doi.org/10.3389/frsc.2024.1327614
Photo: Tiina Merikoski