We love our city street trees – how do local governments make decisions about what trees to plant?

Conway, T.M., A.D. Almas, D. Coore. (2019). Ecosystem services, ecological integrity, and native species planting: How to balance these ideas in urban forest management? Urban Forestry & Urban Greening 41:1-5. https://doi.org/10.1016/j.ufug.2019.03.006

As spring has sprung throughout most American cities, we all love seeing our cities’ trees in in full bloom. Urban trees make our streets more beautiful, provide shade on a hot day, and help us connect with nature even when we live in urban areas. Municipal governments understand the importance of trees to their citizens and many want to help grow and promote the health of the urban canopy in their hometowns. Good thing for us because we love our trees!

Figure 1. A cherry tree in bloom.
https://www.pickpik.com/sakura-cherry-blossom-japanese-cherry-trees-flower-tree-pink-135714

There is a lot of thought and planning from multiple stakeholders that go into building and managing our urban trees. Historically, a majority of city governments have managed their urban trees mainly to promote the beauty of the city. However, as more scientific research on the ecosystem services, or the “goods or services produced by urban forests that contribute to human well-being”, of urban street trees is published, cities are starting to alter how they manage the urban canopy to promote these exciting goods and services. Some of the services provided by urban trees include shading us on a hot day, releasing oxygen and helping to clean our air, and providing habitat for birds and bugs that also call the city home.

What are the experts saying?

In a recent study, a team of scientists explored what management techniques cities have used to build their populations of urban trees. City governments are generally interested in managing the urban canopy in a manner that promotes ecosystem services. To do this, municipalities need to ensure that the trees in the urban environment are able to behave similarly to how they would in a natural habitat and be able to adapt to the challenges trees face living in an urban environment. Some of the limitations a tree can experience in urban areas include: light limitation, limited space, and disturbances from passing cars and people. Trees also want to be surrounded by other tree species that are native to that area, a concept called ecological integrity. Therefore, cities try to incorporate a wide number of native species to mimic a local natural habitat.

Figure 2. Trees line a city street.
https://pixabay.com/photos/city-trees-lights-walkway-urban-4986141/
Urban forest management – a case study

So how are urban forests managed across different cities and how do city governments incorporate the ideas of both ecosystem services and ecological integrity? By analyzing 17 urban forest management plans throughout Ontario, Canada, the research team found that a majority of urban forest management plans focus on promoting both ecosystem services and ecological integrity, while also planting native species. At first glance, this seems great! However, as the authors dug deeper, they found that the lofty goals of trying to attain all three of these urban canopy objectives proved challenging to implement successfully.

Upon further exploration, the authors uncovered some challenges to building an urban canopy that successfully achieves the goals of promoting ecosystem services and integrity, all while maintaining a population of native trees. This is because non-native species are not always the best choice for producing the ecosystem services that a city might desire. For example, a city could want to plant trees that flower in the springtime for its residents to enjoy and to do this, they may need to plant species that would not naturally occur in their part of the world. Additionally, the authors were concerned that, while many urban forest management plans included the broad promotion of ecosystem services, many did not have clear goals on how to achieve the ecosystem services and integrity they had in mind. Without any clear objectives, tracking the success of urban canopy management plans can be difficult. The research team suggested that urban canopy management plans that strive to achieve promoting both ecosystem services and ecosystem integrity may be better served with a transition away from native species.

Figure 3. Some trees and natural habitat outside of Toronto, Ontario.
https://www.pikist.com/free-photo-ikziu
Concluding advice

The authors conclude with the important recommendation that municipal governments should continue to work on managing their urban canopy in a way that promotes ecosystem services and ecological integrity but to do that successfully they may need to move away from a focus on planting native species. Maintaining a diverse urban canopy with many species of trees is important but not all of the trees have to be native because sometimes native trees do not serve the goals of ecosystem services and integrity. Unfortunately, urban canopy management is challenging for city planners and there is no clear-cut instruction manual for how cities should manage their canopy. Therefore, researchers, like this team, are working to help build the knowledge of urban canopy management to help city planners create more robust and healthy urban canopies in cities all over the world.

Reviewed by: Whitney Kroschel

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Jessica Wright

Jessica Wright

I am a third year PhD. student in earth sciences at Boston University in the Department of Earth and Environment. My research focuses on urban infrastructure systems and energy transition policy, specifically focusing on the role of natural gas. I completed my undergraduate studies at Connecticut College in Biology and have worked with a lot of non-profits in and around the Greater Boston area on energy transition policy-making. I love to swim, do yoga, and travel!

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