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So Many Choices… The Challenge of Selecting Trees for the Urban Environment

“With seemingly endless tree species available, how can you decide which one to plant?”

So, you want to plant a tree- great! Between the vast amount of environmental, economic, and societal benefits they provide (Fig.1), trees are becoming increasingly essential for maintaining environmental quality and human well-being in the face of urbanization and climate change.

Tree benefits

Figure 1: Some of the many benefits trees provide; from left to right, general benefits to those more specific to urban areas.

Image credit: Ashley McElhinney, modified from Anna Maloverjan via Pixels.com.

But now you’re wondering… which tree species should I plant? It can be daunting to decide between the seemingly endless amount of species available. There are plenty of resources available to learn all about each species’ characteristics and environmental preferences, but this information can be useless when you don’t know what to consider before selection.

Tree selection

Figure 2: The age-old question.

Image credit: Ashley McElhinney, modified from thegraphicsfairy.com.

Beyond the basic site assessment, several criteria can be considered to help to achieve a healthy, resilient, urban forest.

1. Urban conditions

Urban areas often present unfavorable growing conditions, narrowing the list of species that may otherwise be well-suited to a site’s capacity. Not only must urban trees endure the stress factors that forest trees experience, but they must also tolerate a wide range of anthropogenic (human caused) challenges (Fig.3). It is important to note that although trees do not prefer these adverse conditions, some species may have a higher tolerance for them. If your site assessment indicates a tough planting site, you may consider selecting species with an observed tolerance to these conditions.

Tree stressors

Figure 3: Some of the many sources of stress all trees experience; from left to right, general sources of stress to those more specific to urban areas.

Image credit: Ashley McElhinney, modified from Anna Maloverjan via Pixels.com.

2. Climate change

Analyzing future habitat suitability can help to ensure a tree’s longevity. If climate change projections are accurate, species will have to either adapt, or migrate 3,000 to 5,000 meters per year to avoid extinction, far exceeding the maximum rate of 500 meters per year observed for plant species [1]. Therefore, you may consider assisted migration when selecting which tree species to include, because the silver lining to these altered climate conditions is that they may increase the habitat suitability for some tree species. By choosing to plant these species now, either at the northern edge, or just outside of, their current habitat range, you could help trees keep pace with climate change and prevent possible future tree loss. You can consult the US Forest Service’s Climate Change Tree Atlas for more information.

3. Area of origin

Consider planting native trees, for a number of reasons. Native trees provide substantially more support to native wildlife when compared to their non-native counterparts: they have been shown to support a 50% higher abundance of native birds, 9x higher abundance of rare birds, 3x more butterfly species, and 2x higher abundance of native bees [2]! Non-native plants also present greater risks when planted, as they are 40x more likely to become invasive than native plants [3]. And even if they don’t become invasive, non-native plant imports may act as a Trojan horse for forest pests: an estimated 70% of non-native forest pests, including hemlock woolly adelgid, arrived as contaminants on these plant imports. Therefore, although a well-suited non-native species can make a great addition to the urban forest, it is recommended to limit the amount of non-native species in the landscape.

4. Management issues

Considering species’ various management issues, such as pest susceptibility, may help to avoid safety hazards, excessive costs, and tree mortality. For example, although Ash trees are generally a great urban species, the Emerald ash borer is currently causing widespread loss and mortality among these trees [5], therefore planting them at this time may pose a significant risk. Other possible management issues to consider are species’ invasive potential, messy fruit or leaves, or high susceptibility to branch breakage.

5. Biodiversity

Biodiversity is essential to almost all ecosystem processes, resilience, and stability [6]. Considering that different species are susceptible to different pests, planting a variety of species can help to minimize urban forest canopy loss. The well-known depletion of urban forests across Massachusetts due to Dutch elm disease, compounded by to the over-planting of the American elm, is an example of the risk associated with monoculture [7]. The “10-20-30 guideline” is commonly used to ensure an ideal level of biodiversity; this rule states that in any community, less than 10% of trees should be of the same species, less than 20% should be from the same genus, and less than 30% should be from the same family [8]. However, some experts recommend no more than 10% should be from the same genus, while some even recommend no more than 5%. Considering which trees may be already overplanted in your community could help to maintain high levels of biodiversity.

But, possibly the most important thing to consider is that no tree is perfect - species selection is a site-specific decision, as the popular mantra “Right Tree, Right Place” signifies. And of course, reach out to a professional in your community with any questions you may have. Happy planting!

References

[1] Williams, M.I., and R.K. Dumroese. 2013. Preparing for Climate Change: Forestry and

Assisted Migration. J. For. 111(4):287–297.

[2] Fusco, E.J., J.M. Allen, E.M. Beaury, M.R. Jackson, B.B. Laginhas, T.L. Morelli, and B.A.

Bradley. 2018. Regional Invasive Species & Climate Change Management Challenge:

Why Native? Benefits of planting native species in a changing climate. Environmental

Conservation Educational Materials.

[3] Simberloff, D., L. Souza, M.A. Nuñez, M. Noelia Barrios-Garcia, and W. Bunn. 2012. The

natives are restless, but not often and mostly when disturbed.” Ecology 93(3):598-607.

[4] Mack, R. N., and M.C. Smith. 2011. Invasive plants as catalysts for the spread of human

parasites. NeoBiota(9).

http://dx.doi.org.silk.library.umass.edu/10.3897/neobiota.9.1156

[5] DCR. Forest Pest Fact Sheet. 2017. Massachusetts Dept. of Conservation and Recreation

Forest Health Program.

https://www.mass.gov/files/documents/2017/12/05/MAEmeralAshBorer2017.pdf

[6] Thompson, I.D., K. Okabe, J.M. Tylianakis, P. Kumar, E.G. Brockerhoff, N.A. Schellhorn,

J.A. Parrotta, and R. Nasi. 2011. Forest Biodiversity and the Delivery of Ecosystem

Goods and Services: Translating Science into Policy. BioScience 61(12): 972–981.

[7] Raupp, M.J., A.B. Cumming, and E.C. Raupp. 2006. Street Tree Diversity in Eastern North

America and Its Potential for Tree Loss to Exotic Borers. Arboriculture & Urban

Forestry 32(6):297–304.

[8] Santamour, F.S. 1990. Trees for urban planting: Diversity, uniformity and common sense. https://github.com/thatslifescience/github_dev/blob/gh-pages/images/ pp.57-65. In: Proceedings of the 7th Metropolitan Tree Improvement Alliance

(METRIA): Trees for the Nineties: Landscape Tree Selection, Testing, Evaluation, and Introduction Conference. The Morton Arboretum, Lisle, Illinois, U.S.

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