In the urban planning and design world, the benefits of a strong urban canopy are well known and documented. Urban trees aid in stormwater management, reduce heat island effect, and make cities more livable. With these ideas in mind, many cities across the country—from Sacramento to New York—have implemented tree planting campaigns.
While these campaigns have the potential to be very beneficial, they also pose a risk of being costly public works endeavors that bring little benefit to the city. Currently, in the United States, the average street tree lasts less than 15 years. Studies have shown that 13-year trees planted in traditional structural soils have a net cost of $3,000. Therefore, if New York City planted the trees for its Million Dollar Tree campaign in this traditional way, it could end up costing the city $3,000,000,000. However, if the campaign’s trees lived to 50 years, they would off $9,000,000,000 in benefits over their life cycle.
The key to a beneficial urban tree campaign lies in thoughtful strategies for planting and maintaining urban trees. In a recent talk given at the Greenbuild 2012 conference in San Francisco, Greg McPherson of the USDA Forest Service and L. Peter MacDonagh of the Kestrel Design Group argued that conventional tree pits in sidewalks are a thing of the past. Instead they offered several new strategies for ensuring a great urban forest. Here are some of their tips:
- Use large amounts of loam or bioretention soils that are 65% sand, 20% compost, and 15% clay silt. Their rule is 2 cu. ft. of loam for every sq. ft. of tree canopy.
- Diversify tree species. No single tree species should be >5%, and no single tree genus should be > 10%.
- Set minimum canopy target with deadline of >40%, then find and fill canopy gaps.
- Monitor trees and apply responsive operations and maintenance as needed.
In addition, they offered a few Don’ts.
- Do not plant trees in small pits, compacted soils, sand, or structural soils
- Do not plant lots of a few species.
- Do not plant tree root packages low.
Urban trees have the potential to bring unparalleled benefits to our urban environments, both economically, in terms of ecosystem services and real-estate prices, and in terms of livability. However, it is our responsibility as planners and designers to make sure that they are planted and maintained in a strategic manner for the beauty and benefit of our cities. For more information on tree canopy strategies and resources, please visit the links below:
Harriett Jameson, Biophilic Cities Project Researcher
Harriett is a masters degree candidate for the Urban and Environmental Planning and Landscape Architecture disciplines at the University of Virginia.