By Catherine Clark
The Roseway neighborhood is considered a low tree canopy neighborhood after analysis of street tree inventory data collected by Urban Forestry staff and volunteers in 2015. Although neighbors have planted many trees in the intervening years, we have a lot of open space in parking strips and yards we could fill with more trees. Our street trees are dominated by maples, cherries and plums and the lack of species diversity leaves our urban forest vulnerable to pests and diseases.
In addition to their aesthetic value, trees provide a lot of measurable benefits to our neighborhood. Mature trees produce enough oxygen per year to support two people, and remove carbon dioxide and other air pollutants from the atmosphere. Trees are also important in improving our water quality by intercepting stormwater runoff and reducing the flow to sewer systems. Research has shown that mature tree canopies provide social welfare benefits as they are associated with better health outcomes, and increased community engagement.
Why we need more trees
Trees and plants reflect sunlight and also release moisture through evapotranspiration. This helps cool the environment, and trees planted near homes can reduce the use of air conditioning by up to 30%. The shade and cooling reduce energy use and thus lead to a decrease in release of greenhouse gases, and improved air quality. Neighborhoods that lack canopy cover and parks become hotter, and so more dangerous during heat waves. The great majority of Portland’s population is found east of the Willamette River. Tree canopy cover is approximately 20% on our side of the river, and this rate is lower than that found in New York or Los Angeles. The east side of Portland lacks the parks and natural areas like Forest Park that reduce heat accumulation. Westside neighborhoods have more than twice the tree cover found on the east side.
Portland State University researchers have been working with government and community organizations to study the relationship between tree cover and heat in the environment. They mapped temperatures throughout Portland and show a correlation between increased heat and lack of trees and presence of impermeable surfaces like roads and buildings. They identified urban heat islands throughout east-side Portland, and particularly in far east Portland neighborhoods with very low tree canopy. Heat-related deaths from the ‘Heat Dome’ this past summer were concentrated in neighborhoods with low tree cover (https://www.npr.org/2020/01/14/795961381/racist-housing-practices-from-the-1930s-linked-to-hotter-neighborhoods-today).
Why we should prioritize planting native trees
Oregon native trees are adapted to local conditions including summer drought, soils and temperatures (https://www.portland.gov/trees/native-and-nuisance-trees/native-tree-list).
Native trees provide food and shelter for wildlife such as birds and butterflies, and support for local pollinators. Oregon white oak is the only native oak species found in northern Oregon and it provides critical habitat for a number of endangered species. A major food source for nesting birds comes from the over 500 species of caterpillars that can live on oak trees.
We derive the greatest benefit by planting large-form trees such as Oregon white oak, Western red cedar and Douglas-fir. Other large-form native trees include Western hemlock, Ponderosa pine and Red alder. Vine maple, Cascara and Western crabapple are great trees for smaller spaces. You can find more information about native trees from the Audubon Backyard Habitat Program (https://backyardhabitats.org/) and East Multnomah Soil and Water Conservation District (EMSWCD) (https://emswcd.org/native-plants/native-plant-benefits/).
We are fortunate in Portland to have local government and non-profit organizations that can help us get trees for our properties and provide relevant information. Portland Urban Forestry has been offering free yard trees to Portlanders for the last few years through the Yard Tree Giveaway (https://www.portland.gov/trees/tree-planting/free-trees). Although all of their trees are spoken for this year, you can sign up for notifications regarding next year’s giveaway. Friends of Trees (friendsoftrees.org), a non-profit, partners with local municipalities to plant street and yard trees for $35 or less. That cost includes the tree, transport to your home, and planting by trained volunteers. Finally, the Portland Bureau of Environmental Services offers a one-time credit on your water, sewer and stormwater bill for a tree planted in your yard through the Treebate program (https://www.portland.gov/bes/grants-incentives/about-treebate). I hope this will inspire you to plant more trees!