By Joshua Morris, Seattle Audubon Urban Conservation Manager
Trees and birds go together like peas and carrots. Like peanut butter and jelly. Like top-shelf gin with just a touch of vermouth, up with a twist. More than half of all the birds we’ve documented in Seattle rely on trees. This won’t surprise you then: Seattle Audubon is interested in protecting our city’s urban forest. When the opportunity arose to serve as the NGO Representative on Seattle’s Urban Forestry Commission, we jumped on it. On April 22, Earth Day 2019, Seattle City Council voted unanimously in support of Seattle Audubon’s three-year appointment to the commission. This is the second time Seattle Audubon conservation staff have served the city in this capacity; it’s a great fit organizationally and allows us the opportunity to influence decision making around one of the most important resources for urban birds. We envision an urban forest that is healthy, growing, and valued for the many benefits it provides to people and wildlife in the city of Seattle. We are committed to sharing our skills, knowledge, and passion to keep our city green.
What is the Urban Forest?
The urban forest is the collection of all trees across the city. The Douglas Fir at Discovery Park, the Northern Red Oak in the traffic circle at Federal and Republican, the Yoshino Cherry Trees in the UW Quad, and the Western Redcedar in your neighbor’s yard are all part of the urban forest.
What do we know about Seattle’s Urban Forest?
Our urban forest is species diverse and cosmopolitan. Seattle is a botanical melting pot, with over 600 tree species (including subspecies, cultivars, and varieties) from 143 genera lining our streets alone. Parks, gardens, institutional landscaping, and arboreta add even more diversity. You can meet trees from all six tree-bearing continents (lazy Antarctic…) along our sidewalks. Comparing my calculations to published results from the northeast corner of the country, our community of street trees is approximately seven times more diverse than the average community of street trees in New York state cities, six times more diverse than the average community of street trees in Pennsylvania cities, and five times more diverse than the average community of street trees in New Jersey cities.*
Some of the trees in Seattle’s urban forest are imperiled in their home ranges – including Horse Chestnut (Aesculus hippocastanum), Atlas Cedar (Cedrus atlantica), and Coast Redwood (Sequoia sempervirens). The urban forest includes cultivars unlikely to reproduce on their own (e.g., Leyland Cypress), exotic invasives that can spread with wild abandon (e.g., English Holly), and beloved native species (e.g., Pacific Madrone). There are fewer and fewer of the later: less than 5% of street trees are native to King County.
*(based on Effective Number of Species calculated from Shannon-Weiner Diversity Index and Cowett & Bassuk 2017)
Our urban forest loses its leaves for the winter. While Washington may be evergreen, Seattle isn’t. Seventy-two percent of our canopy is deciduous. Maples are the most common type of street tree in Seattle, accounting for approximately one out of every five street trees you’ll meet. The only cone-bearing genus to make it in the top 20 most common street trees is Thuja. This genus, which includes our native Western Redcedar, comes in at #17, representing just 1.3% of street trees in Seattle. Conifers tend to provide more environmental benefits as they usually grow larger, live longer, and keep their leaves year round.
Our urban forest is mixed-age. It consists of seedlings, saplings, and mature individuals. Trees of different ages, heights, and species provide structural diversity that enhances habitat values. This is particularly true in groves, where multiple layers of vegetation support a greater diversity of wildlife species than individual trees or even-age monoculture stands.
Our urban forest is full of life. Trees support hundreds of species in Seattle. Seattle Audubon community-science volunteers have recorded 132 bird species that rely on the urban forest in Seattle. This is over half of all bird species we’ve recorded in Seattle and includes Western Tanagers, Black-headed Grosbeaks, and Pileated Woodpeckers. The urban forest also supports raccoons, opossums, mountain beavers, and a large number of insects and the odd coyote or two. Sometimes a mountain lion or bear turns up in Discovery Park. Lichens are common and strange and adorn the bark of nearly any tree you care to take a closer look at. Many mushrooms form symbiotic relationships with trees. The unmistakable fly agaric, Amanita muscaria, associates with the roots of pine, spruce, fir, birch, and cedar. Chantrelles grow among our Douglas Fir, Western Hemlock, and Sitka Spruce. Oyster and Reishi mushrooms grow on decaying wood. Profusions of wildflowers thrive in our natural areas. Humans benefit, too. We rely on the urban forest for recreational opportunities, for shade in summer and wind breaks in winter. Trees reduce the quantity of stormwater runoff and improve its quality. They add character to our neighborhoods and remind us of our history.
Who cares for the urban forest?
The urban forest is owned and managed by a variety of entities, public and private, and for a variety of reasons. Most of Seattle’s urban forest (63%) is on land zoned as single-family residential. This includes 52% of our remaining conifers, making private property owners important stewards of some of the most valuable trees in our urban forest. This will have changed slightly as of April 2019; the Mandatory Housing Affordability Act rezoned about 6% of single-family residential to allow denser development. The City of Seattle is the largest “single” urban forest manager. Nine city government departments have a role in managing trees, including Seattle Parks and Recreation, which manages around 20% of the canopy despite occupying just 12% of the city’s land area.
The urban forest is intensively managed and heavily impacted by human activity. From pruning under power lines, to watering during drought and clearing for development, the urban forest is heavily managed. New construction, pests, invasive species, disturbance by trampling, pollution, and increased temperatures and CO2 levels all impact city trees. In 2016, Seattle had about 28% canopy cover, which we are estimated to be losing at a rate of approximately 0.4%, or 215 acres of canopy cover, per year. As Seattle continues to boom, we can expect more canopy loss. It is critical that we work with the city council to strengthen tree protection laws in our city, document loss, and ensure that the trees that are taken down to make room for development are replaced. The decisions we make now will determine both the quantity and quality of urban forest for future Seattleites.
Want to Learn More?
Consider attending the upcoming Urban Forestry Symposium at the Center for Urban Horticulture on May 21, 2019 from 9:00 AM – 4:00 PM. Seattle Audubon will be there to discuss how the urban forest can provide refuge for birds and wildlife as climate change introduces uncertainty in other natural areas.
Get to know the canopy in your neighborhood. How does the canopy cover in your neighborhood compare to others? Which trees are you most likely to encounter on the street in Beacon Hill? Explore the visualization below to find out. Use the drop down menu at the top to select a neighborhood. The map on the left shows the percent canopy cover of each Seattle neighborhood as of 2016. The bar graph displays the number of street trees by species for a selected neighborhood. Note that street trees are a subset of all trees, representing approximately 24% of the urban forest.
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