Seattle has an estimated 150,000 city trees, and of those, SDOT’s Urban Forestry division maintains 40,000. It takes a tough tree to survive the likes of the city’s built environment. By comparison, a tree that can live to be 400-500 years old in a park setting, may have an average life span of 35-50 years in the city’s built environment. City Arborist, Nolan Rundquist, says that besides being an easy target for run-away vehicles and branch shearing by oversized vehicles, trees have other enemies that make their survival difficult.
One of the contributing factors to the tree’s shortened life span in an urban setting, is simply the lack of a sufficient amount of soil volume that it needs to thrive. Rundquist explains that he also selects trees for variety in order to avoid the problems inherent in a “monoculture.” Planting the same type of tree in a long stretch makes them more susceptible to being wiped out by disease and insects.
It’s not uncommon for construction activities to accidentally cut into and damage tree roots. The roots of a tree can also become damaged if the surrounding soil is compacted by excessive weight, such as heavy construction equipment or materials piled on top of the soil around the trunk of the tree. Because tree roots tend to be shallow (within the top 18-24inches of soil ), a change in the grade by adding or removing soil will often be detrimental to a tree’s health. A tree that has had damage to its roots can become unstable, and is also subject to attacks from insects or disease because it is stressed.
Knowing the soil type in your area is important when making a tree choice. In general, trees that are successful in highly compacted urban soils are those that are native to swampy areas where oxygen levels in the soil are lower. Examples of ‘swamp’ trees are Red Maple, Tupelo, Baldcypress and Swamp White Oak.
Fortunately, Seattle’s Mediterranean-like climate is such that we can grow a wide variety of trees here. The current street tree inventory includes 105 genus (such as Maple and Ash) and 310 species or varieties (eg. Red maple, sugar maple, Japanese maple are all species of maple – October Glory, Red Sunset and Bowhall are all varieties of Red maple). SDOT encourages citizens to take advantage of the diverse selection of trees that will grow in the area. If a tree isn’t listed in SDOT’s approved street tree list, check with the Arborist for approval. An excellent resource for making that choice is Great Plant Picks.