A Tale of Two Cities: Urban Tree Cover in Portland and Seattle
Prior to any human settlement of either metropolitan area, Portland and Seattle were primarily covered in old growth forest. The draw of untouched forest led to large settlement and an economic boom for the region. By the mid-19th century in Portland, the city's rapid growth led residents to tracts of land quickly, but tree stumps had a tendency to linger; in some areas, there were so many that people used to jump from stump to stump to avoid the muddy, unpaved roads. Because of this, Portland become nicknamed "stumptown".
Urban trees provide many benefits for cities: they filter urban air pollutants, provide urban heat mitigation, absorb carbon dioxide, regulate stormwater runoff, improve physical and mental health, provide habitat, provide food, and increase surrounding home values. Currently, both cities have an existing canopy cover in the 28% to 30% range and, despite continued development, have goals to increase the total canopy cover in the coming years. Both cities have varying data available on public street trees, private trees and trees in City parks. We combed through these data sets to provide some insight into tree coverage in Seattle and Portland.
Urban trees provide many benefits for cities: they filter urban air pollutants, provide urban heat mitigation, absorb carbon dioxide, regulate stormwater runoff, improve physical and mental health, provide habitat, provide food, and increase surrounding home values. Currently, both cities have an existing canopy cover in the 28% to 30% range and, despite continued development, have goals to increase the total canopy cover in the coming years. Both cities have varying data available on public street trees, private trees and trees in City parks. We combed through these data sets to provide some insight into tree coverage in Seattle and Portland.
Tree Size Distribution
Both cities have completed an inventory of all trees in their public right-of-way. The Seattle inventory has around 150,000 trees and the Portland one has 218,000. Though Portland has a smaller population than Seattle (645,000 versus 725,000), the City's land area is noticeably larger (133 square miles versus 84 square miles). Use the slider below to filter out trees by diameter to see how the two cities compare. Tree diameter is measured as diameter at breast height (DBH) in inches. In Seattle the average DBH for all street trees is eight inches while in Portland it is ten inches.
Notice how in both cities the largest trees are located within the inner neighborhoods near downtown. These neighborhoods also have some of the most expensive home values. If you look at north Seattle, there is an obvious reduction in the density of street trees north of the 85th Street versus south of it. Many of these areas were annexed in the early 1950's and we speculate they had more lax tree protection standards than Seattle did. In Portland, there is a similar reduction in street trees when you go east past Interstate 205.
Portland City Parks Tree Cover
The City of Portland owns and operates 146 developed parks. Along with partners and volunteers, the City is in the process of inventorying every single tree in all of the City parks. We analyzed the 2017 Park Tree Inventory data, which has tree data for 51 City parks. Within these City parks, a total of 190 tree species are found and all trees are rated for their condition: good, fair or poor. Being the Pacific Northwest, the Douglas Fir is the most common species, making up 17% of a total of 5,900 trees found.
References/Notes:
1) Tree inventory data obtained from City of Portland and City of Seattle.
1) Tree inventory data obtained from City of Portland and City of Seattle.