- The City of Seattle 2021 Tree Canopy Assessment Final Report was produced by the City of Seattle’s Office of Sustainability and Environment using findings from the University of Vermont Spatial Analysis Lab report. However, the interpretation of data was done by unnamed people in the city of Seattle, who wrote or reviewed the “final report”. The University of Vermont collected some of the data but are not the authors of the report.
- The report defines tree canopy as “The layer of leaves, branches and stems that provide tree coverage of the ground when viewed from above.” Not mentioned is that the measurement is done at 8 feet above the ground. This is in contrast to US Army Corps of Engineers, who defines the Tree strata as starting at 20 feet in height, and the shrub/sapling layer from 3-20 feet in height https://www.sas.usace.army.mil/Portals/61/docs/regulatory/Workshop_Vegetation_Fall2011.pdf PlanIt.GEO in a recent zoom meeting responded to a question saying that most canopy analysis including ones they do are at done at 12 feet and higher.
- So, the City of Seattle canopy actually includes all shrubs and hedges such as laurel, holly and rhododendron bushes that frequently are over 8 feet high, and smaller saplings. The canopy measured would more accurately be described as “tree and shrub cover.”
- Although asked and confirmed before the report was released that the report would look at tree canopy volume, the report has no such analysis despite LIDARS ability to do so. This is important as tree canopy volume is a more accurate measure of the environmental services trees can provide to cities, as compared to tree canopy area.
- While the report defines a large (exceptional) tree as > 30 inches diameter, it does no analysis of large trees gains or losses, and neglects to compare any changes in large exceptional trees over the time period.
- The report mentions trees planted by the city but provides no data on any survival rates of the trees planted. Also, the number of trees removed by SDOT or Parks is not given, only the number of new plantings.
- The report looked at tree loss during development but only looked at development projects that were begun and completed from 2016 and 2020. The actual tree loss calculation should have looked at all projects that begun between 2016 and 2020 for the most accurate results, not just those that were started and completed. What is missing are projects that begun in that time period but were not completed. This is important as trees are usually removed at the beginning of projects, so the actual tree loss is likely significantly higher than what was reported. Also, if the next 5-year analysis follows the current calculations, projects begun before 2021 but not completed until 2021 or later will not be in the next analysis, leaving a gap in calculating tree loss during development.
- Key data points in Table 4 in the Appendix show that the total redeveloped parcels in the neighborhood residential zone saw a 33.6% loss of tree canopy acres and the total redeveloped parcels in the multifamily zones saw a 49.6% reduction in tree canopy. Issues of concern for future canopy loss include middle housing legislation being considered by the Washington State legislature which would basically convert most of Seattle’s neighborhood residential zone to the multifamily zone. Update – The Washington State Legislature passed HB 1110 requiring Seattle and other large cities in Washington State to allow building of 4-plexes and 6-plexes near frequent transit .Also, ADU legislation was passed in Seattle in July 2019 for the neighborhood residential zone which allows 3 units of housing (main house, attached ADU and detached ADU) o be built on any lot, increasing potential for additional tree loss with increased building lot coverage.
- Figure 8 is in contrast to other references on trees and urban Island heat impacts. Figure 8 is a scatter plot showing the relationship between maximum afternoon temperature and percent tree canopy, and the report somehow concludes that “…at the hexagon scale on a hot day (where a hexagon is the size of several city blocks) hexagons with 26% tree canopy experienced temperatures that were 1-degree lower than hexagons with no canopy.” This interpretation is misleading as to the on the ground results and is an example on how statistical analysis can be misused. See references below for additional information:
- The original report “Seattle and King County Washington Heat Watch Report” done in 2020 reported as high as a 24-degree F temp difference across the county. The analysis of heat impacts is much more nuanced and is affected by a number of things like time of day, tree canopy, buildings and pavement, closeness to water and size of area considered. The “one-degree lower statement” unfortunately is contrary to what many other researchers have found and clearly misrepresents the temperature differences between areas with trees and those not having trees.
- NPR – “In Seattle the difference between the coolest and hottest neighborhoods could be as much as 14.5 degrees, according to a 2019 NPR analysis of surface thermal data from NASA and US Geological Survey satellite imagery from summer days in the last decade.”
- King 5 June 23, 2021 “Areas of King County with more paved landscapes and less tree canopy are feeling the heat more intensely than less urbanized areas, according to a new study from King County and Seattle. More urbanized areas were as much as 20 degrees hotter due to an abundance of hard surfaces like parking lots, rooftops and streets which absorb heat,
- According to Portland State University research – “While testing solutions that reduce urban heat, the study … showed that paving over places that previously had a lot of tree canopy could raise the temperature as much as 25 degrees Fahrenheit on a summer day. Nearby neighborhoods would experience a spillover effect.” Portland State University Study Demonstrates How Plants, Trees, and Reflective Materials Can Reduce Extreme Heat of City Neighborhoods, 2019
- New York Times – Hidden Toll of the Northwest Heat Wave: Hundreds of Extra Deaths, Aug 11, 2021, ” During the deadly heat wave that blanketed Oregon and Washington in late June, about 600 more people died than would have been typical , a review of Mortality data for the week of the crisis shows.”
- Note – A draft of the 2021 Seattle Tree Canopy Assessment was not circulated for public comment prior to its release.
comments by Steve Zemke and Rich Ellison
TreePAC and Friends of Seattle’s Urban Forest
March 6, 2023
updated July 14, 2023