Seattle’s tree canopy studies

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Seattle’s tree canopy has been measured six times in the last 15 years. Maddeningly, each study used different technologies and several contain a large or unknown margin of error. Gauging change over time has proven elusive. A rundown:

  • 2001 — Seattle collected data on its canopy cover for the first time, using LiDAR technology, considered state of the art at the time and found the city 18 percent covered in canopy, with a 5 percent margin of error. This study served as the basis for Seattle’s first urban forestry management plan, which set a goal of 30 percent canopy cover by 2037 and a “reach” goal of 40 percent by 2047.
  • 2009 — Native Communities Development Corporation, a now-defunct Colorado-based imaging group hired by the city, used satellite imagery from 2002 and 2007 in an attempt to learn how the canopy was changing over time. It pegged the city’s tree canopy at 22.5 percent in 2002 and 22.9 percent in 2007. The report does not include the margin of error, but does conclude a “level of uncertainty” about the canopy increasing over that time.
  • 2012 — A federally funded study gauged the health, age and species of trees across the city based on ground sampling in 2010. Run through a US Forest Service program, the $250,000 study came up with an estimate of 26.3 percent tree canopy, but the report does not include the margin of error.
  • 2013 —University of Washington researchers examined problems with uncertainty in canopy cover assessments, and used Seattle’s history as a case study. They found that none of the previous assessments are comparable or they have a clear measure of uncertainty. Researchers carried out their own study to illustrate the challenges of assessing the city’s tree canopy, looking at two types of data from 2009, and one from 2012, based on aerial photos and Google Maps. Using three different methodologies, they arrived at three different numbers: 26.4 percent, 29.6 percent and 28.5 percent, with an unstated margin of error. The researchers concluded that a higher degree of accuracy is needed for tree canopy assessments and provided a list of best practices for how to achieve that.
  • 2015 — The city Office of Sustainability and Environment carried out a comparative study to try to find the long-sought “change over time” The study compared tree canopy from sample Google Maps locations across the city from 2007, 2010 and 2015. But the data was not statistically significant: It showed a 2 percent loss of canopy, with a 3 percent margin of error.
  • 2016 — The most recent study is considered by far the most accurate, thanks to developments in technology and precision. Performed by a University of Vermont lab on behalf of the city Office of Sustainability, it found the city to have 28 percent canopy cover, with a 1 percent margin of error.

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