Tree Canopy Estimate Questioned

Comments at Arlington County Board Meeting On June 16, 2018.

At the April 21, 2018 County Board meeting, civic activist Suzanne Sundburg blasted the County for misrepresenting the health of Arlington’s tree canopy. She said:

“Arlington’s claim of a 1 percent tree canopy increase between 2011 and 2017 is not statistically valid”—due to the wide 6 percent margin of error in the reported statistic.

Christian Dorsey dismissed Sundburg’s criticism of the County’s 2017 tree canopy study that reported this number. He said: “Getting into a misunderstanding about data points of a percentage point or two are not really useful for our public policy.”

Yet the District of Columbia Urban Forestry Administration (UFA) is embroiled in a controversy right now with Forest Service (USFS) researchers over a similar difference in reported statistics.

According to a recent Washington Post story:

“The District reports success: Its analysis of 2016 data found that 38.7 percent of the city’s land area was forested, up from 35.1 percent in 2006.

“But a study published in April by two U.S. Forest Service scientists calls those findings into question. The city lost 850 acres of tree cover — roughly 2 percent of its total surface area — between 2010 and 2015, the researchers reported. That study, which compared the District with states, found that in terms of percentage, the city’s loss of urban forest was among the highest of any jurisdiction over the five-year ­period.

“The authors of the USFS study–estimate that an area of trees equivalent to almost half of Rock Creek Park — or six Malls — had disappeared. In 2015, the city’s tree canopy, according to the researchers, stood at 33.9 percent — more than a percentage point below the assessment when the city’s tree-planting effort began.”

DC disagrees, arguing that their optimistic numbers are backed by state of the art technology, whereas the USFS study was based on a random set of data points drawn from Google Earth and has a margin of error of 1.5 percent.

Unlike Arlington, the District’s UFA has not dismissed its tree canopy critics. Rather it has commissioned a third study to validate its previous findings.