New study paid for by local naturalists shows declining tree canopy

(Updated at 4:35 p.m.) Tree canopy in Arlington County is lower than it was in 2016, according to a new privately-funded study paid for local residents.

The residents, who are involved in Arlington County Civic Federation, Arlington Tree Action Group and EcoAction Arlington, funded the study to how much tree canopy declined since the last county study in 2017.

Based on imaging from 2021, a consultant found that trees cover 33% of land — excluding the Pentagon and Reagan National Airport — down from 41% on the same land six years ago. Coverage ranges by civic association, from 14% in Crystal City to 66% in the county’s northernmost neighborhood of Arlingwood, compared to 26% and 75%, respectively, in 2011.

“It’s really eye-opening,” one of the residents behind the study, Mary Glass, tells ARLnow. “Ideally, the county would have had this, but they didn’t.”

The tree lovers commissioned the study out of frustration with the county for not doing so before beginning work on a Forestry Natural Resource Plan (FNRP).

This updates a 2004 Urban Forest Master Plan and a 2010 Natural Resources Management Plan in one document to address climate change, population growth and threats such as diseases and invasive species, says Dept. of Parks and Recreation spokeswoman Jerry Solomon. It has an eye toward racial equity and environmental justice, to make sure all residents benefit physically and mentally from Arlington’s natural resources.

Combining the plans “allows us to consider Arlington’s ecosystem holistically and craft a more comprehensive set of recommendations for conservation and resource management in the future,” Solomon said.

During this process, and during the Missing Middle housing discussions that concluded with zoning updates earlier this month, preserving trees from redevelopment was mentioned as a top priority for many residents. (The zoning changes approved by the County Board specifies requirements for shade trees on properties redeveloped with Missing Middle housing.)

To help, volunteers from Marymount University and EcoAction Arlington have been planting trees in the hotter, less leafy parts of Arlington.

Marymount University volunteers plant trees in Arlington (courtesy Gaston Araoz)

Now, Glass says, she hopes people will use the new data when the next Forestry Natural Resource Plan draft is published and ready for community input.

“This information is going to be right there so when the next draft comes out, in the next month or so, we’ll be able to make specific comments and recommendations based on the information we have,” Glass said.

The draft could be ready for community feedback this spring or early summer, Solomon said. The parks department spokesperson added that staff have seen the new study and “are excited about the community’s enthusiasm for our urban forests.”

However, she added, “we have not seen the underlying data and don’t have a full understanding of the methodology. As a result, we cannot speak to any discrepancies without adequately assessing it for accuracy, margin of error, or underlying assumptions.”

The department said it felt comfortable starting the plan update based on the overall downward trends in the previous tree canopy studies. Solomon said the current draft acknowledges and has recommendations for reversing the decline in tree canopy.

Despite marginal fluctuations, from a high of 43% in 2008 to a low of 40% in 2011 and a slight uptick to 41% in 2017, the county says tree coverage in parks is offsetting declining tree coverage on residential properties.

“Knowing this, we decided to prioritize the update of the FNRP in order to identify strategies to reverse that trend and address other environmental challenges sooner rather than later,” she said.

Local naturalists, however, have previously grappled with the county for, they say, not properly characterizing and taking seriously this decline.

A draft released last summer claimed that “Arlington’s overall tree canopy has remained stable,” according to a letter from the Forestry and Natural Resources Commission.

In a letter, commission Chair Phil Klingelhofer said this claim was an “incorrect and a strategic mistake,” given that 80% of Arlington’s civic associations registered declining tree canopy, by as much as 20%, between 2008 and 2016.

“In the FNRC’s consultations with tree canopy measurement experts, we learned that the 2016 survey was conducted with a different, less accurate methodology than the 2011 survey (which showed 40% tree canopy coverage),” he said. “It is thus not scientifically defensible to conclude that the tree canopy has not declined substantially since 2011.”

One difference is that the 2017 tree canopy study didn’t use height as a parameter to show distinction between trees and large shrubs, DPR’s Solomon acknowledged.

That partially explains the eight percentage point discrepancy in tree canopy, according to one of the authors of the privately commissioned study, Green Infrastructure Center Director Karen Firehock.

“We are confident this is a much more accurate number to use now, going forward,” she said in a webinar explaining the findings.

Solomon says the county is in the pre-planning stages for another tree canopy study, which will likely begin after the Arlington County Board adopts the Forestry Natural Resource Plan update this winter. This study will include height and imagery data, as well as accuracy requirements, to minimize error, she added.

“We know that this up-to-date tree canopy study will be key to implementing the recommendations of the FNRP — providing granular data about the canopy at a local level to help focus and prioritize our urban forestry work,” Solomon said.

Glass is now organizing tree enthusiasts across civic groups into a new group, Arlington Consortium for Tree Sustainability.

“We’ll be networking, sharing information and resources to try and give the people who are committed to trying to help the canopy move forward — regardless of what comes out of the plan,” she said.