By Nora Palmatier, President
Ever since Tree Stewards’ first tree census project at Ft. Myers Joint Base in 2008, members have completed similar tasks for the Tree Canopy Fund, and in neighborhood parks, schools, churches, and condos. Each project, we kept our feet on the ground, looking up into the trees’ canopy, and recording information with pencil and paper.
This summer, our tree measuring projects were literally turned upside down and we looked down on the tree leaves using Google Earth imagery through i-Tree Canopy. This is a free software tool from the United States Forestry Service that can be used on personal computers to easily and accurately estimate tree and other cover classes (e.g., grass, building, roads, etc.) within a set area. Urban Forest Manager Vincent Verweij used his GIS skills to set up our citizen science project using the boundaries of Arlington County, Virginia (including National Airport and all Department of Defense properties, which affects comparison with past canopy studies). The program is used to estimate results of tree canopy coverage – i.e., how much tree shade is there. Suddenly, we were measuring trees by looking down on the canopy instead of upwards!
Seven Tree Stewards worked on the project, and all together we looked at 6,399 views covering Arlington County. i-Tree Canopy randomly generates sample point views and places a crosshairs marker in the middle of the view. After looking where the crosshairs fell in each scene presented, we clicked on whether it landed on: a) Tree Canopy, b) Pervious grass and vegetation, c) Impervious building rooftop or pavement of roads, sidewalks, parking lot, e) bare soil, or f) water. Each of us did close to 1,000 views.
Above is an example of what we Citizen Scientists saw, minus the crosshairs indicator. The program provided a screen shot with the target point shown, and we clicked on whether the area beneath it is impermeable (red dots on roof and parking lots in lower left corner), tree canopy (green dots along Four Mile Run), or water (blue dot in middle of stream – on larger view water is obvious).
Among my 1,000 views where the crosshairs landed were: on top of an airplane on a runway, the roofs of houses, offices, and apartment buildings, trees, golf courses, and cemeteries (which had many gorgeous trees but a lot more herbaceous cover between graves, so where specifically the crosshairs fell determined the cover I listed). All science projects tell us to beware of operator bias, so I tried very hard to distinguish between tops of trees and shade on ground and not let my bias overcome the purpose – but it was so difficult when I identified Lacey Woods Picnic pavilion rooftop under the crosshairs so had to mark it Impermeable even though it was surrounded by a forest!
Above is the image of all 6,399 points throughout the county. Even when enlarged, it won’t clearly show the lowest canopy areas along major transportation lines as the 2016 aerial study does.
The program report gives us much to consider. Our study estimated that the dollar value of carbon sequestered in county trees is $1,292,714, and that reduced air pollution benefit is $1,798,460 – all this besides providing beauty and shade on hot days. Statistically, having 6,399 points analyzed for a county land mass of 27 square miles is very solid, but it was sampling rather than review of the entire county land.
Comparing the results from our i-Tree Canopy points with previous studies is the proverbial apples and oranges, while comparing studies that exclude DOD and airport is like comparing apples, oranges and puppy dogs. They are all wonderful but not interchangeable. The 2022 i-Tree Canopy reported Tree Canopy at 35% compared to the 2016 aerial study of 38%. Impervious surface in 2022 was 40% and in 2016 was 38%. There was no difference between the two reports in grass and vegetation at 23% and bare soil at 1%. (Numbers for both reports are rounded and do not include sampling error estimates.) The 2016 aerial estimate of 41% canopy coverage did not include the Pentagon parking lot or airport runways while 2022 does, so they shouldn’t be compared.
The 2022 study is county wide and doesn’t provide guidance on where tree canopy has been lost since 2016. It doesn’t provide data to determine: How much tree canopy decrease occurred on DOD land such as the National Cemetery when 700 trees were replaced with columbariums for cremated remains? How much decrease of tree canopy is due to lot clearance on private property when small houses are replaced with large ones? Was percent of increase in impervious surfaces due to new pavement or landing strips as compared to housing? Is there room in parks to plant more trees? We await the next aerial canopy study to be able to see where loss is happening and how best to manage our urban forest. Arlington County is funded to perform another study after the adoption of the Forestry and Natural Resources Plan.
|Land Cover classification||2022 iTree points by Tree Steward volunteers||2016 study including DOD and Airport|
|Impervious surface (building & pavement)||40%||38%|
|Pervious grass & vegetation||23%||23%|
What our i-Tree Canopy study seems to say is that Arlington County tree canopy is definitely not increasing, and it’s most likely decreasing. Our Tree Steward mission to educate the public on the importance and care of trees is even more vital as we know it is on private property that trees can be preserved and more plantings occur. We are already working to decrease canopy loss through planting efforts with the Tree Canopy Fund, tree distributions, and plantings on park land. And we provide public encouragement to plant more trees on private property as well as how to maintain mature trees. We Tree Stewards do a lot, yet this study is a call to increase our efforts; fortunately, we know that is a labor of love.
Additional information you might wish to review
TS work at Ft Myers: https://treestewards.org/2010/10/25/why-count-trees-because-we-value-what-we-know/
TS census of Tree Canopy Fund: https://treestewards.org/2018/01/09/arlingtons-tree-canopy-fund-survey/
About Arlington’s tree canopy https://www.arlingtonva.us/Government/Programs/Sustainability-and-Environment/Trees/Tree-Statistics/Tree-Canopy
Tree removal National Cemetery https://treestewards.org/2013/03/25/trees-endangered-millennium-project-at-arlington-national-cemetery/
Tree Canopy Fund will plant trees for homeowners: https://www.ecoactionarlington.org/community-programs/trees/