By Katherine Reich

Trees are part of the identity and legacy of Falls Church City – and vital to public and environmental health. But they aren’t just another pretty sight: Trees are also very important to our environment. Not only do they absorb carbon dioxide (CO2), but they also shade homes and streets, saving energy and cutting summer cooling bills. And, trees reduce storm runoff and pollutants released downstream into the Chesapeake Bay by absorbing rainfall, filtering water and reducing soil erosion. In short, trees are environmental champions.

Residential development often results in tree removal, but the Neighborhood Tree Program and City Code help preserve and replace trees. The aim is to maintain “tree canopy” – the layer of branches and stems of trees that cover the ground as viewed from above.

A recent study compared tree inventories on residential lots in the City before and after development – where older houses are replaced with new, often larger ones. Dr. David Chojnacky, adjunct professor of Forest Biometrics with Virginia Tech, along with his team, evaluated 21 residential lots representative of 300 redeveloped lots in the city database, between one and 18 years after redevelopment. City code requires retaining or planting enough trees during development to achieve 20 percent canopy cover within 10 years. On most lots, tree cover exceeded that goal; only a few severely cleared lots had less than 20 percent cover. Canopy cover exceeded 40 percent on 43 percent of the lots, but these had either retained more mature trees or were older lots with 15 to 18 years to regrow.

However, there were fewer big trees. On only four redeveloped lots was the average tree size larger than before development; and these lots had lost fewer trees initially. On 13 of the 21 lots, average tree size was less than half what it was before development. This loss was also reflected in the canopy cover: on our study lots, we estimate the average canopy cover before development was 52 percent, with a reduction to 36 percent when measured one to 18 years after development.

Meeting the 20 percent cover goal is good news – but we are concerned about overall loss of big trees from development. Tree canopy is only part of the story. Newly planted small trees grow quickly and soon replace cover, but will take time to achieve the environmental benefits of larger trees for shading, carbon storage and pollution filtering. Ground measurements from the Virginia Tech study can also translate tree cover percentages into tree size and carbon metrics. The average tree diameter before redevelopment was about 17 inches, equating to storage of about 11 tons of carbon per acre for city forest. After redevelopment, average tree diameter was 11 inches, storing about 6 tons carbon per acre. For perspective, carbon taken up by trees on one city lot prior to redevelopment would offset carbon dioxide emissions of 2.5 cars per year, which was reduced to 1.5 cars per lot after redevelopment.

The study raised new questions for tree cultivation. Canopy cover metrics can assess the quantity of canopy cover but not the quality. We are losing mature trees at faster rate than would happen in a natural forest, and we don’t know enough about the lifespan of trees in urban environments to know the ideal rate of canopy replacement. Most of our redevelopment sites will easily meet or exceed our canopy goal in 10 years, but we are less sure what kind of canopy that will achieve.

We’d like to collect more data on to tree growth and regrowth on redevelopment sites to refine a model started in this study. Based on tree diameters and planted tree numbers, this model can predict canopy cover and tree size growth into future, allowing an arborist to assess various tree preservation and planting scenarios for desired canopy cover and tree size. But this requires more research funding which is increasingly scarce. In our area, Chesapeake Bay initiatives dominate, but most federal funding for Bay health funnels through foundations and local governments more focused on restoration activities than on science to determine which of those actions will be most effective.

What can Falls Church residents do to help? Recognize that the city emphasis on trees is not just for aesthetics – trees are essential to the health and well-being of city residents, and to the streams and waters of the Chesapeake Bay. Trees also offset carbon dioxide from thousands of cars commuting on Broad Street. Private property development is a cherished public right, but your trees are important components of the regional urban forest. When landscaping or building, preserving large trees is an excellent way to enhance your own property, preserve local environmental benefits, and help “save the Bay.”

For more information on the tree canopy study and future plans, please contact Katherine Reich, City Arborist at kreich@fallschurchva.gov.

Katherine Reich is the City of Falls Church arborist.