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Mapping project aims to increase Virginia’s urban tree canopy

The term “urban forest” may sound like an oxymoron to some, but to Virginia Tech Forestry Professor Randolph Wynne and Associate Professor John McGee it represents an indispensable community resource. Wynne and McGee are leading an urban tree canopy mapping project for the Department of Forest Resources and Environmental Conservation, a part of the College of Natural Resources and Environment.

   

Trees line the urban landscape of Old Town Alexandria, Va. Trees line the urban landscape of Old Town Alexandria, Va.

The tree canopy is the layer of leaves, branches, and stems of trees that cover the ground when viewed from above.

Urban trees provide economic benefits to communities. They improve water quality, mitigate stormwater runoff, conserve energy, lower air temperatures, reduce air pollution, and enhance property values. The total compensatory value of urban forests in the continental United States is estimated at $2.4 trillion, according to the U.S. Forest Service. But the resource is being threatened by development.

Wynne and McGee say they hope their mapping project will help stop the trend. Working with the Chesapeake Bay Program, the Virginia Department of Forestry, the Virginia Department of Conservation and Recreation, USDA Forest Service, the Virginia Geospatial Extension Program at Virginia Tech, and localities across the state, they use aerial photography and GIS technology to provide local government leaders with tools to assess their urban tree canopy.

   

Trees help mitigate stormwater runoff in an urban setting, as well as bring ambiance to what could be an uninviting landscape. Trees help mitigate stormwater runoff in an urban setting, as well as bring ambiance to what could be an uninviting landscape.

Jennifer McKee, geospatial project developer at Virginia Tech, is another important player in the project. Through her assessment of a locality’s canopy coverage, McKee gives local leaders the information they need to improve their urban forest. As of summer 2010, the project had completed assessments for 17 cities, towns, and counties across Virginia.

To assess the canopy, McKee uses high-resolution imagery from the National Agricultural Imagery Program. The imagery is four-band, meaning it contains data from the red, green, blue, and near infrared wavelengths. Using these images, McKee classifies each pixel as tree canopy, other vegetation, impervious surface, or water.

Next, she completes an accuracy assessment, a process that involves comparing the map classifications she has generated with tree cover observed from 2008 aerial imagery. That measures the reliability of McKee’s classifications, which for use in the project must have an accuracy rate of at least 90 percent.

Finally, she provides each locality with a digital copy of the map and a detailed report of its contents. This report summarizes the data, comparing the locality’s urban tree canopy with that of other communities and identifying areas available for the establishment of new urban forests. For the benefit of planners, the maps integrate other data sources, such as zoning and parcel boundaries.

It is then up to local leaders to fulfill their end of the bargain — to work toward a goal to increase their community’s canopy cover. Though the staff members at Virginia Tech don’t make recommendations for managing canopy cover, they do provide conclusions that can guide local governments’ decisions.

   

Shaded areas offer a place for relaxation and renewal in an urban environment. Shaded areas offer a place for relaxation and renewal in an urban environment.

“Data acquired through this project empower the community decision-making process,” said McGee, who is the state geospatial specialist with Virginia Cooperative Extension. “Because of these data, decision-makers are much more informed and can therefore establish tree canopy targets and identify policies to effectively support the targets.”

McKee said having access to local urban tree canopy data could stimulate change in a variety of forms.

“Changes that local governments are likely to make include protecting existing urban forests, requiring new subdivisions and industrial parks to include trees in their planning, and encouraging and sometimes providing trees for neighborhoods to plant,” McKee said.

Such changes will not only help the local areas but will also fit into larger state and national initiatives to combat problems such as urban runoff, air pollution, and the heat island effect.

  • For more information on this topic, contact Lynn Davis or (540) 231-6157.

Curious?

    The project map for Blacksburg, Va.

You can view completed reports from the project at the Urban Tree Canopy Analysis website.

Audio: About the project

John McGee and Jennifer McKee discuss the Virginia urban tree canopy mapping project.

Another perspective on trees

Eric Wiseman, an urban forestry professor in the Department of Forest Resources and Environmental Conservation, is taking a closer look from the ground at Virginia’s urban forests. Wiseman has partnered with the Virginia Department of Forestry to assess municipal street tree populations throughout the commonwealth to better understand their abundance, composition, and condition.

The project was initially spurred by the threat of emerald ash borer, a nonnative insect pest that has killed native ash trees in urban forests across the Midwest. Wiseman’s research will help forestry and municipal officials plan for an outbreak of the pest in Virginia.

To date, Wiseman and his team have compiled street tree inventories for nearly two dozen localities and conducted additional assessments in half a dozen more. “There is a wealth of information that can improve our understanding and management of Virginia’s urban forests,” Wiseman said.

The value of a tree

The Large Tree Argument, a U.S. Forest Service publication, estimates the lifetime value of a large tree to a community at $4,440. The following are just a few of the benefits:

  • Stormwater abatement: When the canopy intercepts stormwater, it slows down runoff and decreases its overall volume, discouraging erosion and flash flooding.
  • Sound barriers: A dense grove of trees that is 50 feet wide can absorb enough sound to decrease apparent loudness by 50 percent.
  • Sunlight absorbers: Urban trees planted near buildings intercept the sun’s energy before it can contribute to heating the building, which can drastically reduce air conditioning costs. According the U.S. Department of Agriculture, “The net cooling effect of a young, healthy tree is equivalent to 10 room-size air conditioners operating 20 hours a day.”
  • Added value: On average, residential lots with trees sell faster than lots without them and are valued at 5 to 7 percent more.

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