Re-planting our cities; making an urban forest

By Carrie Madren

01/18/2010

Trees among us mark the changing seasons, and help soften bleak urban landscapes. Tree-lined streets make our hometowns friendly and familiar. In summer, trees help shade town buildings from the scorching sun. 'Urban forest' may seem like an oxymoron, but trees can be as integral to a town as buildings.

The benefits are abundant: city tree canopy - tree coverage as seen from an eagle-eye view - offers an intensely green return for the investment of planting, watering and maintaining. Trees slow rainwater from becoming storm water runoff and absorb and store carbon from the atmosphere, combating global warming.

Canopy cover also shades buildings and reduces heating and cooling costs, lowers air temperatures within the city, provides wildlife habitat, reduces air pollution, increases property value (by 15 percent on average) and provides aesthetic benefits - who doesn't love to see a tree's beauty, after all? Urban areas with plenty of trees encourage people to spend time outside, engaging with neighbors - and lowering crime. Finally, roads shaded by trees need less frequent maintenance than their exposed counterparts.

Since the 1970s, however, tree canopies have greatly diminished in Mid-Atlantic towns. Many urban trees have been felled as development creeps into open lots. Other city trees die of old age, get struck by lightning or rot away. Trees near construction projects may suffer, too, if soil is compacted by heavy construction equipment. Trees, which need some 25 gallons of water per week to survive, also succumb to our sometimes dry, always hot Mid-Atlantic summers.

Community trees face exotic pests and diseases that threaten specific tree species, including emerald ash borer and hemlock woolly adelgid. In the 1950s, many American neighborhoods saw street trees wiped out by Dutch elm disease.

The odds are against city gargantuans: the average life of a downtown street tree is just 13 years, according to the non-profit American Forests.

But urban forests may be making a slow comeback. Many towns and cities across the watershed have surveyed their tree canopy and set goals to work towards. Bringing back the canopy, however, is a long game of tug o'war: while newly planted trees take decades to mature, old trees can be felled in a day. In the end, though, the effort to install slow-growing trees will be a win for local ecosystems and communities.

In Maryland, 42 percent of Annapolis is covered by tree canopy, and the city has a goal for 50 percent cover by 2036. To multiply the trees, the city gives away hundreds of trees to residents and sponsors massive tree planting events.

"There's always construction, so there's a loss of trees as well as a gain," explains Jan Van Zutphen, Annapolis' city environmentalist and urban forester. But Annapolis law requires residents to submit a landscape plan any time a tree is impacted, a plan that must detail how the tree or trees will be mitigated for or replaced, Van Zutphen adds.

Canopy covers 35 percent of Washington, D.C., approaching the City's goal of a 40 percent canopy cover. That sounds like a small gap, but that five percent equals 216,000 trees, says Jared Powell of the non-profit Casey Trees, which focuses on planting trees on private property.

"That's 8,600 trees per year for the next 25 years," he says. "It's a challenging, but doable, goal." His non-profit is helping meet the tree canopy by teaching private property owners about maintenance and helping them select and plant trees.

In Virginia, Fairfax County has lost almost 48 percent of its trees to development in the last 30 years, according to the county, though a canopy cover of 41 percent remains. The town of Leesburg's tree canopy was at 27 percent a few years back, but has decreased due to development. The town now has a goal of 40 percent tree canopy coverage in 20 years.

"Increasing canopy is always incrementally very small, because you're putting in small trees, but when you take out trees, those were usually large trees," says Leesburg's urban forester, Jay Banks.

Challenges for all communities include the money to buy, plant, maintain and continually water trees and the space to plant new ones.

Private property owners with open space can help by planting trees, though many homeowners opt for small flowering trees over large shade trees for the aesthetic appeal of the blooming trees. Such smaller trees create a smaller, thinner canopy than mature shade trees. Still, any tree is better than no tree.

The key to making tree canopy plans work, Banks says, is to remember that trees need time to mature. "You have to see what we're putting in the ground today as canopy for future," he says. "It's a long-term investment."