How to bring shade to a city

"Wangari" has begun to spread her roots in Boston's Dorchester neighborhood. The red maple tree Mayor Thomas Menino planted on Arbor Day this spring is named for Wangari Maathai, a 2004 Nobel Peace Prize Laureate and driving force behind the United Nation's One Billion Tree Campaign for 2007. Now more than halfway through 2007, the movement's website shows that although more than 1 billion tree plantings have been pledged from Malaysia to Mexico, only 37 million trees have actually been planted.

Ms. Maathai began her tree-planting movement decades ago to combat soil erosion and deforestation in Kenya's vast terrain. Whether those efforts can be reproduced in America's metropolitan sprawls remains to be seen. But don't be so quick to dig a hole and drop in a tree, warn some arborists.

"The toughest landscape in the world is a city street," says Michael Dosmann, curator of the Arnold Arboretum of Harvard University in Boston. The complex science of urban tree planting must take into account poor soil quality, pollutants, and the limited root space in sidewalk grass plots.

"It's always been very popular to plant trees, but to do it right, it takes a bit of expertise and knowledge," says urban tree expert Nina Bassuk. "You have to do it right from the start so that you don't have to come back in and play catch-up."

At the April 7 planting of "Wangari," Mayor Menino announced the "Grow Boston Greener" effort to plant 100,000 trees by 2020.

A coalition of city and environmental leaders says the projected 20 percent increase in tree canopy will reduce Boston's "urban heat island effect," which causes temperature spikes of five to 20 degrees in areas with excessive concrete and asphalt.

Ms. Bassuk, who heads the Urban Horticulture Center at Cornell University, says trees can have a "tremendous" reduction effect on urban temperatures, which reach their peak from 3 p.m. to 5 p.m. but still exert heat late into the evening.

"An asphalt or concrete sidewalk really heats up and radiates heat long past when the sun goes down," she says. "If we can shade those pavements, you can do a huge amount to reduce that type of reradiation."

Using satellite mapping to set goals

Boston is the fourth US city, behind Baltimore; Annapolis, Md.; and New York, to employ the US Forest Service's latest forest mapping technology called the Forest Opportunity Spectrum. FOS fuses satellite images with a flexible model for classifying land parcels and analyzing cities' environmental goals, says Morgan Grove, research forester for the Northern Research Station of the US Forest Service in Burlington, Vt.

The resulting detailed satellite images – close enough to identify individual trees and cars – allow cities to determine current urban tree canopy cover and set goals for environmental improvements, such as Baltimore's desire to improve water quality and New York's efforts to curb air pollution. The four cities are the first to set specific tree canopy cover goals. Mayor Bloomberg in April announced New York City will plant 1 million trees during the next 10 years.

Boston's 29 percent urban tree canopy cover, as determined by the FOS survey, is already above most of its East Coast neighbors. New York City has a 24 percent cover while Baltimore has only 20 percent. Annapolis, however, boasts a 41 percent cover. If Boston's $15 million tree campaign goes according to plan, the trees will be in the ground by 2020 and expected to increase canopy cover to 35 percent by 2030, says Sherri Brokopp, sustainable cities program director of the Urban Ecology Institute. The city has just $3.5 million committed in funding but is seeking additional commitments from the private sector, says Jim Hunt, Boston's chief of environment and energy.

The societal benefits of trees

According to city and environmental officials, the $15 million spent on trees is expected to have social benefits as well. Trees are touted as miracle workers for city problems. Boston officials cite studies claiming that trees reduce energy costs, ease stress, and lower crime rates. Increased vegetation is also credited for decreases in aggressive street behavior, boosts in neighborhood pride, and more people spending more time outside, which serves as increased outdoor surveillance.

Boston is even using the FOS system for a venture into "environmental justice," in other words, equal opportunity trees.

"The question is, does everyone, in terms of race and income, have similar levels of canopy cover in their neighborhoods?" Mr. Hunt says. According to Boston's tree inventory statistics, the crowded and poorer East Boston neighborhood has only 6 percent canopy cover in comparison to the wealthier West Roxbury's 49 percent.

Several Boston environmental organizations are already stepping up their efforts. On this typical Thursday in early July, members of EarthWorks and JP (Jamaica Plain) Trees plant trees in various yards in the Jamaica Plain neighborhood. EarthWorks is a member of Boston's urban forest coalition and reports all tree plantings to the Urban Ecology Institute.

Matt Walter, a certified arborist and JP Trees volunteer, is planting a kousa dogwood in the yard of Jamaica Plain resident David Baron. [Editor's note: The original version misidentified the organization that Mr. Walter volunteers for.]

"They've given me a lot more guidance than I would have gotten just from walking into some nursery," says Mr. Baron as Mr. Walter explains how often he should water the dogwood. "I really wanted to get a tree, and I heard this program was free."

On this same afternoon, Walter examines a black tupelo tree planted two weeks ago. Upon examining the tree and its dried brown leaves, Walter unearths it and pointedly says, "Some trees make it. Some trees don't."

He soon determines the tree grew a double trunk flare, likely caused from repotting in the nursery. The double flare caused a water shortage to the deep roots, he says, and promises the homeowner a new tree will be on its way.

Challenges for city trees

Tom Ward, Dana Greenhouse manager at the Arnold Arboretum, says root damage is a common cause for city tree deaths. Once a tree is unearthed and replanted, as is typical in the greenhouse transferring process, it loses 90 percent of its roots and needs time to recover. Overeager lawn mowers can sometimes disrupt newly planted trees and permanently derail its growth.

But roots are not the only issue for city trees. Biodiversity, including a variety of species and age, increases survival rates and resistance to natural processes, Mr. Ward says. A notorious example is the Dutch elm disease, which decades ago wiped out US populations of the Dutch elm tree. Ward still fields calls from city planners desiring to plant elm trees on their Elm streets.

"They want to know, 'Should we plant it all in elms?'" he says. "I tell them, 'No.' "

Ward praises Boston officials for undertaking a "very ambitious" 100,000 tree project, but cautions that a city tree's life – especially that of an urban sidewalk tree – can be stunted by factors from drought to human interaction.

"These trees take a coalition of people to basically babysit, water, and tend to," says Mr. Ward. "In the end, aftercare makes all the difference."