How to bring shade to a city
Four East Coast cities are using satellite mapping to set environmental goals and plant more trees.
"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.Skip to next paragraph
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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.
Tips for urban tree survival
1. Consult with a certified arborist before planting. He or she will advise about the best tree species for the soil, space, and overall environment of where you want to plant.
2. Improve the landscape before you plant. Soil particles need to have optimum moisture and air movement, but soil compaction often causes urban trees to have poor aggregation. This can be helped by adding organic matter such as compost or peat moss.
3. Prepare the tree appropriately before it's planted. Almost all trees used to be planted using the "balled and burlap" method, where the rootball is wrapped in soil and burlap. Now, trees can also be successfully planted using a "bare root" method. Consult with your arborist for the best planting method for your tree and environment.
4. After planting, mulching is crucial for survival. Water and cover the upturned soil area with wood chips about 4 inches deep and at least 6 inches away from the trunk.
5. Make sure the mulching protects the tree from lawn mowers and trimmers.
Source: Information compiled from Arnold Arboretum and Boston's Urban Forest Coalition