Cities go hi-tech to shade their streets

— -- "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% 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