These green streets - environmental advantages of urban forests

Sierra, July-August, 1993 by Marc Lecard

Imagine a city where the saplings outnumber the citizens, where you can't see the buildings for the trees; a cool, shady metropolis where the cliche "urban jungle" has taken on an entirely new meaning. Typical tree-hugger fantasy? Perhaps. But it might also be wise city planning.

The phrase "urban forest" sounds like an oxymoron. Cities grow asphalt, not trees; they're concrete farms, auto zoos, forests only of skyscrapers and power poles.

But the trees that do manage to grow in cities are more than a reminder of real forests elsewhere. Consider this: one city tree will soak up an estimated 13 tons of carbon dioxide (the main culprit in global warming) a year--1 69 tons over its average 13-year lifetime. (Because of the stress and strain of their environment, urban trees don't live as long as their rural relations.) Given the number of street trees in U.S. cities--New York City has more than 600,000--it's clear that curbside trees can be as important to the global atmosphere as their country cousins.

And their leaves cool more than overheated urbanites. The bare brick, asphalt, and concrete of many city spaces give rise to something called the "heat-island effect," a temperature increase that occurs in artificial, unshaded areas. Buildings and pavement not only absorb heat, raising daytime temperatures, but hold on to it and continue to warm the air after sundown. According to Growing Greener Cities by Gary Moll and Stanley Young (Living Planet Press, Los Angeles, 1992) cities are 5 to 9 degrees warmer than surrounding rural areas. An "adequate canopy" of trees, say Moll and Young, can keep things cooler, making city temperatures closer to those in the surrounding countryside, which means less energy expended on air-conditioning, less fossil fuel used to generate electricity, and, ultimately, reduced emission of greenhouse gases. According to EPA studies, 3 to 8 percent of the electricity cities use in summer goes to compensate for heat-island effect. (And while trees keep buildings cooler and more efficient in the summer, they have the added advantage of moderating the wind-chill factor by blocking the cold gusts of the winter season.)

Cooling concrete isn't the only work done by curbside trees. Their root systems help reduce runoff water--which in cities tends to be polluted with motor oil and other toxics. Their leaves absorb particulates floating in the air and help combat smog by soaking up hydrocarbons.

The psychic benefits of the urban forest may be hard to quantify, but the pleasure derived from masses of leafage softening an otherwise harsh and barren cityscape is an important part of what trees add to quality of life. It's as if trees soaked up fears and anxieties as well as smog and [CO.sub.2].

In spite of their many contributions, the number of city trees is declining. Over the past ten years or so, municipalities have been cutting back on "amenities" like tree planting and maintenance; this, combined with the shorter arboreal lifespan in built-up areas, has meant that urban trees have been disappearing faster than they are being replaced. Global Releaf, a project of American Forests, took a national inventory and, according to vice-president for urban forestry Gary Moll, found that "between 1975 and '85 we lost four trees for every one planted."

Given what's at stake, an increasing number of concerned citizens have begun devoting themselves to keeping the urban forest in leaf. There are several national urban forestry organizations that cooperate with federal, state, and local governments, businesses, and nonprofit and volunteer groups to educate people on the importance of city trees and to help plant and maintain urban greenery. "You need people with different skills, activists as well as treeplanters," says Moll. "Cities need to be reminded that trees are part of the urban infrastructure."

If you'd like to install your own curbside foliage system, contact your local urban forest group; they can tell you which trees are best for your area and give planting advice. If there is no such group in your neck of the woods, get in touch with one of the national organizations--they'll advise you on how to set up a hometown branch.