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To grow, trees need light, water _ and smog

For a tree, what's a better place to put down roots, the gritty city or the green, surrounding countryside?

Go with the gritty, suggests an article in today's edition of the British journal Nature.

The reason lies with ground-level ozone, or smog, which can be especially hard on certain types of trees and plants. While city air is, on the whole, dirtier than that of the suburbs, the situation is reversed for smog because of a complex chemical reaction known as "scavenging."

This phenomenon is well documented in New York, where the study took place.

But what took the study's authors by surprise was the dramatic effect of smog, compared to other pollutants, on certain trees.

Researchers planted cottonwood trees and found they grew twice as big in the urban air of the Bronx and Queens as in eastern Long Island and the Hudson River valley. The trees were planted in urban and rural soils in eight different locations, with similar water and light exposure.

"Whichever soil they were in, they always grew twice as much in New York City," said Jillian Gregg, who conducted the study with colleagues at Cornell University and the Institute of Ecosystem Studies in Millbrook, N.Y.

Using statistical analyses and experiments with lab-grown trees, researchers were able to rule out temperature differences and most types of pollution as possible causes for the city-country difference.

The one factor that really hindered tree growth was ozone, for which cumulative exposure was 50 percent higher outside the city, Gregg said. The corrosive gas is thought to retard tree growth in part because it damages cell membranes in leaves.

While the lower-ozone city environment is better for cottonwoods, that doesn't mean it is better for people.

Lower ozone levels would theoretically make life easier for some asthma sufferers, but ozone is just one of a complex web of environmental factors linked to breathing problems, many of them higher in cities.

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