Better in the city?

Scientists studying urban pollution have come forth with the jaw dropper that trees in New York City's concrete jungle grow twice as large as those in the countryside. But if you think pollution is good for tree growth, you'd be wrong.

Researchers blame the rural trees' stunted growth on higher sustained levels of ozone originating in the city. The study by Cornell University and the Institute of Ecosystem Studies appeared in the journal Nature. It traced the growth of identical cottonwood trees in both kinds of locations.

Ground-level ozone, a precursor of smog linked to respiratory ailments in humans, is known to slow the growth of some plants. Even though one-hour peak ozone levels can be high in New York City, the study revealed that cumulative ozone exposures at. the rural sites were up to 50 percent higher than in the city.

Once ozone is blown out of the city and into the countryside, where nitric oxide concentrations are low, the ozone remains in the atmosphere longer. And as ozone's precursors drift toward the country, they produce even more ozone. Scientists not involved in the research said the findings suggest scientists need to look more closely at the secondary effects of urban pollution on rural areas.

Scientists planted seven sites with genetically identical cottonwood trees grown from cuttings taken from the same tree. Four sites were in New York City's boroughs; the other three were in rural Long Island or upstate New York.

Over three growing seasons, trees grew robustly in New York City, while those in rural areas lagged far behind. Later experiments in controlled settings found that the same trees, when exposed to high levels of ozone, grew haft as large, according to the article.