- The Washington Times - Wednesday, July 9, 2003


Scientists studying urban pollution have discovered to their amazement that trees in New York City’s concrete jungle grow twice as large as those in the countryside, far from the billowing smokestacks and crowded streets.

The findings illustrate what scientists have only recently realized — that pollution from urban areas can have its biggest effects far from cities.

The researchers blame the rural trees’ stunted growth on higher sustained levels of ozone originating in the city. The study traced the growth of identical cottonwood trees in both kinds of locations.

“In the country, the trees were about up to my waist. In the city, they were almost over my head — it’s really dramatic,” said Jillian W. Gregg, the study’s lead author.

The findings appear in today’s issue of the journal Nature. The work was done by a team from Cornell University and the Institute of Ecosystem Studies in Millbrook, N.Y.

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, Mrs. Gregg said cumulative ozone exposures at the rural sites were up to 50 percent higher than in the city.

Rural ozone starts in cities, where “ozone precursors” from auto and industrial emissions interact with sunlight to form ozone. In the city, high concentrations of one of those precursors, nitric oxide, can have what Mrs. Gregg called a “scavenging reaction” that scrubs ozone from urban air and keeps overall ozone levels moderate.

But once ozone is blown out of the city to 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 that scientists need to look more closely at the secondary effects of urban pollution on rural areas.

“To me the lesson is that in the places where most plants grow — and plants are trees, shrubs and crops — ozone can be a serious problem,” said Eva J. Pell, a professor of agricultural sciences at Penn State University.

Mrs. Gregg and her colleagues planted genetically identical cottonwood trees, from cuttings taken from the same tree, at seven sites — four in New York City’s boroughs and three in rural Long Island or upstate New York.

To eliminate the possibility that soil variations could account for any growth differences, soil from rural sites was moved to some of the urban test plots and urban soil was moved to rural areas. The trees were planted in pots buried in the ground and were watered the same amount.

All the trees were grown near pollution-monitoring equipment so the researchers could gauge pollutant levels.

Over three growing seasons, saplings grew robustly in New York City, while lagging far behind in the rural areas.

“No matter what soil I grew them in, they always grew twice as large in New York City,” said Mrs. Gregg, who was initially perplexed by the results.

Later experiments in controlled settings found the same trees, when exposed to high levels of ozone, indeed grew half as large.

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