The results clash with assumptions that cities filled with concrete, asphalt and polluted skies are inhospitable to plants. But they don't mean New York City is a plant paradise, either.
Instead, the researchers attribute the trees' stunted growth in rural areas to higher sustained levels of ozone at those sites in comparison to 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 the July 10 issue of the journal Nature. The work was done by a team from Cornell University and the Institute of Ecosystem Studies in Millbrook, New York.
Ground-level ozone, a precursor of smog linked to respiratory ailments in humans, can also slow the growth of some plants.
Even though one-hour peak ozone levels could be high in New York City, Gregg said cumulative ozone exposures at the rural sites were up to 50 percent higher than in the city.
Rural ozone starts in cities as "ozone precursors" from auto and industrial emissions that form ozone by chemically interacting with sunlight. In the city, high concentrations of one of those precursors, nitric oxide, can have what Gregg called a "scavenging reaction" that scrubs ozone from urban air once it forms 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, miles from traffic jams and smokestacks.
Other scientists who did not contribute to the research said the new study's conclusions suggest that scientists need to look more closely at the secondary effects of urban pollution on rural areas.
"It's one of the most compelling negative correlations between plant growth and ozone concentration that I've seen," said Eric Kruger, an associate professor of tree physiology at the University of Wisconsin at Madison.
Gregg and her colleagues planted genetically identical cottonwood trees rooted from cuttings taken from the same tree at seven sites — four in New York City's boroughs, and the three others 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 the urban test plots and urban soil was moved to rural areas.
The trees were planted in pots buried in the ground and were irrigated with the same amount of water, Gregg said.
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 Gregg, who said she was initially perplexed by the unexpected results.
Later experiments in controlled settings found the same trees, when exposed to high levels of ozone, indeed grew half as large.
David F. Karnosky, a professor of forest genetics at the Michigan Technological University in Houghton, Michigan, said the next step should be long-term studies aimed at assessing how high ozone levels impact other tree species in other regions.