This is a story about a city tree and a country tree. Each started as an identical sapling of a common cottonwood, a fast-growing poplar. Scientists wanted to analyze the impact of multiple pollutants in the two settings. And they found to their amazement that the city tree grew twice as large as its country clone.

Jillian Gregg set about to find the worst, most polluted environment in New York City. There she planted a tree. She planted another in the Hudson River Valley 80 kilometers from New York. She and a team of scientists at Cornell University and the Institute of Ecosystems studies in New York followed the test sites over three years to understand why the city trees grew bigger.

In both locations the plants grew next to atmospheric pollutant monitoring stations. "What we did was to separate factors above ground Vs below ground. So, we went to a series of urban and rural sites and collected the soil and took them all to the same place," she says. "We could [then] ask the question within each soil type, do plants grow less in New York City?"

The answer was no. No matter what soil they were grown in, they got the same results. City trees were double the size of their country cousins. "That result was consistent for 11 different soils, 8 different sites, 2 different transects and 3 consecutive growing seasons," she says.

The scientists then set out to study above ground factors that would account for the difference. When a number of experiments ruled out warmer temperatures and higher concentration of nitrogen and carbon dioxide, scientists turned their attention to the stunted trees in the country.

Jillian Gregg says the answer is a difference in chemistry in the atmosphere that favors city trees. "We found that the cumulative ozone exposures were higher outside the city center," she says.

Rural ozone starts in the city. Automobile and industrial emissions interact with sunlight to form ozone, which the wind blows into the country.

The difference between the city and the country hinges on nitric oxide - one of the primary precursors of ozone, which was found in high concentrations in the city.

Jillian Gregg explains that it produces a chemical reaction that causes urban ozone levels to drop to nearly zero at night and in the winter. "So ozone is continually created and destroyed and created and destroyed within the city, but when that same air mass goes to the country, the high ozone concentrations remained in the atmosphere for a longer period. So you have a higher cumulative ozone exposures."

Which Jillian Gregg says translates into stunted trees. She says the work is a cautionary tale of the effects of urban pollution. "It is important for us to understand that we cannot escape the urban pollutants by going to the country, the effects can have an even greater impact there," she says. "So, if we want to get away from all of these pollutants, we need to curb them at their source."

Jillian Gregg and colleagues from Cornell University and the Institute of Ecosystem Studies write about the work in the Journal Nature.