TREES grow better in cities than in the countryside.
Ecologist Jillian Gregg at Cornell University in Ithaca led a study of cottonwood saplings in New York city and the surrounding countryside. 鈥淭hey were always twice as big in the city,鈥� she says. 鈥淚t鈥檚 very dramatic.鈥�
The effect is caused by traffic fumes creating the toxic gas ozone, Gregg鈥檚 team found. In the cities, ozone levels are kept in check as the gas is destroyed by other pollutants such as nitrogen oxides.
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But ozone also drifts into the countryside where nitrogen oxide concentrations are low. There, because the ozone lingers longer in the atmosphere, trees experience prolonged exposure to the gas (Nature, vol 424, p 183).
鈥淚t makes perfect sense now, but was a real surprise at the time,鈥� says Gregg, who expected the trees to grow worse in the urban environment.