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Trees grow faster in the city

July 10, 2003 By Michael Hopkin This article courtesy of Nature News.

Ozone pollution stunts suburban saplings.

Things really are bigger in New York. Even the trees. Those in the Big Apple's core grow larger than those in the surrounding suburbs, researchers have discovered1. Surprisingly, the difference is down to pollution.

For three years Jillian Gregg of the US Environmental Protection Agency in Corvallis, Oregon and her colleagues tracked the growth of identical eastern cottonwoods (Populus deltoides) planted in New York City and at nearby rural sites on Long Island and the Hudson valley.

"They started about six inches high," recalls Gregg. "The ones in the city ended up taller than me; they were only up to my waist in the country." Variables such as soil composition, temperature and carbon dioxide levels could not account for the difference.

The crucial factor, calculates Gregg's group, is exposure to ozone. Levels of this pollutant frequently fall almost to zero in a city's heart, while remaining higher in the country.

"Ozone is probably the most important plant pollutant in the United States," agrees John Lawrence of the US Department of Agriculture's Forest Service in Corvallis, Oregon. The highly reactive form of oxygen stunts plant growth, and can prevent flowering.

Ironically, ozone is generated when sunlight reacts with pollutants, such as the gases in car exhausts. But because it's so reactive, leftover pollutants can scrub city centres clean of ozone, says Gregg. Oxides of nitrogen, for example, react with ozone, reducing levels to below those in the South Pole's famed ozone hole.

In New York's leafy environs, on the other hand, ozone can climb to high levels. In the absence of other pollutants with which to react, it hangs around for longer.

Tens of kilometres up in the stratosphere, ozone protects us from the Sun's ultraviolet radiation. But ground-level ozone can damage the environment and our health. The gas can exacerbate human respiratory ailments such as asthma, for example.

Any place where you have lots of automobiles generates ozone
Eva Pell
Pennsylvania State University

The US East Coast is a known hotspot for ozone pollution. But will cities elsewhere inflict a similar fate on their suburbs? Not necessarily, says plant physiologist Eva Pell of Pennsylvania State University in Philadelphia.

"Any place where you have lots of automobiles generates ozone, but in some places you will have bigger problems than others," Pell predicts. A city's prevailing air movements can determine whether the local countryside is swamped with its pollutants.

Mountains surrounding Los Angeles, for example, may spare much of southern California the worst ravages of ozone pollution. "Los Angeles' air is hemmed in and tends to stagnate," Pell says.


  1. Gregg, J. W., Jones, C. G. & Dawson, T. E. Urbanization effects on tree growth in the vicinity of New York City. Nature, 424, 183 - 187, (2003).


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