Rural trees suffer from urban smog

Most people might assume that big-city smog affects urban trees and vegetation the most, but a new study reveals that ozone in city smog increases with exposure to sunlight as it drifts to rural areas where it does more damage.

Scientists this week used New York to make the case that the city, not the most environmentally friendly place in other respects, is a great place to grow trees. Poplars fare better in the Big Apple than in the countryside.
Ozone is an important element in the stratosphere where it protects against harmful UV radiation, but close to the ground it’s a major component of the kind of air pollution that stunts tree growth. It’s produced when primary pollutants are emitted (e.g., coal-fired generating stations, car exhausts) and react under the influence of sunlight. But by the time ozone has formed, the largest part of the air mass has drifted to rural areas.

A scientist at Cornell University in New York planted fast-growing cloned poplars in various sites in and around New York City and in rural areas of the Hudson River Valley and Long Island. The comparison of how the genetically identical trees fared in the urban and rural ecosystems was reported in the science journal Nature.

After comparing growth over three seasons, the scientist (who is now with U.S. EPA) in Oregon, discovered that the city trees thrived. In fact, they grew twice as much as their country counterparts. Exposure to light, soil factors and precipitation could not account for the differences in growth, leaving ozone as the culprit.

Another factor is nitrous oxide (NOx) which is present in urban air but not prevalent in the countryside. NOx "eats up" ozone in the city, enhancing the positive effects for vegetation. The results from the New York study are deemed applicable to all large cities.