Scotch pines emit nitrogen oxides into the air. (Fallen Trees?).

Even pristine forests can contribute to air pollution. In fact, researchers now say that northern pine forests exude a family of nitrogen oxides and do so in quantities that may rival those produced worldwide by industry and traffic.

Nitrogen oxides can react with hydrocarbons to yield nitric acid, a primary ingredient in acid rain. They can also help produce smog-causing ozone. Scientists generally peg automobiles as the prime source of nitrogen oxides. Trees, on the other hand, are usually credited with sopping up air pollutants.

Forests and industrial pollutants sometimes interact unpredictably, however. For example, researchers found that a hydro-carbon released by oak trees in the Sierra Nevada of California exacerbates ozone production from industrial nitrogen oxides (SN: 6/1/02, p. 346).

Now, forest ecologist Pertti Hari of the University of Helsinki and his colleagues add another layer of complexity to the relationship between trees and air pollution. The researchers suspected that pine trees growing in a southern Finland forest might use atmospheric nitrogen oxides as a source for nitrogen, an essential plant nutrient. To find out, they enclosed branches of forest Pinus sylvestris, or Scotch pine, in chambers that are transparent to ultraviolet (UV) light. Then they measured the change in air concentrations of nitrogen oxides.

In the March 13 Nature, the team reports that the branches emitted, rather than absorbed, the pollutants. Seconds after the researchers closed a chamber, concentrations of the gases doubled. When branches were shielded from the sun's UV light, they emitted less nitrogen oxides. That might explain why earlier studies missed the nitrogen oxides that plants release, Hari says. Scientists often measure tree emissions under lab conditions that lack normal UV exposure or in chambers that block UV, he explains.

Arboreal emissions of nitrogen oxides are "evidently an important component of the nitrogen cycle," Hari says. Other ever greens--and perhaps even all plants--might also release the compounds under many natural conditions, he suspects.

Ambient concentrations of the air pollutants may be a deciding factor, Hari says. In the forests of Finland, air concentrations of nitrogen oxides can fall below I part per billion. That's less than one-fiftieth of the U.S. air-quality standard for the chemicals. Plants living in such clean air may release nitrogen oxides, while the same plants living in polluted air might absorb the chemicals, Hari suggests.

"The study makes clear that vegetation may indeed be a significant source for nitrogen oxides," says forest biologist Russell K. Monson of the University of Colorado at Boulder. As the thinning ozone layer allows more UV light to reach Earth's surface, plants' contribution to nitrogen oxides and smog might even increase, he adds.