Rather than being a global warming solution, pine trees may be inducing smog and acid rain by releasing vast amounts of nitrogen oxide into the air, researchers have discovered.

Under some circumstances, needles from Scots pine trees can release nitrogen oxides directly into the atmosphere, according to a report in today's issue of the journal, Nature.

A team led by Professor Pertti Hari of the University of Helsinki in Finland, conducted experiments with new pine shoots from Pinus sylvestris, measuring the amount of nitrogen oxides they emitted when exposed to sunlight.

Nitrogen oxides (NOx), along with hydrocarbon gases and sunlight, are the three ingredients needed to make photochemical smog that clogs the skies of many urban centres, as well as acid rain.

The researchers were surprised to find NOx produced by the pine needles, and that levels rose when shoots were exposed to ultraviolet radiation from the Sun. While the amount produced was insignificant on a local scale, "our findings suggest that global NOx from boreal coniferous forests may be comparable to those produced by worldwide industrial and traffic sources," said the authors.

Other scientists were cautious. "I would like to see a more qualitative estimate of how significant that might be in comparison to all the other sources of nitrogen oxide in the atmosphere," said Associate Professor David Griffith of the atmospheric chemistry research group at Australia's University of Wollongong.

While the gases produced by pine trees could in theory deliver as much air pollution as worldwide industrial emissions, in reality most trees do not grow in air that has the very low background levels of nitrogen oxides needed for the trees to emit NOx in the first place.

"If there are already nitrogen oxides in the air, there will be a net uptake, they will be taken up by the trees," he told ABC Science Online.

The study did show for the first time that forests can produce two of the three critical ingredients for smog, and it had been previously established that trees produce hydrocarbons. "No-one has recognised before that they also produce the nitrogen oxides," said Griffith. "Once you put both those ingredients together you can form photochemical smog."

But the amount of nitrous oxides required for this to happen are in the order of tens of parts per billion, and the research has only found levels of one to three parts per billion, he said.

"What I can't judge is how significant this would be globally," he added. "It's going to be a local effect. Nitrogen oxides don't have a long lifetime in the atmosphere." By comparison, nitrous oxides have a very long lifetime, and can spread all over the world.

Australia does not have a lot of boreal or 'needle tree' - forests, because they tend to exist in high latitude areas. But the findings could be relevant on a local scale in Australian pine plantations, Griffith said.

Large scale planting of pine trees to act as 'carbon sinks' has been promoted by some, the theory being that fast-growing trees absorb and retain carbon dioxide from the air. Carbon dioxide is widely cited one of the major contributors to global warming.

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