Ottawa — Coniferous forests around the world may be emitting more smog-causing nitrogen oxides than traffic and industry combined, suggests a report in the prestigious journal Nature.
The report, released Wednesday, flies in the face of the accepted view that forests reduce pollution by absorbing it — a theory Canada relied on in demanding credit for forests as pollution "sinks" under the Kyoto climate change accord.
But environmentalists aren't about to blacklist Scotch pine trees. They note that forest emissions are part of a natural balance that has existed since pre-industrial times and say man-made emissions are behind most pollution and global warming.
Scotch pine needles release nitrogen oxides directly into the atmosphere when exposed to ultraviolet light, says a study led by Perrti Hari of the University of Helsinki.
Nitrogen oxides are smog precursors: They combine with other pollutants to form ground-level ozone, a major component of smog.
The emissions from Scotch pines increase in proportion to the amount of ultraviolet radiation they receive, says the study.
"Although this contribution is insignificant on a local scale, our findings suggest that global NOx emissions from boreal coniferous forests may be comparable to those produced by worldwide industrial and traffic sources," says the report.
Former U.S. president Ronald Reagan caused an uproar in 1980 when he said trees cause just as much pollution as cars.
Quentin Chiotti, a scientist with Toronto-based Pollution Probe, acknowledged that forests contribute a variety of chemicals to the atmosphere, but was surprised at the comparison with traffic and industry.
"If I walk through a forest am I going to be at risk for my cardiac and respiratory health? Unless it's a very unusual forest ... I can't imagine our health being at risk."
Even though the emissions may seem large when calculated for the globe they have no local health effects because they are diluted in the global atmosphere, he said.
Henry Hengeveld, a scientist with Environment Canada, said emissions from vegetation are part of a balance that doesn't change much over time.
"They should be pretty steady at a pre-industrial level," he said.
What matters for human health is the increments to that background level, he said.
Kevin Percy of the Canadian Forest Service in Fredericton said he has problems with the suggestion that emissions from coniferous trees could exceed those from traffic and industry.
"From my perspective, that would be pure conjecture at this point."
