Trees toxic?
Can't see Canada's Kyoto strategy for the trees
NEIL REYNOLDS
From Wednesday's Globe and Mail
theglobeandmail.com
OTTAWA — In his 1980 U.S. presidential campaign, amid loud hooting, Ronald Reagan said it first. Trees pollute. In the intervening 25 years, science has vindicated him.
Nature has always been recognized as a primary source of pollutants, of course, from volcanic eruption, from forest fires, from methane swamps. But now we know that shade trees are spewing vast quantities of chemical pollutants into the air. We know that they are doing so at an accelerating rate. And we know, worst of all, that these emissions peak on hot summer days -- precisely when people seek the shade of their branches for refuge from the sun. Count one more win for the Gipper.
In the most exhaustive North American research program of its kind, scientists studied data collected by the U.S. Forest Service from 2.7 million trees on 250,000 separate plots of land across the United States. (One of the scientists was University of Toronto ecologist John Caspersen.) Their findings, released in 2004, confirm that tree leaves emit harmful volatile organic chemicals (VOCs), a class of hydrocarbons normally associated with the tailpipes of cars and trucks that reacts with other chemicals to form smog. They found that the primary culprit was isoprene, a chemical -- in the words of Popular Science magazine -- that "young, fast-growing trees churn out like chimneys." Isoprene production occurs most prodigiously in summer heat. Evergreens are significant emitters, especially in the southern states. But so are hardwoods. In Missouri, oak groves emit as much as 300 tons of isoprene a day.
In Britain, scientists at Lancaster University have now developed a method to measure VOC emissions from different species of trees. (In Europe, scientists identified deciduous trees -- oak, elm, maple -- as the biggest polluters.) Using their Urban Tree Air Quality Score, urban planners can now select the most appropriate "biogenic emitter" to plant at any particular site anywhere in the world. It's not only trees, though, that pollute. Australian scientists have determined that grass itself is "a potent emitter of reactive hydrocarbons," especially when freshly mowed. Among the chemical compounds released into the atmosphere by vegetation: acetaldehyde, acetone, methanol, methyl butanol and a number of hexanols.
Governments will presumably come under pressure in the years ahead to require emission-control testing of trees and shrubs (using biogenic breathalyzers) along urban streets and in urban parks. Warning labels will presumably be needed. ("This tree may be hazardous to your health." "Best before 2040.") Nurseries will presumably be alert to the competitive advantage of different species. ("Nine out of 10 doctors choose mountain ash.") Genetic manipulation will presumably follow. ("Oak trees with half the isoprene.") The consequences for literature will presumably be significant as well. To paraphrase Joyce Kilmer, I think that I shall never see a poem toxic as a tree. Life is always a close-run thing between action and ignorance. Each successive generation thinks itself sophisticated and smart, yet each finds itself humbled by the apparently more advanced people in the next. This is especially true with governments, which is why central planning doesn't work. By the time a bureaucracy acts, the erroneous assumptions it has made are obvious even to the bureaucracy itself. By then, though, it's too late to stop. Thus with the Kyoto accord, which allows countries to meet carbon emission targets by planting forests to soak up carbon instead of actually cutting emissions.
A few years ago, people thought that the environmental risk from plants was limited to pollen. No longer. The scientific study of biogenic hydrocarbons continues apace. In 2002, hundreds of scientists from around the world met at Oxford University -- plant scientists, atmospheric chemists, ecophysiologists -- to assess "plant emissions of hydrocarbons and the impact on the environment." The Geoenvironmental Research Centre at Cardiff University in Wales cited estimates that hydrocarbons from plants account for 66 per cent or more of all hydrocarbons entering the atmosphere. (Ronald Reagan's actual words were these: "Approximately 80 per cent of our air pollution stems from hydrocarbons released by vegetation.") None of this begins to deal with the biogenic pollution that people themselves naturally "emit" into the atmosphere. Along with plants and animals, people produce vast quantities of biogenic aerosols, particles of stuff, living and dead, that we shed every minute, every hour, every day. Scientists have calculated that the mere act of changing one's clothes can propel 10,000 bacteria a minute into the air where, carried aloft in little spherical bubbles, they join the fungi, viruses, pollens, spores and dust particles that are a natural part of the air we breathe.
It's a fact. Humans are messy. Nature is messy. People are probably best advised to leave tree pollution to nature and concentrate instead on chimney pollution, including the tailpipe kind, for which they are responsible. At the same time, though, they might want to reconsider the Kyoto accord, which forgives carbon emissions in some circumstances provided -- as Canada intends -- you produce more biogenic pollution. With all these isoprenes in the air, however, Canadians shouldn't count on hiding behind the trees too much longer.
neilreynolds@rogers.com |
