Even though he also said “Facts are stupid things” during a 1988 interview, it turns out his claim about dirty trees was partly correct.
There are a number of ways in which plants contribute to air pollution. The piney-woods aroma that we so appreciate in our Christmas trees or on a walk among evergreens is courtesy of a mixture of volatile chemicals, primarily terpenes and isoprenes. On hot days, conifers release lots of these compounds, which combine with car exhaust and contribute to ground-level ozone, a respiratory irritant. In the stratosphere, ozone protects us from dangerous UV radiation, but down low it’s harmful.
Bryan Duncan, an atmospheric scientist at NASA’s Goddard Space Flight Center, explains that “What Reagan neglected to indicate is that unhealthy levels of ozone wouldn’t form without nitrogen oxides (NOx), pollutants emitted when gasoline and coal are burned.” Agricultural emissions also react withisoprenes to form surface ozone. If not for human-caused pollutants, this wouldn’t happen of course, but the fact remains that trees in modern times do contribute to a certain type of air pollution.
A lesser-known fact is that trees also create greenhouse gases. No doubt this would have delighted the rabidly anti-nature President, who as Governor of California strove to block the expansion of the Redwood National Forest, saying “A tree is a tree. How many more do you have to look at?”
It seems that all woody plants can liberate methane, a greenhouse gas 32-45 times more potent than carbon dioxide, from the soil into the air. The mechanism by which this occurs is not well understood, but it clearly happens. In damp woods and swamps the amount can be significant, while forests on higher elevations release very little methane.
As if that wasn’t bad enough, some tree species, cottonwood (Populus deltoides) in particular, actually produce their own methane thanks to microbes that colonize their tissues. We’ve long known that dead trees – and vegetation of all sorts – create this planet-warming gas. That’s how methane digesters and cows work. But since 2018, living forests now must be factored into global greenhouse-gas estimates.
According to an April 9, 2021 article in The Conversation by Luke Jeffrey, a Postdoctoral Research Fellow at Southern Cross University in New South Wales, Australia, about half the methane coming out of the Amazon basin is from its tropical forests. This sounds dire – no wonder Brazilian President Jair Bolsonaro is in such a hurry to burn down his country’s rainforests! Maybe we should donate matches in the interest of stopping climate change. Hold on – it’s not that simple. Australian researcher Dr. Jeffrey and his team were the first scientists in the world to document methane-eating bacteria in trees, finding in 2020 that certain bark microbes consumed about 36% of tree methane.
The impact on planetary warming of methane generated from rainforests is a drop in the ocean compared to the effect of all the carbon dioxide released when they are torched. As Dr. Jeffrey emphasizes, “Trees are in no way shape or form bad for our climate, and provide a swath of priceless ecosystem benefits. The amount of methane emitted from trees is dwarfed by the amount of carbon dioxide they will take in over their lifetime.”
Writing for the journal American Forests in July 2017, Melanie Friedel says “U.S. forests alone store 14 percent of all annual carbon dioxide emissions from the national economy.” That’s pretty significant when you consider that the US, at 4.4% of the Earth’s population, burns 24% of its fossil fuels. Peter Ellis, a researcher at The Nature Conservancy in Arlington, Virginia, led a 2017 study which found that worldwide, forests can supply more than a third of the total carbon-dioxide reductions needed to keep global warming below 2°C through to 2030.
Admittedly, gauging the overall effect that trees have on climate is more knotty than simple carbon calculations. It’s good to bear in mind their other gifts as well. Trees reduce air pollution by absorbing noxious gases through their stomata, or leaf openings, into the leaves where toxins are broken down.
Airborne particulate matter, which we now know is much more harmful to us than was once thought, is captured on leaf surfaces and later washed to the ground. This is kind of like the way expensive state-of-the-art industrial air filters work, but it’s free and doesn’t run on electricity. A 2002 US Forest Service report out of Syracuse, NY states that in contiguous forest stands, trees reduce sulfur dioxide by 14%, particulate matter by 13% and nitrogen dioxide by 8%.
There may be no limit to the benefits of trees. In 2018, The Green Heart project, a National Institutes of Health-funded initiative run by the University of Louisville, began a multi-year study on exactly how and why trees reduce cardiovascular disease. Maybe planting more trees can restore the hearts of callous world leaders in addition to helping curb the rate of climate change.
Read more about trees in New York State.
Photo of Eastern Cottonwood courtesy Wikimedia user Laurent Bélanger.