by Elizabeth Gardner
It’s not just cars and factories that can have a negative effect on air quality. So can plants and trees. Assistant Professor Mark Potosnak of the environmental sciences program studies how the biosphere and the atmosphere interact and specifically how the gases from plants affect the overall chemistry of the air we breathe.
“Plants don’t cause pollution, but plant gases interact with other types of pollution to create ozone–the kind that promotes asthma in kids and can affect anyone at a high enough concentration,” Potosnak says. He and his students study how leaf emissions may affect, and be affected by, global climate change–a very intricate relationship that’s not simple to model or predict. They traveled to Alaska over the summer to measure leaf emissions in the wild.
“The Arctic and Alaska are showing rapidly accelerating climate change,” Potosnak says. “Everyone thought change would happen more quickly there, but it’s happening even faster than our models predicted.” Spring is coming earlier and fall is lasting later, and the amount of ice is close to a record low.
One of the group’s primary interests is the gas isoprene, a hydrocarbon emitted by many species of trees and bushes, including cottonwoods, oaks, aspens and willows. By itself, isoprene doesn’t constitute a harmful pollutant; it just makes the air a little hazy, but it can react with human-generated pollutants to form ozone. Isoprene emissions help a tree respond to temperature changes, and temperature sometimes determines, to some extent, how much isoprene the tree gives off. The emissions are also affected by the amount of carbon dioxide in the atmosphere, which is increasing and is one major reason for rising temperatures globally.
Potosnak’s group has found that higher temperatures tend to increase isoprene emissions in some plant species, whereas higher levels of atmospheric carbon dioxide tend to decrease the same emissions. The question is, can extra carbon dioxide reduce the emissions enough to offset the increase caused by rising temperatures? (Spoiler: Apparently not.) Partly funded by a National Science Foundation grant, Potosnak’s group visited the Toolik Field Station, managed by the University of Alaska Fairbanks’ Institute of Arctic Biology, near the Arctic Circle. Their target organism was a dwarf willow that’s unique to the area and produces large amounts of isoprene. Enclosing individual leaves in small chambers, they adjusted variables like light, carbon dioxide, humidity and temperature and measured how each variable affected isoprene emissions.
Undergraduate Lauren LeStourgeon, whose senior thesis is based on isoprene observations, has been working with Potosnak for the past year. She relished the opportunity to get out of DePaul’s greenhouse and into the field. She’s studied aspens and oaks in addition to willows and has found marked differences among the species. For example, oak isoprene emissions don’t seem to vary with carbon dioxide concentration. Her work is partially supported by a scholarship from the Illinois Space Grant Consortium, a NASA program.
Despite the rapacious mosquitoes and the sleep disruption caused by the “midnight sun,” the trip was a highlight of LeStourgeon’s DePaul career, and Alaska was so unexpectedly warm that she didn’t need her cold-weather gear. She estimates she studied five or six willow leaves per day, each for an hour or more, over the course of three weeks in mid-June and collected data on almost 200 in all. “There was a lot of standing around,” she adds.
LeStourgeon presented her research this fall at the 2011 Great Midwestern Space Grant Regional Meeting at the University of Illinois at Urbana-Champaign, where she won an award for best undergraduate poster. So far, her findings suggest that rising temperatures increase isoprene production faster than extra carbon dioxide can suppress it.
With a body of leaf-level findings about emissions, the next step is to see whether the numbers scale up and how they plug into evolving models of global climate change. Potosnak says figuring out the role of plant emissions in atmospheric change is a key component for determining environmental policy. “We’re not going to chop down trees, but when we do controlled studies to understand how pollution occurs, we can make better decisions about how to cut back our own emissions,” he says.
Potosnak began his academic career as a physics major at Harvard, but switched his focus after one class in earth science, which showed him how physics could help measure the ways humans impact Earth. At the time, one of his professors was studying the causes of the atmosphere’s ozone hole.
Potosnak got a chance to study isoprene-emitting cottonwood trees during his graduate study at New York’s Columbia University, which at the time was managing the scientific research site Biosphere 2 near Tucson, Ariz. Originally built to study how humans could live in a sealed environment, the three-acre facility had been converted to a lab for studying the effects of carbon dioxide on plants, and the fast-growing cottonwood had a major presence there. “It was such a large facility that we could take entire trees and subject them to different carbon dioxide levels,” he says.
Potosnak’s students take a variety of imaginative approaches to environmental issues: measuring lead levels in the leaves of urban trees, sending balloons into the upper atmosphere to measure how carbon dioxide concentrations change with altitude, and running energy conservation competitions in DePaul’s residence halls.
Despite persistent skepticism in a few sectors of the general public, Potosnak says climate scientists, as a group, harbor no doubt about the existence and origins of global climate change. “People might fight over how quickly it’s happening or what it might mean for hurricanes, but the broad idea that the Earth is getting warmer because of human activity is as close to 100 percent [accepted] as you can get,” he says. “We have to move forward and start reducing emissions, accepting that there’s going to be climate change and figuring out what we need to do to adapt. Some adaptations might take 20 years to accomplish, so having good scientific models is crucial.”
Freelance writer Elizabeth Gardner has covered science, business and technology topics for such publications as University Business, Internet Retailer and Modern Healthcare. She is based in Chicago.