The word "Arctic" tends to conjure images of glaciers, polar bears, icy waters, and frozen tundra. And indeed, all of those are common features of Alaska's North Slope. But as climate change alters the atmosphere and landscape in the Arctic, another image might need to be added to that list: fire.
Researchers at Toolik Field Station, the National Science Foundation's Arctic long-term ecological research site located about 150 miles south of the Arctic Ocean, are studying the impact of severely burned tundra on the local and global environment.
Though scientists have for years examined burns in Alaska's boreal forest, where fire is a natural part of the growing cycle, the idea of a blazing fire in the tundra is hard to imagine. After all, not too long ago, it was a rarity to see a lightning strike—the source of most forest fires—that far north. John Hobbie, one of the founders of Toolik, remembers a professor at Dartmouth College, where he was an undergraduate in the mid-1950s, telling the class there were no cumulous clouds in the Arctic.
But as the temperature in the Arctic has risen, the number of lightning strikes has increased 20-fold. Because the area is so cold and wet, a strike generally has trouble causing much trouble. In July 2007, lightning hit an area north of camp, near the Anaktuvuk River, starting a fire that smoldered for several weeks before surprising everyone and taking off.
Evidence of the fire was seen—and felt—at Toolik. "When the wind wasn't blowing, you could see a big wall of fire," says Syndonia Bret-Harte, a plant community and ecosystem ecologist at the University of Alaska, Fairbanks. "It was awesome and beautiful and disturbing at the same time."
By the time it had gone out, about 400 sq miles of land had been burned, with large swaths severely damaged by the fire. "The Anaktuvuk fire happened late in the season," Bret-Harte says. As a consequence, a lot of soil had thawed and plants had months to grow, creating plenty of kindling for combustion.
The site now provides an opportunity for several scientists at Toolik to look at how fire affects the landscape and how an increase in fires might ultimately contribute to further warming. For example, if a severe fire burns off the mineral soil, exposing the permafrost, it could accelerate melting of ground that has been frozen for hundreds or thousands of years.
Further, without plant cover, the albedo of the surface changes, in theory further warming the soil and providing microbes access to carbon- and nitrogen-based nutrients that had long been locked away in permafrost. The microbes break down those nutrients into carbon dioxide and other greenhouse gases. Adrian V. Rocha, a postdoctoral researcher at Marine Biological Laboratory, an ecology research center in Woods Hole, Mass., is studying that carbon balance.
Rocha has installed eddy-covariance towers, which record meteorological data; soil temperature and moisture; energy exchange; and carbon dioxide and water flux, which can be used to calculate carbon exchange in the area. There is a tower in a severely burned area, a moderately burned area, and an undisturbed area, allowing him to compare how much and for how long the fire disrupts carbon balance.
Meanwhile, Bret-Harte is looking at how the abundance and composition of plant species might change in the wake of the fire and how that altered landscape is linked to changes in the nitrogen cycle. She uses ion-exchange resins to get an index of ammonia, nitrates, and phosphorus concentrations in the area.
She is also collecting foliage that will be reduced to a powder and then analyzed by mass spectrometry for a nitrogen-15 signature. The idea is that as deeper layers of soil become available, the plants that subsequently grow should be richer in nitrogen-15 than plants in areas that weren't burned.
So far, there has been a fair amount of regrowth. Last summer, or one year after the burn, plants covered about 25% of the ground, Bret-Harte says. Indeed, in late June, now two years after the burn, evidence of the resiliency of plants was abundant; a severely burned area had become a sea of cotton grass. However, she notes that the normal mosses and lichens have yet to return to the area, and in fact a plant usually found in the boreal forest, marchantia, has cropped up.
The scientists working at the burn site have also reported anecdotal evidence of an increase in the number of thermokarsts—features that occur when the permafrost melts and the ground collapses—in the severely burned areas. "Fire is consuming a lot of the insulating organic layer, and one thing we see are these huge thermokarsts in the burn," notes Michelle Mack, a plant ecologist at the University of Florida.
Thermokarsts have, in turn, been known to greatly affect the surrounding environment, causing an influx of nutrients into nearby lakes and streams and shrubbier vegetation to thrive in the area. Past seasons have demonstrated that a shrubbier North Slope leads to a shorter time interval between fires, Mack says.
Based on measurements made last summer—by Mack; Bret-Harte; Gaius R. Shaver, Rocha's adviser at the Marine Biological Laboratory, and others—there is an early sense of how an increase in tundra fires could impact the Arctic and contribute to climate change.
The group recently presented data from studies of the fire conducted in 2008 and found that the amount of carbon burned off was actually not as old as they had expected. About 10 cm of soil organic matter was lost in the fire, and on average the carbon coming out of the fire was 50 years old. "It's surprising to me that even with a high-severity burn, we didn't really burn off that much soil organic matter," Mack says.
With the burn just a short helicopter flight from Toolik, the scientists will continue to study the landscape in the coming years and eventually construct a picture of what the Arctic might look like if fires become more prevalent.