To learn more about the history of wildfires in an area, it helps to look in the water - at the bottom of lakes, to be specific.
Charcoal records taken from lake bed sediment core samples can show layers of ash and cinders deposited in the lakes by wildfires. Distant events tend to deposit smaller particles, since ash can float a long way on the wind, whereas fires nearby deposit larger particles of fire debris - twigs and pine needles.
"Core samples from lake beds tell the historic tale of fires," said Cathy Whitlock, a Montana State University professor in the Department of Earth Sciences Paleoecology Lab. "They tell us how large-scale changes in climate have affected fire history."
The samples are taken from boats in the summer, or from atop the ice in winter. Using a hand-operated piston corer, researchers remove a yardlong cylinder of mud 2 inches in diameter. They keep pulling samples from the same hole until they reach gravel on the bottom of the lake, where the boring stops.
In Yellowstone National Park, where Whitlock has been doing research, samples typically go back about 15,000 years, when glaciers first started receding from the park and lakes were forming. In one lake in Eastern Washington, Whitlock's crew has drilled down to deposits made 130,000 years ago.
Researchers end up with a sausagelike tube of mud that they take back to the lab, where it's split down the middle. Once the core is revealed, samples of organic matter are removed. In some cases, they find twigs, pine needles and parts of insects thousands of years old.
"When you get the cores, the whole history is laid out in front of you," Whitlock said. "It's pretty amazing. It's a layer cake of what's happened historically in that watershed."
The organic matter can be radiocarbon-dated with a mass spectrometer. Pollen samples are also taken to see what type of vegetation existed in the area.
Whitlock said understanding fire history is critical for understanding present trends.
To that end, 100 scientists in the Global Paleofire Working Group have taken core samples from 406 sites on six continents. The results of that research were published in September in the scientific journal Nature Geoscience.
University of Oregon researcher Jennifer Marlon was the lead author of an article that correlated the global core samples from the past 2,000 years. The study found fewer fires from 1 A.D. to 1750, in a cooler climate. From 1750 to 1870, when European-Americans expanded westward and cleared woodlands for farming, and the Industrial Revolution took hold, the number of fires climbed markedly around the globe. From 1870 to 1970, there was an abrupt decline, despite increasing temperatures from industrialization and a larger human population.
Marlon and her fellow researchers point to more intensive grazing, agriculture and fire management as causes for the decline.
"The impact of continued human settlement has been to decrease burning," Whitlock said.
Researchers have also been able to examine the impact of human activities on wildfires historically. Stories are told of American Indians regularly burning areas so that vegetation would regenerate. But finding evidence of human-caused fires is difficult. How could researchers separate a lightning-sparked fire from a human-caused blaze?
Such specific instances may not be measurable, but looking at long-range data, researchers could see the number of forest fires climb as European-Americans settled the continent.
Studies in New Zealand showed the most evident impact of humans and fires, since the South Pacific island was initially unoccupied. Lake bed core samples in the country revealed that when the Maori arrived from Polynesia in the late 1200s, the number of fires and deforestation initially increased and then leveled off. The number of fires rose again when Europeans arrived in the 1800s, changing the vegetative makeup of the island and leading to greater deforestation.
Fires in the ecosystem
Whitlock's MSU crew has taken measurements in New Zealand, Patagonia, Colorado and Yellowstone National Park. What has emerged is a record of fires over hundreds of years.
"Large fire years correspond with dry years over the last 800 years," Whitlock said. "That's not surprising, I guess."
According to the studies, big fire years ebbed and flowed with the climate, with large fire years cycling about every 180 to 300 years. The period from 1690 to 1730 saw big fires in Yellowstone; 1250 and 1550 also were big fire years. Going back further, a core sample taken from Yellowstone's Cygnet Lake looks back 11,000 years.
"For the last 11,000 years, Yellowstone has been a lodgepole pine forest with little change," she said. "Prior to that, it was a colder climate, probably tundralike."
Whitlock said studies have shown that there was a 16,000-year period of fires globally up until about 2,000 years ago, when the climate became cooler and wetter.
"What we're seeing now is nothing unique," she said of large wildfires in the West. "We've had a lot of fires over the last 16,000 years."
Whitlock said fire seasons in the northern Rocky Mountains track closely with certain atmospheric patterns. A well-developed upper-level ridge of high pressure that moves in from the south during the summer means little rain to parts of Montana, Wyoming and Idaho.
"Fires are really in our future, that's just a reality," Whitlock said. "The projections for the future are quite different than anything we've seen. Fire-climate linkages are strong on all time scales."
Contact Brett French at firstname.lastname@example.org or at 657-1387.