One of the most flammable high-altitude regions of the world, a 2,000-square-kilometer zone in the Yukon Flats of interior Alaska has seen dramatic increases in both the frequency and severity of fires in recent decades. A research team led by the University of Illinois reports the area has seen higher wildfire activity more recently than at any other time in the past 10,000 years.
The findings were reported in Proceedings of the National Academy of Sciences
and add to existing evidence that relatively frequent and powerful fires are converting the conifer-rich boreal forests of Alaska into deciduous woodlands. Deciduous trees shed their leaves each fall and are more resistant to burning than the black spruce and white spruce that once dominated the Yukon Flats. The research team said it is still unknown if the shift to deciduous forests will overcome the fire-inducing effects of a warming climate.
“We reconstructed the fire history by picking charcoal fragments out of sediments preserved over thousands of years,” said University of Illinois doctoral student Ryan Kelly
. “And from what we can tell, the fire frequency at present is higher than it has been at any time in the past 10,000 years.”
The research team, including Illinois plant biology professor Feng Sheng Hu
and scientists from the University of Idaho, the University of Minnesota and the University of Washington, looked at the charcoal and pollen content of mud collected from the bottoms of 14 deep lakes in the Yukon Flats.
“We chose this area because today it is one of the most flammable boreal ecoregions of North America,” Hu said. “So we are focusing in an area that is highly flammable and we are focusing on periods of climate fluctuation during the Holocene. We’re trying to figure out what happened in the past to help us to project what may happen in the future.”
The Holocene epoch runs from the present day to around 11,700 years ago.
The researchers focused on a particularly warm period in the Holocene called the Medieval Climate Anomaly (MCA), which occurred around 1,000 to 5,000 years ago. During the MCA, global temperatures and tree species in the Yukon Flats were similar to those being observed today.
“This period probably wasn’t really as warm as today, definitely not as warm as it’s bound to get in the future, but may be the most similar to today,” Kelly said. “There was lots of burning, almost as much as today, and the fires were particularly severe.”
The scientists discovered the tree species composition in the Yukon Flats underwent a gradual shift during the MCA – from forests dominated by coniferous trees to woodlands populated by relatively fire-resistant deciduous trees.
“That feedback from the vegetation prevented fire frequency from increasing much more than it already was,” Kelly said. “So there was a limit to fire frequency during the Medieval Climate Anomaly.”
Kelly said the same kind of change-over in tree species is occurring today. Much of the area examined for this study has burned in the last decade, with young deciduous trees now growing where black spruce once stood.
According to Kelly, the current wildfire activity has already surpassed the limit seen during the MCA. During the last 3,000 years, the average fire frequency in this region was nine or 10 fire events per thousand years. In the last 50 years, however, the number of wildfire events has doubled, to almost 20 per 1,000 years.
“That’s like a fire every 50 years, whereas in the past it was closer to a fire every hundred years,” Kelly said.
The study’s findings are important because boreal forests cover more than 10 percent of Earth’s land surface and contain a vast amount of carbon, primarily in the soil.
“There is more carbon in the boreal forests than in the atmosphere,” Kelly explained. “And one of the main ways that the carbon that’s accumulated over thousands of years gets out of the soil is through burning.”
The carbon released from these fires adds to the greenhouse gases in the atmosphere. This could potentially lead to additional climate warming.
“The Yukon Flats region appears to be undergoing a transition that is unprecedented in the Holocene epoch,” said Hu. “And the transition may be indicative of what will happen throughout much of the North American boreal forests in the decades to come.”
“Ryan’s study area is already covered by deciduous forest because so much spruce has burned recently – it’s already different than the vast majority of boreal forests,” Hu said. “The climate today appears to be warmer than in the past 10,000 years in that region, and we know that the climate is continuing to warm up.”
Hu said as the warming continues, it is plausible that even deciduous forests will become highly flammable.
“It’s wood,” he said, and “if it’s dry, it will burn.”