• The magenta-flowered fireweed, which springs up after a burn, dominates a landscape once covered in black spruce in Alaska's Yukon Flats. (Photo: Feng Sheng Hu)

Wildfire frequency in the northern boreal forest is entering untrodden territory as the climate warms, forest composition changes and more intense fires happen more often. According to a study published Monday in the Proceedings of the National Academy of Sciences, fire frequency is surpassing a high only once seen in the current geological epoch, the Holocene.

Using data collected from lakebeds in Alaska’s Yukon Flats, which border on the Yukon, researchers from the University of Illinois were able to look back ten thousand years at past fire regimes. What they found was an ecosystem that adapts to fire frequency with changes in tree types, but which has not yet seen temperatures like those predicted for the coming century.

“We’re already beyond the limit suggested by the (ancient) data, and that’s where we get into uncharted data,” says the study’s lead author, Phd student Ryan Kelly.

Kelly studied the pollen and charcoal deposited in layers of sediment at the bottom of lakes — a common method of looking back in time — and compared those records to modern fire records that start in about 1950.

In the past 60 years, each of Kelly’s study areas in the Yukon Flats have burned once, suggesting a frequency of 20 burns per 1,000 years. Looking back in time, he found that the fire frequency has only once been close to what it is today, during a warm period 500 to 1,000 years ago known as the Medieval Climate Anomaly.

During that period, the forest adapted to the increased frequency of fires. Usually dominated by spruce trees, which burn very easily, less flammable deciduous trees, such as birch and aspen, began to take over. That same change is happening again today, and the researchers believe that this might reduce the severity of the fires.

“The deciduous trees seemed to have put a damper on the fire cover,” Kelly says. However, with climate change predicted to be especially pronounced in higher latitudes, even the less flammable trees may not be enough to hold back the fires.

“Generally, the models predict that there will be more and more burning on the boreal landscape,” Kelly says. “Even though you’re going to get more deciduous trees, the climate is going to be conducive to burning. You may just start to see deciduous fires.”

The burns don’t just level the forests. They also release carbon dioxide from the soil and vegetation into the atmosphere, further contributing to climate change. A 2011 study estimated that the world’s boreal forests contain 22 per cent of the world’s terrestrial carbon storage, putting away about 3.5 times Canada’s total carbon emissions that year alone. In North America, the boreal forest extends from Alaska to Newfoundland.