Research finds past burns can limit Alaska wildfire spread

Research finds past burns can limit Alaska wildfire spread

Previously burned areas have played a major role in limiting the spread of wildfires across Alaska's boreal forest, according to new research led by University of Alaska Fairbanks scientist Ben Gaglioti.

Gaglioti, a researcher with the Water and Environmental Research Center at the UAF Institute of Northern Engineering, and his collaborators examined 568 instances between 1984 and 2020 in which an Alaska wildfire encountered a previously burned area.

Their findings, recently accepted for publication in the Journal of Geophysical Research: Biogeosciences, show that those previous burns can act as a powerful natural control on subsequent wildfire spread. However, this limitation tends to weaken during hot, dry and windy conditions.

Using historical observations to model wildfire activity, the researchers estimate that burning rates could have been several times higher without the fuel limitations imposed by previous fires.

The reason lies in what happens after a wildfire.

When the boreal forests of Alaska burn, a fire often consumes much of the vegetation and other fuel material. This natural disturbance triggers a period of forest and fuel regeneration where recently burned areas are temporarily less flammable and, on average, more resistant to reburning.

Researchers call this process fuel self-regulation.

"An initial rise in fire occurrence during a shift to warmer, drier years can create a patchwork of younger, fire-averse forests that can temporarily slow down a further rise in burning," Gaglioti said.

To measure the strength of this fuel limitation effect, Gaglioti and his collaborators studied Alaska's wildfire record from 1984 through 2020.

They identified 568 locations where a newer wildfire encountered the mapped perimeter of a previous burn and analyzed whether the fire continued into the younger vegetation or was stopped or significantly limited at the boundary. Gaglioti calls the fuel-limited situations "puzzle pieces" because their boundaries tend to fit together on maps, but not overlap.

Whether these fire-spread interactions resulted in reburning for fuel limitation depended on the age of the previous burn, fire size and weather.

A fire had about a 25% probability of spreading into an area that had burned the previous year. That probability increased to approximately 63% after 30 years, as vegetation recovered and more fuel accumulated.

These probabilities significantly increased during more extreme fire weather conditions, especially during the first decade of fuel regeneration.

"More extreme fire weather can create conditions where fires are up to two times more likely to spread into younger fuels that would otherwise be resistant to burning in normal weather conditions," Gaglioti said.

But even when fires entered previously burned areas, reburning rates in areas less than 35 years old remained dramatically lower than burning rates in older fuels. Although reburning is increasing in the fire record, the researchers estimate that the effect of a previous fire still persists for decades. Reburning rates approach those of older forests more than 40 years after a fire.

The findings could be particularly relevant to Alaska officials, land managers and communities confronting the effects of increasing wildfire activity.

Managers can use their results as a look-up table to assess whether an active fire will continue to spread beyond the footprint of a recently burned area, Gaglioti said.

"Our results can help them use the age of the burn scar, the size of the fire and the fire weather conditions to make an informed prediction on whether an active fire will become fuel limited and stop," Gaglioti said.

Climate warming is expected to increase wildfire activity in northern forests. But wildfire also changes the conditions encountered by the next fire.

More burning creates more young forests. Those young forests contain less available fuel and can make it harder for subsequent fires to spread.

Gaglioti and his collaborators found that this negative feedback has historically exerted a powerful stabilizing influence on Alaska's boreal wildfire regime, and, although this limitation will weaken in a warmer future, it will continue to limit the area burned annually in Alaska.

When they modeled historical fire activity without the fuel limitations created by previous burns, the estimated rate of burning increased by approximately 4.9 times.

Extreme fire weather can weaken the effect. Under severe conditions, fires were more likely to cross into younger burn scars. But the researchers found that fuel limitations remained significant even during hazardous fire conditions.

The researchers do not suggest that previous fires will completely offset the effects of a warming climate. Instead, their work identifies another major factor that should be considered when scientists and land managers assess how Alaska's wildfire regime will change.

Co-authors of the study are Colin Quinn of the University of Maryland and NASA Goddard Space Flight Center; Sam Mark of UAF and the University at Buffalo; Winslow Hansen and Jazlynn Hall of the Cary Institute of Ecosystem Studies; and Dan Mann of the UAF Institute of Arctic Biology.

ADDITIONAL CONTACT: Ben Gaglioti, [email protected]

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