Deciduous trees offset carbon loss from boreal fires in Alaska – sciencedaily

More severe and frequent fires in Alaska’s boreal forest release vast stores of carbon and nitrogen from burnt trees and soil into the atmosphere, a trend that could accelerate global warming. But new research published this week in the journal Science shows that deciduous trees replacing burnt spruce forests more than compensate for this loss, storing more carbon and accumulating it four times faster over a 100-year fire interval. The study, led by a team of researchers from the Center for Ecosystem Science and Society at Northern Arizona University, suggests that these faster-growing, less flammable hardwood forests may act as a stabilizing “ firebreak ” against increasing fires and loss of nutrients in the Region.

The study began following the dramatic 2004 fire season in Alaska, when an area seven times larger than the long-term average burned down. Historically, more than half of this woodland has been dominated by black spruce, but after fires, faster-growing aspen and birch replaced some of these stands. The team, made up of researchers from the University of Northern Arizona, the University of Alaska Fairbanks, Auburn University and the University of Saskatchewan, investigated 75 black spruce stands that burned in 2004 and followed their recovery for the next 13 years. They also collected a range of data from trees and soils of different ages and burn severities to build a chronosequence, a kind of scientific time-lapse that allows researchers to quickly move forward through a fire cycle of 100. years to see how forests are recovering and changing.

“In 2005, I thought there was no way for these forests to recover the carbon they lost in that fire,” said Michelle Mack, professor of biology at Northern Arizona University and senior author of the ‘study. “The literature is replete with articles suggesting that deeper, more severe fires burn more carbon than can be replaced before the next fire. But not only did we see these deciduous trees making up for these losses, they did so quickly.

The team found that new aspen and birch poplars where black spruce burned accumulated carbon and nitrogen faster than spruce, storing most of it in their wood and leaves as opposed to the top layer. organic soil. And by the end of a projected 100-year cycle, hardwood stands had recovered as much nitrogen as it had lost in the fire and more carbon than it had lost, resulting in a increase in the ecosystem’s net carbon footprint. Calculating this balance is critical as scientists strive to understand how these northern forests are changing and the effects of those changes on the global carbon picture.

“I was surprised that deciduous trees could replenish lost carbon so effectively and efficiently,” said Heather Alexander, assistant professor of forest ecology at Auburn and one of the co-authors of the article. “Even though a considerable amount of carbon is burned and emitted into the atmosphere when black spruce forests are severely burned, the deciduous trees that often replace them have an astonishing ability to scavenge and store carbon in their leaves and wood above ground. “

“In a region with only five common tree species, this study shows how changes in tree composition can dramatically alter patterns of carbon storage in boreal forests,” said Jill Johnstone, Nordic researcher at the University of Alaska-Fairbanks and study co-author.

“Carbon is only one piece of the puzzle,” said Mack, who said hardwood forests have other important feedbacks, or interconnected effects, on climate. “We know these forests help cool the regional climate, and we know they are less flammable, so fires are less likely to spread. Taken together, these effects create a relatively strong set of stabilizing climate feedbacks in the boreal forest.

But there’s a lot researchers don’t know about the plight of boreal deciduous forests in a warmer world.

“As mature deciduous trees die, will they be replaced by trees with the same structure, composition and carbon storage capacities?” Alexander asked. “And will they recover from a fire with the same carbon storage capacities?”

“The shift from slow-growing black spruce to fast-growing deciduous species could balance the impacts of the intensifying fire regime in the boreal forest,” said Isla Myers-Smith, global change ecologist at the University of Edinburgh which did not participate in the study. “But it remains to be seen how the carbon gains will offset the losses in the future with accelerated warming at high latitudes.”

Mack said continued global warming could reverse the carbon sequestration gains these trees represent. “Carbon should reside in the landscape longer because deciduous forests are less flammable. But flammability is not a constant. The climate will pass a threshold where things get so hot and dry, even deciduous forests will burn. So a question we need is to ask how strong will be the mitigating effect of low flammability and how long will it last? “

The carbon in permafrost also complicates the picture. Although many sites in this study do not have permafrost near the ground surface, permanently frozen soil is found in the boreal biome. As it thaws, permafrost releases carbon and methane stores, potentially offsetting the storage gains of deciduous trees, Mack said. “We will eventually pass a temperature threshold where negative feedback is not enough.”

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