Deciduous trees compensate for carbon loss from Alaskan boreal fires, new research reveals

Sharper and more frequent fires in the Ares boreal forest are releasing huge reserves of carbon and nitrogen from burned trees and soil into the atmosphere, a trend that could accelerate global warming. But a new study published this week in the journal Science shows that deciduous trees that replace burned spruce forests more than make up for that loss, storing more carbon and accumulating it four times faster over a period of 100 years of fire. The study, led by a team of researchers from the Center for Science and Society on Ecosystems at the University of Northern Arizona, suggests that these faster-growing, less flammable deciduous forests may act as a stabilizing ‘fire breakthrough’ against escalating fire patterns and nutrient loss. region.

The study began after a dramatic fire season in Alaska in 2004 when the area burned seven times more than the long-term average. Historically, more than half of this wooded terrain was dominated by black spruce, but after the fire, the 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, the University of Auburn and the University of Saskatchewan, investigated 75 black spruce stands that burned in 2004 and tracked their recovery over the next 13 years. They also collected data from trees and soils of varying ages and burns to construct a chronosequence, a kind of scientific interval that allows researchers to quickly rewind through a hundred-year fire cycle to see how forests recover and change.

“In 2005, I thought there was no way these forests could recover the carbon they lost in this fire,” said Michelle Mack, a professor of biology at the University of Northern Arizona and lead author of the study. “The literature abounds with papers suggesting that deeper and more serious fires burn more carbon than can be replaced before the next fire. But not only have we seen these hardwoods make up for those losses, they’ve worked so fast.”

The team found that new aspen and birch trees in which black spruce burned burned carbon and nitrogen faster than spruce, storing most of it in their wood and leaves as opposed to the organic layer of soil. And at the end of the predicted 100-year cycle, the deciduous stands restored as much nitrogen as was lost to the fire and more carbon than was lost, resulting in an increase in the net carbon balance in the ecosystem. Calculating this balance is crucial as scientists work to understand how these northern forests are changing and the effects of these changes on the global carbon picture.

“I was surprised that the leaves could so effectively and efficiently compensate for the lost coal,” said Heather Alexander, an assistant professor of forest ecology at Auburn and one of the paper’s co-authors. “Although significant carbon is burned and emitted into the atmosphere when black spruce forests are much worse, deciduous trees that often replace them have an incredible ability to return and store carbon in their above-ground leaves and wood.”

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

“Carbon is just one piece of the puzzle,” said Mack, who said deciduous forests have other important feedback or interrelated effects on climate. “We know that these forests help the cold regional climate and we know they are less flammable, so fires are less likely to spread. Together, these effects create a relatively strong set of stabilizing climate feedback in the boreal forest.”

But there is much that researchers do not know about the fate of deciduous boreal forests in the warm world.

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

“Switching from slow-growing black spruce to fast-growing deciduous species could balance the effects of strengthening the fire regime in the boreal forest,” said Isla Myers-Smith, a global change ecologist at the University of Edinburgh who was not involved in the study. “But it remains to be seen how carbon gains balance future losses by accelerating warming at high latitudes.”

Mack said that continuous warming of the climate could cancel out the carbon sequestration gains that make up these trees. “Carbon should stay in the landscape longer because deciduous forests are less flammable. But flammability is not constant. The climate will cross the threshold when things get so hot and dry, even deciduous forests will burn. So one question we need to ask is how much will be strong to alleviate the effect of low flammability and how long will it last?

Coal in permanent ice also complicates the picture. Although many sites in this study did not have permafrost near the soil surface, permanently frozen soil is located over the boreal biome. As it thaws, permafrost releases carbon and methane supplies, potentially offsetting gains from hardwood storage, Mack said. “Eventually we will cross the temperature threshold where negative feedback is not enough.”


This research was supported by the U.S. Department of Defense Strategic Environmental Research and Development Program, NASA’s Arctic Boreal Vulnerability Experiment, the U.S. Joint Fire Science Program, and the Bonanza Creek Long-Term Environmental Research Program, the U.S. National Science Foundation program, and the USDA Forest Service.

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