New-Growth Alaskan Forests May Store More Carbon After Wildfires

A new study of Alaskan forests growing back after fire finds they are regenerating quickly. The forests are actually on track to store more carbon than they did before, offering a rare note of optimism from the frontlines of climate change, reports Nathanael Johnson for Grist.

Alaska is home to vast, mountainous tracts of forest dominated by evergreen conifers such as spruce. Over decades, these oceans of trees pulled huge amounts of carbon dioxide from the atmosphere and locked the carbon from those molecules away in their trunks, roots and surrounding soils. But when fire ravages a forest, that carbon busts out and reenters the atmosphere in plumes of smoke.

As climate change has heated up the planet, forest fires in Alaska—like the ones that scorched an area the size of Massachusetts in 2004—have burned so-called boreal ecosystems more frequently and more severely than at any point in the last 10,000 years, write the authors of the new research in the Conversation.

In theory, warming causes more wildfires which then causes further warming through the release of carbon dioxide. This pattern suggests an alarming positive feedback loop that could further accelerate climate change.

When the authors of this new paper, published last week in the journal Science, studied 75 sites recovering from those devastating 2004 blazes they saw fast regrowth and a shift toward forests with more deciduous species, such as aspen and birch trees.

“This has two important effects when it comes to climate change and wildfires,” the authors write in the Conversation. “The deciduous trees store more carbon, and they don’t burn as quickly or as severely as dry, resinous black spruces and their needles do. The result is that these changing forests could mitigate the fire-climate feedback loop, and maybe even reverse it—at least for now.”

Prior to the 2004 fire season, black spruce was the dominant tree in more than half of the forest area that burned. By 2017, researchers found 72 percent of the 75 regenerating wooded areas in the study were dominated by deciduous trees or had a mix of deciduous and conifer trees, reports Ibrahim Sawal for New Scientist.

Michelle Mack, an ecosystem ecologist at Northern Arizona University and the study’s lead author, tells New Scientist that the fires burned especially deeply at these sites, which ended up exposing more nutrient-rich soils that are favored by faster growing deciduous trees.

Mack and her team also measured soil carbon and tree growth at the 75 sites to estimate how much carbon they accumulated. These measurements revealed aspen-dominated forests accumulated carbon four times faster than the old spruce forests had, per Grist. This pace suggests that by the end of a 100-year span, these deciduous forests might store 160 percent more carbon than the coniferous woods they replaced.

“In 2005, I thought that there was no way these forests could recover the carbon they lost in this fire,” says Mack in a statement. “The literature is full of papers suggesting deeper, more severe fires burn more carbon than can be replaced before the next fire. But not only did we see these deciduous trees make up for those losses, they did so rapidly.”

Beyond the potential for increased carbon storage, the other climate benefit of the deciduous forests is that their higher moisture content makes them more resistant to burning in the first place. Additionally, in a forest dominated by aspens, most of the carbon gets stored in the trees themselves, as opposed Alaska’s conifer forests, which store much of their carbon in the moss-matted ground.

Mack tells Grist she sees these findings as “one tiny positive in a great field of negatives.”

However, also speaking with Grist, University of New Mexico forest ecologist Matthew Hurteau notes this boost in carbon storage after forest fires may be specific to Alaskan forests.

It also remains unclear whether the new forest types will maintain their carbon-storing prowess in the long term.

“As mature deciduous trees die, will they be replaced with trees with the same structure, composition, and carbon storage abilities?” asks Heather Alexander, a forest ecologist at the University of Auburn and one of the paper’s co-authors, in the statement. “And will they recover from fire with the same carbon storage capacities?”

The deciduous forests’ resistance to fire may also run out as climate change continues to crank up the heat on Earth.

“The carbon should reside longer on the landscape because deciduous forests are less flammable. But flammability isn’t a constant,” says Mack in the statement. “The climate will pass a threshold where things get so hot and dry, even deciduous forests will burn. So, one question we need to ask is, how strong will the mitigating effect of low flammability be, and how long will it last?”

Alex Fox | | READ MORE

Alex Fox is a freelance science journalist based in California. He has written for the New York Times, National Geographic, Science, Nature and other outlets. You can find him at Alexfoxscience.com.