Site icon Conservation news

U.S. mature forests are critical carbon repositories, but at risk: Study

A protected old-growth forest in Sequoia National Park. Image by Joan Maloof/Old-Growth Forest Network.

A protected old-growth forest in Sequoia National Park. Image by Joan Maloof/Old-Growth Forest Network.

  • A new study quantifies the amount of carbon in a sampling of publicly held U.S. forests, demonstrating the importance of mature and old-growth stands.
  • As much as two-thirds of the carbon held in the large trees in these forests is at risk because the trees lack legal protection from logging.
  • In addition to the carbon benefits provided by the country’s mature and old-growth forests, which the authors say could help the U.S. meet its emissions reductions targets, the older trees found in them support vibrant ecosystems, regulate water cycles, and are resistant to fires.

Large trees in older forests that hold significant amounts of carbon located within U.S. national forests are vulnerable to logging, according to a new study published Jan. 6 in the journal Frontiers in Forests and Global Change.

Forest protection efforts often single out old-growth forests because of the carbon they keep out of the atmosphere, along with the complex ecosystems they anchor — and rightly so, Dominick DellaSala, a study co-author, told Mongabay.

“You’re walking through these magnificent forests with these giant trees that are hundreds, if not thousands, of years old,” said DellaSala, chief scientist at the Earth Island Institute’s Wild Heritage Project. Protecting them is “a no-brainer.”

Mature hardwood forest in Florida’s Upper Pithlachascotee River Preserve. Image by Dominick DellaSala.

But long-lived forests, even if they’ve been logged too recently to meet the strict definition of old growth, provide many of the same benefits.

“It’s not just primary or old growth, but it’s that stage just before old growth that matters,” DellaSala said. “We need to get back to our old-growth forests that were degraded and lost over centuries of logging.”

For the study, DellaSala and his colleagues used publicly available data from the U.S. Forest Service to calculate the carbon contents of mature and old-growth forests in a sample of 11 national forests across the different geographic regions of the contiguous United States. They found that the trees in those forests hold nearly 561 million metric tons of carbon, roughly equivalent to the CO2 emissions of 551 coal-fired power plants, according to the U.S. Environmental Protection Agency.

But their analysis also revealed that one- to two-thirds of that carbon stock on U.S. Forest Service land is at risk because it is held by large trees in mature forests that aren’t protected and potentially at risk of being logged.

“We hear all the time, at least from federal agencies, that they’re not logging large trees,” DellaSala said, “but no one’s been able to define what ‘large’ is.”

Threatened mature forest in the Yaak Valley of Montana. Image by Dominick DellaSala.
Threatened mature forest in the Yaak Valley of Montana. Image by Dominick DellaSala.

First off, it can depend on where those large trees are located: An older tree that’s relatively large compared to others in the forest in one part of the country might be considerably smaller than trees of the same age in another part of the country. But no matter where they’re located, as these trees age, they often become increasingly vital members of a functioning forest ecosystem: They’re fire-resistant, hold oodles of carbon both in their trunks and beneath the soil, support biodiversity, and help maintain a functioning water cycle.

Similarly, as the entire stand matures, it becomes more complex and a more robust repository of carbon and species.

“The problem becomes … how do you know it’s mature?” DellaSala said.

And that’s a problem the team sought to solve: In addition to quantifying the carbon held in these trees, the researchers also worked on a new way to define what a large tree is based not just on its diameter but on its age in relation to the maturity of the stand and the point at which it’s pulling the most carbon from the atmosphere.

Again, the age range of these trees varies depending on the type of forest, but it’s generally between 35 and 75 years, the authors found. To make things simple, they suggest protecting all trees that are 80 years or older.

Forest in Olympic National Forest, Washington. Image by Sarah Adloo/Old-Growth Forest Network.

“Our approach shows that logging protections grounded in a straightforward, age-based cutoff — such as 80 years, as many are calling for — would protect significant amounts of carbon, accommodate forest growth differences, and be readily usable in the field,” Carolyn Ramírez, a staff scientist with the Natural Resources Defense Council’s Forests Project and a co-author of the paper, said in a statement.

Tara Hudiburg, an associate professor of forest, rangeland and fire sciences at the University of Idaho, who was not involved in the study, said integrating more than just a tree’s diameter in the decision of whether to cut it down was “a good idea.”

And she noted the importance of these stands to the climate.

“Do we need mature and old-growth forest to stay intact in the biosphere so that we keep that CO2 out of the atmosphere?” she asked. Her answer was unequivocal: “Yes.”

The study by DellaSala and his colleagues is the third in a four-part series that aims to provide a comprehensive inventory of old-growth and mature forests in the United States. In part, the research is a response to President Joe Biden’s Earth Day Executive Order for such an assessment. But DellaSala also said the work has been on his “professional bucket list for a couple of decades.”

“Why don’t we have a basic inventory and mapping of mature and old-growth forest in the United States?” he asked. “We don’t even have a basic map. I couldn’t even tell you how much old growth, for example, that we have nationwide.”

A path through Mt. Baker-Snoqualmie National Forest, Washington. Image by Sarah Adloo/Old-Growth Forest Network.

The research series began with an assessment of the carbon stocks in Alaska’s Tongass National Forest, revealing that 89% of the Tongass’s productive forests are old-growth. The Tongass, about the size of the U.S. state of West Virginia or almost the size of Ireland, is the largest in the National Forest system. On Jan. 25, the Biden administration announced it was reinstating restrictions on roadbuilding in much of the Tongass in an effort to protect wildlife, watersheds and the climate benefits the forest provides.

The second part of the series, published in September, mapped out the country’s mature forests, revealing that less than a quarter are fully protected. The fourth study in the series will overlay the map-based results of the second study with the findings from this latest analysis, DellaSala said. Combined, they make a critical finding clear, he added: “As the forest gets older, most of the carbon capture and storage is in the larger trees.”

The findings also demonstrate the role these long-standing forests can play in meeting the Biden administration’s climate mitigation goals — namely, to cut carbon emissions by at least 50% of 2005 levels by 2030, DellaSala said.

“A lot of us are saying now that we’ve got less than a decade to really turn this thing around,” he said. “Here, we have a solution to the problem.”

DellaSala noted that ending fossil fuel usage is key, but that protecting mature forests will also make a difference.

“These older forests are our best nature-based climate solution,” he said. “They buy us time, they give us hope, they give us a chance to turn the corner.”

Banner image: A protected old-growth forest in Sequoia National Park. Image by Joan Maloof/Old-Growth Forest Network.

John Cannon is a staff features writer with Mongabay. Find him on Twitter: @johnccannon

Correction: A previous version of this article misstated the proportion of the Tongass that is old-growth; 89% of its productive forests are old-growth. The article also misstated the origin of the banner image; it is from Sequoia National Park.


Birdsey, R. A., DellaSala, D. A., Walker, W. S., Gorelik, S. R., Rose, G., & Ramírez, C. E. (2023). Assessing carbon stocks and accumulation potential of mature forests and larger trees in U.S. federal lands. Frontiers in Forests and Global Change5. doi:10.3389/ffgc.2022.1074508

DellaSala, D. A., Gorelik, S. R., & Walker, W. S. (2022). The Tongass National Forest, Southeast Alaska, USA: A natural climate solution of global significance. Land11(5), 717. doi:10.3390/land11050717

DellaSala, D. A., Mackey B., Norman P., Campbell C., Comer P. J., Kormos C. F., … & Rogers, B. (2022). Mature and old-growth forests contribute to large-scale conservation targets in the conterminous United States. Frontiers in Forests and Global Change5. doi:10.3389/ffgc.2022.979528

FEEDBACK: Use this form to send a message to the author of this post. If you want to post a public comment, you can do that at the bottom of the page.

Exit mobile version