How old is mature? New definitions could inform federal forest policy
Not all forests are equal in carbon sequestered, mature forests are pulling more weight
Howland Forest, Maine
photo by Miles Grant
A new study, published in the peer-reviewed journal Forests and Global Change, presents the nation’s first assessment of carbon stored in larger trees and mature forests on 11 national forests from the West Coast states to the Appalachian Mountains. This study is a companion to prior work to define, inventory and assess the nation’s older forests published in a special feature on “natural forests for a safe climate” in the same journal. Both studies are in response to President Biden’s Executive Order to inventory mature and old-growth forests for conservation purposes and the global concern about the unprecedented decline of older trees.
Scientists have long demonstrated the importance of larger trees and older forests, but when a tree is considered large or a forest mature has not been clearly defined and is relative to many factors. This study develops an approach to resolve this issue by connecting forest stand age and tree size using information in existing databases. This paper also defines maturity by reference to age of peak carbon capture for forest types in different ecosystems. But the approach is readily applicable across forest types and can be used with other definitions of stand maturity.
Map by Christina Shintani
Key findings include:
- The minimum age at which forests may be considered mature, according to peak carbon capture, ranged from 35 to 75 years among the regions and forest types studied.
- The carbon stored in all trees of the 11 forests totaled 561 million metric tons, of which 73% is in larger trees and 60% is in unprotected larger trees in mature stands.
- For 8 of the 11 forests that had carbon accumulation data, the annual rate was 4.7 million metric tons per year, of which about half is in unprotected larger trees in mature stands.
- Forest stands with the greatest carbon stored and highest annual carbon accumulation were mainly in the Pacific Northwest, and all but one of the national forests (Black Hills in South Dakota due to drought and insect related tree mortality) experienced an increase in above-ground carbon over recent years.
Researchers used thousands of forest plots obtained from the U.S. Forest Service “Forest Inventory and Analysis” (FIA) dataset to determine the amount of carbon absorbed from the atmosphere that accumulates and is stored in individual trees as they mature. As trees age, they absorb and store more carbon than smaller trees, making them uniquely important as nature-based climate solutions. Additionally, as the entire forest matures, it collectively accumulates massive amounts of carbon over centuries in vegetation and soils. The study identified the forest age at which carbon accumulation is greatest, and used that as the threshold for defining a “mature” forest. Scientists also determined the median diameter of trees at this threshold age and how much of the forest carbon of the larger trees in mature forests is unprotected from logging. The amount of carbon in unprotected larger trees in mature stands of the 11 forests studied, representing only 6% of federal forest land, is equivalent to one-quarter of annual emissions of carbon dioxide from fossil fuels in the U.S. This is consistent with prior work.
According to lead researcher, Dr. Richard Birdsey of Woodwell Climate Research Center, “our study determined when an individual tree in a forest can be considered mature and when the forest itself is at an optimal rate of carbon capture and storage for conservation purposes. It is directly responsive to the president’s executive order.”
The Biden administration has set bold emissions reduction targets of 50-52% of 2005 levels and recently announced a “roadmap for nature-based solutions” as part of this effort. However, the roadmap neglects to connect the importance of protecting older forests to the climate targets. Federal agencies are proceeding with an inventory of mature and old-growth forests in response to the executive order, but policies regarding their management have not yet been established. By protecting older forests and trees on federal lands from avoidable logging, the Biden administration can help close the gap on its emissions reduction goals. The methodology in this paper provides a readily implementable path for critical policy solutions.
According to Dr. Dominick DellaSala, Chief Scientist at Wild Heritage, “there seems to be a big disconnect between what the White House is wanting and how federal agencies are responding to the president’s forest and climate directives. While the Forest Service recently withdrew a controversial timber sale in older forests on the Willamette National Forest in Oregon (“Flat Country Project”) because it was inconsistent with the president’s directives, dozens of timber sales in older forests remain on the chopping block.”
Dr. Carolyn Ramírez, Staff Scientist with the Forests Project at the Natural Resources Defense Council, pointed to the findings as supporting the push by over 100 conservation groups–the Climate Forests Campaign–for a national rulemaking to protect mature forests and big trees from logging for their superior climate and biodiversity benefits: “This work reinforces how essential mature forests on federal lands are to securing our climate future. It’s now up to the agencies to protect these carbon storing champions from the chainsaw with formal safeguards. 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.”
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