How much U.S. forest is old growth? It depends who you ask | Science

Last spring, President Joe Biden surprised forest scientists when he ordered the U.S. Forest Service (USFS) and Bureau of Land Management (BLM) to inventory their holdings of mature and old-growth forests by Earth Day 2023. The order triggered a scramble for the United States to, for the first time, formally define what constitutes “mature” and “old-growth” forests and to apply those definitions across millions of hectares of land.

Now, the agencies have delivered their findings: Of the nearly 72 million hectares of forest managed by the two agencies, 45% are mature and 18% are old growth, according to a report released last week. The figures far exceed previous estimates published by nonfederal researchers and are likely to add fuel to an already raging debate about how to manage older forests and make them resilient to climate change.

The government’s tally has drawn a divided response. “It’s a very significant piece of work,” says Cristina Eisenberg, an ecologist and traditional ecological knowledge expert at Oregon State University who was involved in early conversations about the effort and reviewed the report. She applauds the research team for eschewing conventional definitions that limited old growth to densely packed stands with towering trees and a closed canopy. That is a “settler-colonialist construct,” Eisenberg says, reflecting a forest type that came into existence only after Indigenous people were removed from the land.

But Greg Aplet, a senior forest scientist with the Wilderness Society, says “the methods [the agencies] used are not transparent.” In January, Aplet co-authored a paper finding that only 37% of USFS and BLM forests are mature or old growth. The new report classified as mature many forests that most researchers would consider young, Aplet says, a result that “strains credulity.”

Over the past year, USFS and BLM officials asked tribal groups, scientists, and others to weigh in on how best to define old-growth and mature forests. The agencies then assembled a research team tasked with capturing the complexity of the nation’s forests in a few defensible numbers; some team members worked nights and weekends and skipped vacations to meet the April deadline. One of the team’s top challenges was coming up with universal definitions that could span the vast diversity of forested ecosystems, from lush rainforests of the Pacific Northwest to dry forests of the Southwest, where even old trees remain small and grow sparsely. “They were given a nearly impossible task,” Eisenberg says.

The team first turned to past assessments of specific forest types, which defined old growth based on metrics such as the age of a stand, the size and arrangement of trees, the number of canopy layers, and the amount of dead wood on the forest floor. The researchers then applied those metrics to identify old growth in more than 200 forest types across the country, using data from USFS’s long-standing inventory of forest characteristics, drawn from hundreds of thousands of plots across the country.

Federal scientists had never previously attempted to define “mature forests.” The team settled on a definition that included forests exceeding a set threshold of the structural characteristics that are associated with old-growth forests. The goal was to identify “a stage of development prior to old growth,” says Christopher Woodall, a research forester with USFS and one of the report’s lead authors. Under their definitions, mature and old-growth forests collectively cover some 46 million hectares of USFS and BLM land, an area larger than California, the team found.

Woodall cautions that the “mature” definition is a work in progress and is likely to be further refined as scientists continue to study and debate the definitions. “There is no magical equation to tell you when a forest moves from mature to old growth,” says Woodall, whose team is planning to submit a paper detailing their methods to a peer-reviewed journal.

Several other research groups have recently published their own methods for defining old-growth and mature forests. Aplet and his colleagues used equations to estimate how much carbon is contained in a forest based on its age, type, and site quality. They defined old-growth forests as those that had reached 95% of their potential total carbon accumulation and mature ones as those that had reached the age of peak average carbon accumulation. A different study published in September 2022, which used satellite data to estimate canopy height, canopy cover, and above-ground living biomass, found that mature and old-growth forests combined occupied 23 million hectares, or 30%, of USFS and BLM forestland.

The federal definitions produced some unexpected findings. For example, the analysis found that pinyon-juniper woodlands, which grow mostly in the western U.S., contain more old-growth and mature forest than any other forest type, with 9 million hectares across BLM and USFS lands. Such forests, which don’t reach the towering heights or evoke the cathedral-like  feeling of coastal redwoods or northwestern cedar-hemlock rainforests, have rarely been categorized as old growth. The report also found old and mature stands in other open forest types, such as oak woodlands of the Southwest and longleaf pine savannas of the southeast.

The report is likely to intensify debate over how the federal government should manage older forests. Some scientists and activists argue such forests should be strictly protected because they store more carbon than younger forests and are often biodiversity hot spots. “I would love to see a moratorium on cutting of mature and old-growth forests on federal land,” says Joan Maloof, an emeritus professor at Salisbury University and founder of the Old-Growth Forest Network.

Others counter that so few national forests have been logged in recent decades that species such as ruffed grouse and bobcat, which need open space and young, shrubby forest, are losing habitat. An initiative called the Young Forest Project, which is supported by some conservationists as well as hunting and forestry groups, argues for more logging in older forests.

With its inventory complete, the government is now turning toward an even more daunting challenge: devising ways to help its mature and old-growth forests adapt to climate change. Eisenberg, who is co-leading a report on that topic, says her team plans to bring both Western science and traditional ecological knowledge to bear. Indigenous groups have long used practices such as fire and careful selective harvesting, for example, to keep forests open, which makes them less susceptible to fire and drought. A revival of traditional practices could help stressed forests survive, Eisenberg says. As it is, “Our forests are in trouble.”

Last spring, President Joe Biden surprised forest scientists when he ordered the U.S. Forest Service (USFS) and Bureau of Land Management (BLM) to inventory their holdings of mature and old-growth forests by Earth Day 2023. The order triggered a scramble for the United States to, for the first time, formally define what constitutes “mature” and “old-growth” forests and to apply those definitions across millions of hectares of land.

Now, the agencies have delivered their findings: Of the nearly 72 million hectares of forest managed by the two agencies, 45% are mature and 18% are old growth, according to a report released last week. The figures far exceed previous estimates published by nonfederal researchers and are likely to add fuel to an already raging debate about how to manage older forests and make them resilient to climate change.

The government’s tally has drawn a divided response. “It’s a very significant piece of work,” says Cristina Eisenberg, an ecologist and traditional ecological knowledge expert at Oregon State University who was involved in early conversations about the effort and reviewed the report. She applauds the research team for eschewing conventional definitions that limited old growth to densely packed stands with towering trees and a closed canopy. That is a “settler-colonialist construct,” Eisenberg says, reflecting a forest type that came into existence only after Indigenous people were removed from the land.

But Greg Aplet, a senior forest scientist with the Wilderness Society, says “the methods [the agencies] used are not transparent.” In January, Aplet co-authored a paper finding that only 37% of USFS and BLM forests are mature or old growth. The new report classified as mature many forests that most researchers would consider young, Aplet says, a result that “strains credulity.”

Over the past year, USFS and BLM officials asked tribal groups, scientists, and others to weigh in on how best to define old-growth and mature forests. The agencies then assembled a research team tasked with capturing the complexity of the nation’s forests in a few defensible numbers; some team members worked nights and weekends and skipped vacations to meet the April deadline. One of the team’s top challenges was coming up with universal definitions that could span the vast diversity of forested ecosystems, from lush rainforests of the Pacific Northwest to dry forests of the Southwest, where even old trees remain small and grow sparsely. “They were given a nearly impossible task,” Eisenberg says.

The team first turned to past assessments of specific forest types, which defined old growth based on metrics such as the age of a stand, the size and arrangement of trees, the number of canopy layers, and the amount of dead wood on the forest floor. The researchers then applied those metrics to identify old growth in more than 200 forest types across the country, using data from USFS’s long-standing inventory of forest characteristics, drawn from hundreds of thousands of plots across the country.

Federal scientists had never previously attempted to define “mature forests.” The team settled on a definition that included forests exceeding a set threshold of the structural characteristics that are associated with old-growth forests. The goal was to identify “a stage of development prior to old growth,” says Christopher Woodall, a research forester with USFS and one of the report’s lead authors. Under their definitions, mature and old-growth forests collectively cover some 46 million hectares of USFS and BLM land, an area larger than California, the team found.

Woodall cautions that the “mature” definition is a work in progress and is likely to be further refined as scientists continue to study and debate the definitions. “There is no magical equation to tell you when a forest moves from mature to old growth,” says Woodall, whose team is planning to submit a paper detailing their methods to a peer-reviewed journal.

Several other research groups have recently published their own methods for defining old-growth and mature forests. Aplet and his colleagues used equations to estimate how much carbon is contained in a forest based on its age, type, and site quality. They defined old-growth forests as those that had reached 95% of their potential total carbon accumulation and mature ones as those that had reached the age of peak average carbon accumulation. A different study published in September 2022, which used satellite data to estimate canopy height, canopy cover, and above-ground living biomass, found that mature and old-growth forests combined occupied 23 million hectares, or 30%, of USFS and BLM forestland.

The federal definitions produced some unexpected findings. For example, the analysis found that pinyon-juniper woodlands, which grow mostly in the western U.S., contain more old-growth and mature forest than any other forest type, with 9 million hectares across BLM and USFS lands. Such forests, which don’t reach the towering heights or evoke the cathedral-like  feeling of coastal redwoods or northwestern cedar-hemlock rainforests, have rarely been categorized as old growth. The report also found old and mature stands in other open forest types, such as oak woodlands of the Southwest and longleaf pine savannas of the southeast.

The report is likely to intensify debate over how the federal government should manage older forests. Some scientists and activists argue such forests should be strictly protected because they store more carbon than younger forests and are often biodiversity hot spots. “I would love to see a moratorium on cutting of mature and old-growth forests on federal land,” says Joan Maloof, an emeritus professor at Salisbury University and founder of the Old-Growth Forest Network.

Others counter that so few national forests have been logged in recent decades that species such as ruffed grouse and bobcat, which need open space and young, shrubby forest, are losing habitat. An initiative called the Young Forest Project, which is supported by some conservationists as well as hunting and forestry groups, argues for more logging in older forests.

With its inventory complete, the government is now turning toward an even more daunting challenge: devising ways to help its mature and old-growth forests adapt to climate change. Eisenberg, who is co-leading a report on that topic, says her team plans to bring both Western science and traditional ecological knowledge to bear. Indigenous groups have long used practices such as fire and careful selective harvesting, for example, to keep forests open, which makes them less susceptible to fire and drought. A revival of traditional practices could help stressed forests survive, Eisenberg says. As it is, “Our forests are in trouble.”