The report is the federal government’s first estimate of America’s oldest trees. Environmentalists hope it is a step for protecting them from logging.

In a first-ever finding that could increase protections for remaining U.S. forests, the federal government estimated Thursday that more than 100 million acres of old-growth and mature timberlands are still standing on public lands, despite decades of commercial logging, wildfires and climate threats.

The findings, the result of a year-long review ordered last year by President Biden, are likely to inflame tensions with the timber industry over which forests — especially those in the western United States — should remain unlogged. But they are energizing many conservation activists, including those who argue that old-growth forests are vital for storing carbon dioxide that contributes to climate change.

“It’s extremely encouraging that the Biden administration is recognizing the value of mature and old-growth trees,” said Blaine Miller-McFeeley, senior legislative representative at Earthjustice. He said the environmental law group supports rules “that will protect and restore climate forests for future generations from the threats they face today, including unnecessary logging.”

The report by the U.S. Forest Service and the Bureau of Land Management is the result of an order Biden issued last year to protect older forests from wildfire, climate change and other threats. While the order itself was controversial — environmentalists and the timber industry disagree over what counts as an “old” tree — the findings are likely to fuel debate over which forests deserve more protection.

The report found that more than 32 million acres of old-growth forests remain on public lands in the United States, representing about 18 percent of all forested land managed by the two agencies. The ages and sizes of these trees vary by species and region, but most are well over 100 years old. Scientists and environmentalists view these trees as vitally important to fighting climate change because they store vast amounts of carbon in their trunks, branches and roots. The study also concluded that there are around 80 million acres of mature forest — about 45 percent of the agencies’ forested land.

The agencies’ work suggests a much higher estimate of old-growth and mature forest than previous scientific studies have shown, a departure the report’s authors attributed to their inclusion of parts of Alaska, which other studies excluded, as well as “differing goals and methodologies.” Other research cited in the federal government’s report estimated the country’s remaining old forests to be between 53 million and 59 million acres.

Another difference that could contribute to the varying figures: the agencies’ decision to include 23 million acres of older trees in pinyon-juniper forests, which cover hot, arid land in Western states. Unlike California’s towering redwoods and the red cedars of Alaska’s Tongass National Forest, these trees built to survive in the high desert are often left out of discussions of iconic old growth.

Environmental groups praised the report’s release and said they hoped it would lead the Biden administration to enact new protections for the oldest and largest trees.

Steve Pedery, conservation director of the nonprofit Oregon Wild, said conservation groups have been pushing the two agencies to map and protect the country’s oldest forests since the 1970s. “Looking ahead, what is key now is how the Forest Service and the BLM use these maps and inventory,” he said, “and whether or not they will adopt strong permanent rules to protect these forests.”

Yet safeguarding older trees is likely to be hugely controversial.

There is no scientific consensus on how to define old-growth and mature trees — and logging companies are likely to push back against any new limits on their access to the most valuable timber.

The Federal Forest Resource Coalition, a timber industry group, released a statement Thursday describing the report as “lots of data that doesn’t help the Forest Service do what needs to be done: manage unreserved forestlands to reduce fire danger, create wildlife habitat, and provide jobs in economically distressed rural areas.”

“America has no shortage of public forestland, most of which is lightly managed, if it’s managed at all,” said Bill Imbergamo, the group’s executive director. “Today’s inventory shows we’re not running out of mature trees.”

The report also notes the growing danger that climate change-fueled wildfires pose to older forests. But the government’s plans to protect trees from fire often call for chain saws — an intervention known as thinning that is supposed to restore forests to a time when natural fire cycles regularly cleared away underbrush and small saplings.

Experts said any new protections would have to achieve a delicate balance between protecting large trees and allowing agency land managers to use techniques that keep ecosystems healthy. While many forestry experts support targeted thinning of overgrown forests, some conservationists have accused loggers of using the projects as cover to cut large, old trees.

Biden’s executive order calls for the agencies’ next step to be crafting policies that protect old-growth and mature trees from wildfire and other threats. The Interior Department published a proposed rule last month to “promote ecosystem resilience on public lands" that would elevate conservation to a formal use of public lands, possibly equal to mining, grazing and oil and gas projects. On Thursday, the Forest Service announced that it is asking for public comment on how the agency should manage forests before issuing any proposed regulations.

Earlier this year, the Biden administration banned logging in much of the Tongass, restoring protections that were rolled back under the Trump administration. The Forest Service also canceled a planned timber sale in Oregon’s Willamette National Forest that conservation groups said would have largely felled trees ranging in age from 80 to 150 years old.