The big-tree hunters chased rumors into southwestern Colorado’s San Juan Mountains. They’d heard about massive trees hiding in rugged terrain, and over many visits, they spotted and then hiked to treetops that rose above the rest of the forest. The work, done in 2014 and 2015 by a cohort of arbor enthusiasts, broke a string of records for Colorado’s tallest leaf bearers. They found a 169-foot-tall Douglas fir, a 165.3-foot ponderosa pine, and a world-record-setting blue spruce at a soaring 180.6 feet. The marvel of it, says Austin Rempel, Durango-based senior manager of reforestation for American Forests, a 150-year-old conservation organization focused on preserving forest ecosystems, is that “even in this day and age, we don’t know where all the old growth is.”

A recent executive order could change that. In April, President Joe Biden signed a document calling for new, standardized definitions of old-growth and mature forests and a nationwide inventory of those forest types by April 2023. The project aims to protect the last, relatively untouched stands of complex forests for the sake of the biodiversity they preserve and the carbon dioxide they pull out of the atmosphere.

In Colorado, where little old growth remains, the effort might make the most difference by protecting mature forests—younger forests that are beginning to show signs of becoming old growth—and reversing a centuries-long trend of old growth vanishing. But the protections won’t come easily because there’s not a consensus on which trees to include in either category.

“Creating a definition is very complex for old-growth systems,” says Sara Husby, executive director of the Great Old Broads for Wilderness, a nationwide grassroots organization that advocates for wilderness and is headquartered in Durango, “even in Colorado, because it will vary based on latitude, elevation, vegetation, disturbance, history, and precipitation that the different forests get here.”

Foresters, natural resource managers, and conservation groups—here in Colorado and across the country—have questioned how the same labels and criteria can apply across vastly different ecosystems and species. Even the public is weighing in, posting feedback (you can provide feedback here through August 30) for the U.S. Bureau of Land Management and the U.S. Department of Agriculture, which oversees the U.S. Forest Service. “We’re all kind of scratching our heads at what a national definition could look like,” says Marvin Brown, who leads the forest management committee for the National Association of State Foresters. (Colorado’s state forestry office declined to comment until the new definition is complete.) “One of my folks in Oregon said, ‘It’s not rocket science. It’s a lot more complicated.’”

Tall ponderosa pines in the Hermosa Creek Wilderness area in the San Juans. Photo by Robert T. Leverett/American Forests

The complications arise because trees aren’t so different from people: “Mature” and “old” can be entirely distinct traits. For example, a few centuries would be a long lifetime for a ponderosa pine, but that amount of time would barely qualify as being beyond the toddler years for a bristlecone pine, which can live for millennia. In fact, a Rocky Mountain bristlecone pine located in central Colorado has been dated at 2,435 years old, according to the OldList, a database of ancient trees kept by Rocky Mountain Tree-Ring Research, a Fort Collins–based nonprofit research organization. When questions arose about how to reconcile disparities like these during a public information session put on by the U.S. Department of Agriculture, Linda Heath, director of inventory, monitoring, and assessment research for the U.S. Forest Service, suggested the definition might speak to age, size, or types of species as concepts, without giving many specifics.

“Old growth” did, in fact, dawn as a concept in the 1940s. Then, it meant slower-growing older forests and those with the largest-diameter living trees. Since the 1980s, though, the U.S. Forest Service has used a broadly framed definition that speaks to “structural attributes,” including tree size; accumulation of dead, woody material; a layered canopy; species present; and ecosystem function. “The problem with [the current definition] is that it’s so generic that it doesn’t really allow you to discriminate readily between what would qualify as old growth or what would not,” says Gregory Aplet, a Denver-based senior science director for the Wilderness Society, a Washington, D.C., nonprofit that advocates for land conservation. And, generic or not, Aplet says that old growth has “never even been quantified, much less mapped, for Colorado.”

Rethinking mature forests, some say, could really be beneficial in the Centennial State. What counts as mature? That’s not entirely clear, either, but in general, mature forests don’t yet host the decadent tumble of trees and layered canopy that might be recognized as old growth—but they could someday. Some experts recommend drawing the line for mature simply based on tree age; others suggest using biomass, such as woody debris or downed trees, as a delineator. (Ask a timber industry professional, and “mature” is the age at which it makes sense to turn a tree into lumber.)

Conservation groups in Colorado see Biden’s executive order as an opportunity to extend protections—mostly from incidental losses to roads and through wildfire mitigation projects—to mature forests for the first time. For instance, if certain forests are defined as mature, fire mitigation crews could be asked to leave certain clusters of trees intact or to prioritize protecting the biggest trees while still reducing risks to nearby communities. Reversing losses of old-growth forests and increasing Colorado’s crop of big, old trees—which pull more carbon dioxide out of the atmosphere and can store it for the duration of their centuries-long lives—starts with preserving these mature forests.

“If we cut down all the trees before they’re 80 or 100, we’re never going to re-grow any more old growth,” says Ellen Montgomery, public lands campaign director with Environment America, a network of 30 state environmental groups that’s advocating for the new definition of mature to draw the line at 80 years old. Montgomery, who lives in Colorado, adds, “The situation we’re in now is, even with a lot of it being protected, we’re losing old growth. It’s kind of being chipped away at. So, the idea here is to protect the old-growth-in-waiting stands and forests.”

But new definitions are just an early step in the process. “I don’t think the existence of a definition by itself is going to make any difference at all—it’s what you do with it,” Aplet says.

That would mean making policy, either federally or locally, once the definitions are set. Land managers at the Forest Service and Bureau of Land Management, for example, could use these new definitions to revise their management plans and reshape their priorities. But those plans are only updated every decade or so and can take years to complete. Other strategies could include establishing tree reserves or restricting tree cutting, perhaps through administrative actions, state policy, or federal legislation. Or it might yield nothing at all. After all, the president’s order ends at an inventory; it doesn’t say what has to be done with that inventory.

Maybe awareness at least will edge mature and old-growth forests toward better protections. In Colorado, now, Rempel says, threats don’t mean chainsaws and logging as much as they mean insect outbreaks, diseases, wildfires, and drought. Soon after they were found by the big-tree hunters, some of those record-breaking specimens were lost in the 416 fire in 2018. An aerial drop of water on those behemoths in the San Juans might have saved them—so perhaps once forest managers know where the oldest, most important trees are (and policy more thoroughly supports preserving them), they can take action to help more survive, even as climate change makes it harder for trees to sprout and thrive.

“How can it not help,” Rempel says, “if they really apply it?”