What Do We Mean by “Old Growth”? (U.S. National Park Service)

Definitions underpin everything people do, and agreeing on them is essential for joint action. But how we define our nation’s forests is especially important because of their global and cultural significance.

By Ann M. Gallagher and Carolyn A. Copenheaver

A short tree sits on a gray plain with blue skies and distant mountains in the background — Despite their relatively small stature, the bristlecone pines at Great Basin National Park in Nevada are among the oldest living things on Earth. The oldest are 4,000-5,000 years old. But they aren't as old as clonal organisms like the remarkable Pando, a vast quaking aspen clone that scientists estimate began growing at the end of the last ice age.
Image credit: NPS

An old-growth forest is something foresters will say they’ll know when they see it. Yet when asked to create a formal definition, many find it impossible. One of our favorite definitions, shared by an attendee at a 2002 writer’s workshop in Boothbay Harbor, Maine, is attributed to University of Arkansas professor Dave Stahle, a tree-ring scientist. After speaking to a group of science writers and realizing that all his definitions of “uneven-age distributions” and “steady state biomass” had fallen upon deaf ears, Stahle reputedly burst out, “Old-growth forests are where fairies would live!”

Although this definition certainly captured the writers’ imaginations, it won’t do for researchers or forest managers. Nor will it address President Biden’s Executive Order 14072 issued in 2022. This order directed the Bureau of Land Management and the U.S. Forest Service to create definitions for “old growth” and “mature,” apply them, and catalog public lands meeting them by Earth Day, 2023.

Should human values dominate how we define or delineate these forests? Or should we make our definitions less human-centric?

Prior to that executive order, the Urban Ecology Research Learning Alliance (UERLA) had been considering definitions for old-growth forests for months. We wanted to understand how science, public policy, land management, ecological processes, and law influence how federal agencies view old-growth forests. Should human values such as aesthetics and recreation dominate how we define or delineate these forests? Or should we make our definitions less human-centric?

Bureau of Land Management director Tracy Stone-Manning has praised mature and old-growth forests for sustaining biodiversity and mitigating climate impacts. In a statement released in 2022, she said, "Old-growth and mature forests are critical to ensuring resilience in our forests in the face of climate change, and they play a key role in storing carbon.” Her remarks emphasize the value of our discussions as we wrestle with defining, inventorying, and protecting these globally critical landscapes.

A lush, green forest with a creek running through it and a wooden footbridge. — The Tongass National Forest in Alaska is one of the world's largest temperate rainforests and the largest national forest in the U.S. It's home to some of oldest living trees on earth.
Image credit: U.S. Forest Service

Searching for Meaning

Inspired by EO 14072, UERLA worked with collaborators from Virginia Tech’s College of Natural Resources and Environment on reviewing the scientific literature to see how scientists used the terms old-growth and mature. What different aspects of the forest did they consider to make those definitions? For this project, Ethan Frye, a Virginia Tech student and UERLA summer intern, read over 200 peer-reviewed scientific research papers. His unpublished work (in press) documents what those papers showed about how scientists view and measure forest research. Along with Professor Eric Wiseman, we supervised Frye’s work.

Morrison and Steury emphasized the importance of including cultural values in the definitions of old-growth and mature forests.

Frye also visited parks in the National Capital Region, narrowing his research objectives and targeting his literature review through conversations with park resource managers. He spoke to Nick Bartolomeo from Rock Creek Park, Matthew Morrison from National Mall and Memorials, and Brent Steury from George Washington Memorial Parkway. Morrison and Steury emphasized the importance of including cultural values in the definitions of old-growth and mature forests through visits to the National Mall’s Japanese cherry trees. They and Frye also looked at trees that were part of historical monuments at Theodore Roosevelt Island in George Washington Memorial Parkway.

A Japanese stone lantern stand inside an urban forest of Japanese cherry trees — An urban cherry tree forest in National Mall and Memorial Parks. This significant cultural landscape memorializes historical events surrounding World Wars I and II and symbolizes friendship and diplomacy between the U.S. and Japan. Both the Japanese stone lantern and the trees were gifts from Japan.
Image credit: NPS

Context Matters

Although parks in the National Capital Region no longer contain old-growth forests (by any definition), there are many that still have mature forests. Frye’s conversations helped Virginia Tech researchers delineate what differentiates mature from old-growth forest. Rock Creek Park is one of several national parks designated by Congress in 1890, along with Yosemite and Sequoia. At the time, that designation drew researchers to witness some of Rock Creek’s “majestic”—meaning large and old—trees. Frye found that to be a common distinction in scientists’ definition of old-growth and mature forests.

One notable change in how scientists used the term coincided with the controversy stemming from protection of the northern spotted owl. During that time, use of ”old growth” increased by 751 percent.

Frye also documented an increase in the term “old-growth forest” in research papers spanning 123 years. One notable change in how scientists used the term coincided with the controversy stemming from protection of the northern spotted owl in the Pacific Northwest. During that time, use of ”old growth” increased by 751 percent, with many articles focused on old-growth forest and the northern spotted owl. Following the 1997 Kyoto Protocol, a need to understand the scale of carbon sequestration led to an uptick in dead wood sampling for carbon storage calculations. This resulted in the publication of numerous papers discussing the correct way to sample forests to measure carbon storage, improving the value of climate change modeling.

A brown owl with striking markings sits on a tree branch in a forest with a furry animal in its talons — Northern spotted owl with prey at Point Reyes National Seashore. This species is listed as threatened under the Endangered Species Act. Scientists consider this owl an indicator of old-growth forest health.
Image credit: NPS / C. Arreglo

The Conversation Isn't Over

The Bureau of Land Management and the U.S. Forest Service released its joint-agency report in response to EO 14074 in April 2023. Both agencies manage more than 178 million acres of forest with ecological, social, Tribal, and economic value. Forest types vary across the country. Juniper-Pinyon pine forests in the West are vastly different from temperate rainforests in the Pacific Northwest or oak-dominated deciduous forests in the Northeast. So it’s not surprising that in the end, the agencies used existing field plot data, which focused largely on tree measurements such as diameter at breast height, stem volume, or tree height.

A tall tree stands in front of granitic rocks — This 55-foot tall pinyon pine is the largest in Idaho and is located in City of Rocks National Reserve. The estimated age of old pinyons like this one is 350-450 years. Wildfire, insect pests, and deadly pathogens threaten the park's pinyon pines, which occupy this species' northernmost range.
Image credit: NPS

But the significance of tree height, for example, varies from species to species. A pinyon pine in Idaho is impressive at 55 feet, a height which is much less impressive for a sycamore tree in West Virginia or the giants of Olympic National Park. A definition based on height disregards important differences between various tree species and conditions of their forest communities. Basing a definition on height can therefore be misleading. The report stated that both agencies manage about 32.7 million acres of old-growth forest, or 18 percent of U.S. forests. And they manage about 80.1 million acres of mature forest—about 45 percent.

On December 20, 2023, the Forest Service invited public comment on a proposal to amend all 128 land management plans for units of the National Forest System. According to the notice, the aim of these amendments would be to “include consistent direction to conserve and steward existing and recruit future old-growth forest conditions.”

These perspectives have bearing on how we as a society measure mature and old-growth forests and which we decide to protect and by how much.

Frye’s research documents the influence of science, policy, and culture on old-growth and mature forest definitions. It adds an important perspective and creates a benchmark by which to view the April 2023 report. Scientists still disagree on what “old growth” and “mature” mean, and others warn that such definitions come with cultural baggage. So we suspect that the next few decades will see changes in how people use these terms in response to EO 14074. These perspectives have bearing on how we as a society measure mature and old-growth forests and which we decide to protect and by how much. Given the global environmental and cultural importance of our nation’s forests, this discussion isn’t simply academic.

About the authors

Woman with brown hair, glasses, and a black shirt smiling for the photo in front of a packed bookshelf.

^{Ann M. Gallagher is the science education coordinator for the Urban Ecology Research Learning Alliance, a unit of the National Park Service, National Capital Region. Image courtesy of Ann M. Gallagher.}

^{Carolyn Copenheaver is a professor in the College of Natural Resources and Environment at Virginia Tech.}

What Do We Mean by “Old Growth”?

Definitions underpin everything people do, and agreeing on them is essential for joint action. But how we define our nation’s forests is especially important because of their global and cultural significance.