Mature and Old-Growth Forests

The Mature and Old Growth Definition and Inventory report was published May 2024 and is the first national inventory of these forests on Forest Service and Bureau of Land Management lands. The report is part of an overarching strategy by the Forest Service to address changing conditions on the nation’s forests, including mature and old growth. The report found that both mature and old-growth forests are generally widely distributed on Forest Service and Bureau of Land Management landscapes across the nation.

For these two agencies, there are 33 million acres of old growth and about 80 million acres of mature forests. Collectively, old growth represents 19 percent and mature forest another 45 percent of all forested lands.

On Forest Service lands alone, there are 67 million acres of mature forests (roughly 47 percent of the total forested area) and 24 million acres of old-growth forests (about 17 percent of the total forested area).

The report outlines the following key areas:

Definition and Characteristics: Mature and old-growth forests are defined by their unique structures, age and ecological functions.

Mature and old-growth forests look dramatically different from coast-to-coast, state by state and locally. For instance, old-growth sequoias in California can be thousands of years old and upwards of 250 feet tall with a 30-foot or greater trunk diameter, while an old-growth stand of dwarf pitch pine in New Jersey may include trees that are hundreds of years old, roughly 14 feet tall, and only several inches in diameter. These differences underscore the complexity of defining these forests.

• Mature Definition: Mature forests are the stage of forest development immediately before old growth. They are generally defined as forests that have reached a certain level of age and structural complexity but may still grow more.

• Old-growth Definition: Old growth encompasses later stages of development, beyond the mature phase. They are typically, older than 100 years and characterized by large, old trees, a diverse mix of species, multi-layered canopies, and offer a rich array of habitat and biodiversity.

Inventory development: Existing mature and old-growth definitions for each Forest Service region were applied to Forest Inventory and Analysis data to generate the national-scale inventory. The Forest Inventory Analysis regularly samples forests systematically (about one plot per 6,000 acres).

Cultural Significance: Old-growth forests have cultural and historical significance throughout many communities and Indigenous peoples and connect people to these landscapes. Many Indigenous communities, for instance, have deep cultural ties to old-growth forests, viewing them as traditional and sacred spaces.

The integration of Indigenous Knowledge along with evolving scientific understanding are important to effectively understanding and managing these forests into the future.

Ecological Importance and Biodiversity: The document explains the critical roles these forests play in ecosystems, including habitat for wildlife, carbon sequestration and water quality. Mature and old-growth forests also provide essential resources for many species, including those that are rare or endangered.

A high level of biodiversity is found in mature and old-growth forests and emphasizes their importance for maintaining healthy ecosystems and supporting a wide variety of species.

Community Collaboration: The report conveys the importance of partnerships among Indigenous communities, federal, state, and local agencies, non-profits, and private landowners to create comprehensive management plans.

Research and Monitoring: Ongoing research and monitoring programs are necessary to understand the dynamics of these forests and track their health and changes over time to conserve and manage them well.

The Way Forward: The Forest Service’s approach to defining mature and old growth has paved the way for estimations on the abundance of all forest age groups, including early growth and young trees. The agency is also exploring additional technologies alongside partners and the science community to gather aerial and satellite imagery to map forests at more resolute scales and create a long-term monitoring system. These efforts will occur alongside further dialog on the social, cultural, and economic values attributed to these forests by tribal nations, local communities and various stakeholder groups.

The Mature and Old Growth Threat Analysis Report, published June 2024, is the first-ever analysis of threats to these forests across Forest Service and Bureau of Land Management lands.

The report is part of an overarching strategy by the Forest Service to address changing conditions on the nation’s forests, including mature and old growth.

The analysis focused on threats to forests identified the Mature and Old Growth Definition and Inventory report, published May 2024.

The threat analysis report outlines the following key findings:

Key threats: Mature and old-growth forests have high exposure to a variety of threats and future projections show this will likely increase. Currently, wildfire and fire exclusion are the leading threat to mature and old-growth forests, followed by insects and disease in the West. In the East and Alaska more varied disturbances such as increasing temperatures, insects and disease and invasive species, threaten older forests. The analysis also found that two-thirds of mature forests and just over half of old-growth forests are also vulnerable to these threats.

Tree cutting (any removal of trees) is currently a relatively minor threat despite having been a major disturbance historically given these practices were the primary reason for loss of old-growth forests from 1950 to 1990.

On Forest Service lands since the year 2000:

Wildfires were associated with a net decrease of:

• 2.2 million acres of mature forest – 3.2 percent of mature 

• 655,000 acres of old-growth forest – 2.6 percent of old growth

Insects and disease corresponded with a net decline of:

• 1.6 million acres of mature forest – 2.3 percent of mature

• 273,000 acres of old-growth forest – 1.1 percent of old growth

Tree cutting was associated with a net decrease of:

• 220,000 acres of mature forest – 0.3 percent of mature

• 6,000 acres of old-growth forest – 0.02 percent of old growth

Where no severe forest disturbances have occurred, mature forests had a net increase of 2 million acres of mature and 821,000 acres of old growth. Combined, there has been a 2.1-million-acre net decline of mature forests and a net decline of 140,000 acres of old growth.

These estimates were generated using data from national Forest Inventory and Analysis plots that were measured more than once between 2000 and 2020.

Future projections: Over the next 50 years the growth of young and mature forests may result in an increase of older forests, despite increased disturbances. However, gains lessen with each passing decade and housing growth in the wildland-urban interface complicates mitigation of threats. Projections of increasing mature and old-growth forests are tempered by the reality that American forests are experiencing changing conditions which is altering where and what types of mature and older forest can persist.

Complex factors: The introductory threat analysis is a first step towards understanding the myriad interacting biophysical and social factors that threaten the persistence of older forests on public lands across the nation. 

Definitions: For this analysis, threats were defined as disturbances or stressors, either current or projected, that can contribute to the enduring loss or degradation of the characteristic conditions, functions, or values of existing mature and old-growth forests.

Cultural Significance and socioeconomic factors: Certain unique characteristics of old-growth forests have cultural and historical significance throughout many communities and Indigenous groups that connect them to these landscapes.

Research and Monitoring: The initial threat analysis, and future monitoring of status, trends, and disturbances of forests of all ages will inform further understanding of various patterns and the development of associated adaptive management strategies. The analysis suggests the current and future conditions, rather than historic conditions, should guide forest management considerations.

The Way Forward: The Forest Service will continue to work to understand what conditions make these forests either more or less susceptible to damage and will identify the appropriate management strategies. The agency also plans to conduct ongoing management, monitoring and analysis of its forests in collaboration with national forests, partners and communities through the forest planning process.