Mature and Old-Growth Forests

Older forests have characteristics that can provide important wildlife habitat, contribute ecosystem services such as clean water and carbon storage, and harbor significant social and cultural values for many human communities. The U.S. Forest Service has defined old-growth forests since the 1980s, and the BLM relies on these regional definitions when managing old-growth forests on public lands. 

Old-growth forests are dynamic systems distinguished by old trees and related structural attributes. Old growth encompasses the later stages of stand development that typically differ from earlier stages in a variety of characteristics, which may include tree size, accumulations of large dead woody material, number of canopy layers, species composition and ecosystem function. 

Climate change has spurred more frequent and longer lasting disturbances in forest ecosystems, including wildland fire, severe drought, and insects and disease. Increasing the area of old-growth forests in the United States would contribute to the climate resilience and landscape health of public lands. To meet this objective, the BLM and U.S. Forest Service are putting further emphasis on management to promote the mature stage of forest development.  

Mature forests are defined ecologically as the stage of forest development immediately before old growth. The mature stage generally begins when a forest stand moves beyond self-thinning, and is often marked by abundance of large trees, vertical canopy layers, aboveground biomass accumulation and stand height, as well as other attributes. Mature forests vary widely in character with age, geographic location, climate, site productivity, relative sense of awe, characteristic disturbance regime and the values people attribute to or receive from them. 

The majority of mature and old-growth forests managed by the BLM are pinyon and juniper woodlands. 

The BLM manages 19 million acres of pinyon and juniper woodlands, of which about 6 million acres are estimated to be old-growth. The BLM and U.S. Forest Service define old-growth pinyon and juniper woodlands to be 150-250 years old, depending on site productivity, the geographic location and other factors. Some juniper trees can exceed ages of 1,500 years, and pinyon trees more than 900 years. 

The following chart presents the initial inventory of mature and old-growth forests on BLM lands only by BLM State Office Administrative Unit: 

*The initial inventory does not include all BLM-managed lands in Alaska. The inventoried area excludes 27.5 million acres of potentially forested land managed by the BLM in Alaska that is not included in the U.S. Department of Agriculture’s Forest Inventory and Analysis Program.