What Is an Old-Growth Forest?

The term "old-growth" refers to forests that have developed over a long period — typically centuries — without significant human disturbance. These are not simply old trees. Old-growth forests are complex, layered ecosystems defined by multiple characteristics that take generations to develop:

  • Large, ancient trees with wide, irregular canopies
  • A multi-layered understory of younger trees, shrubs, and ferns
  • Abundant standing dead trees (snags) providing critical wildlife habitat
  • Deep accumulations of fallen logs in various stages of decay
  • Rich, biologically active soil communities built up over centuries
  • Canopy gaps created by natural disturbances like windthrow or lightning

These structural features don't exist in younger, managed forests — even those that are decades old. An old-growth ecosystem takes hundreds of years to reassemble after disturbance, which is why what remains is irreplaceable on any human timescale.

How Much Old-Growth Remains?

The precise figures are difficult to pin down and vary by region and definition, but ecologists widely agree that the vast majority of the world's original old-growth forests have been lost to logging, agriculture, and development. In the contiguous United States, only a small fraction of the original temperate old-growth forest cover survives — concentrated primarily in the Pacific Northwest and isolated patches in the Appalachians. Similar patterns hold across Europe, Asia, and South America's temperate zones, though tropical old-growth forests — particularly in the Amazon, Congo Basin, and Southeast Asia — remain significantly larger but are disappearing at alarming rates.

Why Old-Growth Is Ecologically Irreplaceable

Biodiversity Hotspots

Old-growth forests support an extraordinary range of species that depend specifically on the structural complexity these ecosystems provide. Cavity-nesting birds like the Northern Spotted Owl and Pileated Woodpecker require large, ancient snags. Marbled Murrelets nest only on wide, mossy branches found on ancient conifers. Hundreds of invertebrate, lichen, and fungal species exist exclusively within old-growth environments. When the trees go, those species go with them.

Carbon Storage

Old trees store enormous amounts of carbon in their wood, roots, and the surrounding soil. A single large old-growth tree can store more carbon than many younger trees combined. When these forests are logged or burned, that carbon is rapidly released into the atmosphere. Protecting existing old growth is one of the most immediately effective tools available for climate mitigation.

Water Regulation

Old-growth forests act as natural sponges. Their deep, fungal-rich soils absorb rainfall slowly, recharging groundwater and regulating stream flow. This buffering effect reduces flood risk downstream and maintains clean, consistent water supplies through dry seasons — services that benefit both wildlife and human communities.

The Fungal Network

Beneath old-growth forest floors lies an intricate underground network of mycorrhizal fungi connecting tree root systems — sometimes called the "wood wide web." This network allows trees to share nutrients and chemical signals across vast distances. Young, replanted forests lack this established network, making them fundamentally different ecological environments regardless of how the surface appears.

Threats and What's Being Done

Commercial logging remains the primary direct threat to remaining old-growth, but wildfire suppression policies, climate-change-driven drought, and invasive pests also play significant roles in old-growth decline.

On the conservation side, a number of meaningful efforts are underway:

  • Federal protections in several countries have placed remaining old-growth areas under legal protection, though enforcement and policy continuity remain challenges.
  • Indigenous land stewardship has demonstrably protected old-growth in regions where Indigenous communities maintain territorial rights — research increasingly shows these are among the best-preserved forest areas globally.
  • Old-growth restoration ecology is an emerging field focused on accelerating structural complexity in younger forests through selective management practices.

What You Can Do

Individual action matters, even if it can feel small against industrial-scale forces. You can support land trusts and conservation organizations working to protect remaining forest corridors, advocate for strong protections in local and national forest management policies, and — most simply — spend time in old-growth forest. Those who experience these places firsthand tend to become their most committed advocates.

Old-growth forests are not a renewable resource on any timescale that matters to us. What exists today is what we have to protect.