Definition: What Counts as Old-Growth?

Old-growth forests are late-stage forest ecosystems characterized by large, old trees and an abundance of structural complexity: layered canopies, canopy gaps, diverse understory patches, and substantial deadwood. The specific age or diameter thresholds vary by region—so the most reliable definition is based on structure + ecological function rather than a single number.

• Age: often old, but not the only factor
• Structure: layered canopy, gaps, variable tree sizes
• Deadwood: snags and down logs are key habitat components
• Continuity: long-term habitat stability supports specialist species

How to Recognize Old-Growth in the Field

If you want a practical checklist, look for three or more of the traits below. Old-growth stands vary by climate and history, but these signals show up across many regions.

Important: Old-growth identifiers are regional. A high-elevation or dry forest may show “old-growth” traits with smaller trees than a coastal rainforest—structure is still the key.

Why Old-Growth Forests Matter

Old-growth forests act like ecological infrastructure. Their complexity supports species that can’t thrive in simplified stands, and their stable soils and microclimates can buffer drought and heat extremes.

• Biodiversity: habitat for fungi, lichens, cavity nesters, and shade specialists
• Carbon: long-term storage in trunks, roots, deadwood, and soils
• Water cycling: canopy interception, fog capture, infiltration, and stable stream flows
• Resilience: structural diversity can reduce “single-point failure” risk

If you’re planning restoration or planting, it helps to treat old-growth as the reference condition: it shows what forests look like when allowed to mature with minimal simplification.

Pacific Northwest Old-Growth Context

In many Pacific Northwest ecosystems, old-growth structure is commonly associated with long-lived conifers and shade-tolerant understory species. These guides connect the dots across the region’s forest types.

Explore: American Forests • Spruce Trees • Western Red Cedar • Arbutus Trees • Pacific Yew

Old-Growth Forest FAQ

What is an old-growth forest?

Old-growth forests are long-developed forest ecosystems with complex structure—large old trees, multiple canopy layers, deadwood (snags and logs), and diverse microhabitats. Age matters, but structure and ecological function are the key identifiers.

How can I tell if a forest is old-growth?

Look for multiple canopy layers, large diameter trees, abundant deadwood (snags and down logs), uneven ages, canopy gaps, and rich understory diversity. Many sites also show stable soils, mossy logs, and long-term habitat continuity.

Are all old forests old-growth?

Not necessarily. A stand can be old in years but still lack old-growth structure if it was heavily logged, simplified, or repeatedly disturbed. Old-growth is about ecological complexity, not just tree age.

Why do old-growth forests matter for carbon?

Old-growth forests can store very large amounts of carbon in living trees, deadwood, and soils. Protecting existing carbon stocks is often as important as planting new trees.

What species are common in Pacific Northwest old-growth?

Common old-growth associates vary by site, but may include western red cedar, Douglas-fir, spruce, hemlock, and understory species like pacific yew and arbutus in suitable habitats. Local conditions and elevation strongly influence composition.

Can plantations become old-growth?

Plantations can develop some old-growth-like traits over time, but it often takes many decades to centuries and requires management that encourages structural diversity—snags, down wood, varied ages, and mixed species.