What is old growth?

An old-growth forest is what a forest becomes when nothing interrupts it for long enough.

Not a stand of old trees planted in rows. Not a “mature” forest that has been recently thinned. Old growth is a system: the trees, the standing dead, the rotting logs on the floor, the soil, the fungi, the salamanders, the warblers, all having developed together for several centuries with no clear-cut, no industrial fire, no plough.

The first thing to understand is the time scale. Most temperate old-growth forests need 150 to 500 years of relative quiet before they look like themselves^[1]. A single human lifetime can witness a young forest become a mature forest, but it cannot witness the development of true old growth. Whoever sees old growth today is seeing the work of every generation that did not interrupt it.

How to recognise it

The visible markers of old growth are learnable. A 14-year-old can be taught to recognise them in an afternoon walk; an experienced ecologist can read the fine grain. Five indicators carry most of the signal in any region of the world^[1][2]:

1. Big living trees, near the regional maximum

Every region has a “champion-class” tree size. Old-growth Douglas-fir in the Pacific Northwest reaches 200 cm DBH (diameter at breast height) and 80 m tall^[1]; old-growth eastern hemlock in the Appalachians pushes 100 cm and 30 m; old-growth Sitka spruce on the British Columbia coast is among the largest living things on Earth^[4]. The number scales by region; the structural fact is the same. The canopy is anchored by individuals you couldn’t fit your arms around.

2. Standing dead trees (snags) of every size and decay class

A working forest gets the dead trees taken out. Old growth keeps them. A snag is the forest’s library: at least three quarters of cavity-nesting birds and a long list of mammals depend on standing dead wood^[3]. Spotted owls, pileated woodpeckers, flying squirrels, fishers, marbled murrelets — none of them are accidents in old growth, all of them are scarce in young forests, and the difference is the snags.

3. Heavy coarse woody debris on the floor

The downed logs you trip over in old growth take 100 to 500 years to fully decompose, depending on species and climate^[3]. A big rotting hemlock log in old growth is, in a real sense, still ecologically functioning: a slow-release nitrogen reservoir, a salamander habitat, a fungal substrate, a seedbed for the next cohort of trees. (“Nurse logs,” in the literature.) Walk fifty steps in old-growth Pacific rainforest and you will trip over four centuries of accumulated wood. Walk fifty steps in plantation forest and you will trip over almost nothing.

4. Multi-layered canopy structure

Old growth is vertically stratified. Emergent giants poke above a continuous main canopy. Mid-canopy trees stand in their reproductive prime. Suppressed understory trees wait for a gap. A shrub layer. A herbaceous layer. Mosses, liverworts, and lichens cover everything that is not moving. Plantation forest has one canopy. Old growth has five or six^[2].

5. Diverse age structure within the stand

Trees of multiple generations. Evidence of small gap-phase regeneration where one or two big trees fell decades ago and a younger cohort filled in^[5]. Old growth is not uniformly old — it is old at the canopy, with a continuous cycle of recruitment underneath.

Who saw this first

The five indicators above are not folklore. The modern structural definition of old growth — what we just walked through — was assembled mostly in one place: the USDA Forest Service Pacific Northwest Research Station and the H. J. Andrews Experimental Forest in the Oregon Cascades. Mostly in one decade, the 1970s and 1980s. And mostly under the leadership of one ecologist: Jerry Franklin.

Franklin and his collaborators — Kermit Cromack, William Denison, Chris Maser, Jim Sedell, Fred Swanson, Glenn Juday, and many others, joined later by Tom Spies and Mark Harmon — walked the Cascade and Olympic old growth with tape measures and notebooks. They counted snags. They weighed downed logs. They traced how salmon-bearing streams interact with riparian forest. They climbed into the canopy on rope rigs that had to be invented for the purpose. In a foundational 1981 USDA Forest Service technical report^[1], they distilled all of that into a precise structural definition that had previously existed only as “you know it when you see it.” That report is still the document the rest of the world cites.

Franklin’s subsequent career added several pieces that became central to modern forest ecology. He led the long-term research at Mount St. Helens after the 1980 eruption and documented biological legacies — the surviving trees, downed wood, intact soil microbiology, and buried seed banks that allowed the blast zone to reforest itself far faster than anyone had predicted. That single observation reframed how the field thinks about disturbance recovery everywhere: a forest is not a thing that grows from bare ground after a calamity, it is a thing whose memory of itself survives the calamity in pieces of wood and patches of dirt. He testified to the U.S. Congress during the Pacific Northwest old-growth debates of the 1990s and helped design the Northwest Forest Plan that protected most of the remaining federal old growth. He spent decades training new ecologists at the University of Washington.

Most of the contemporary literature on old growth, including every source cited in this primer, traces back to Franklin in one or two steps. If you want to read further, his foundational report is open access (cited above). His more accessible recent synthesis, written with Norm Johnson and Debora Johnson, is Ecological Forest Management^[6].

A name worth knowing. Most kids who grow up in a country with a Forest Service have never heard of him; the next generation of foresters and ecologists will keep his work alive only if someone tells them who he is.

What old growth is not

A plantation. A plantation is a tree farm: same species, same age, evenly spaced rows, scrubbed of dead wood, periodically harvested. From the outside it looks “forest-like.” Internally it lacks every indicator above. Tree count is not forest, in the same way that warehouse population is not city.

A “mature second-growth” stand can carry several of the indicators above — big trees, some snags, some downed wood — without yet being old growth. The line is fuzzy by design: ecologists distinguish “old-growth in development” from fully-developed old growth on the basis of how many of the indicators are present at a structurally meaningful density^[2]. The fuzziness is not a problem. It is a reminder that old growth exists on a continuum of accumulation, and that protecting an “almost-old-growth” stand today is also protecting a real old-growth stand a few generations from now.

Why old growth specifically

Old growth carries a special burden under modern conditions because there is, by definition, no fast version of it. Once cleared, the system is gone for the lifetime of every person alive when the chainsaw arrived, and probably the lifetime of every grandchild they will have. Replanting trees, even native species, even at high density, even with the best intentions, produces a young forest. Whether that young forest eventually becomes old growth depends on whether anyone interrupts it again for the next four hundred years.

This is the first reason old growth deserves disproportionate care. Most environmental damage is reversible on human time scales: a polluted river can be cleaned, a cleared field can be re-meadowed, a depleted aquifer can be (slowly) refilled. Old growth cannot be repaired. It can only be protected, or replaced.

What to do with this

If you find old growth near where you live — and if you live in a country with any temperate or tropical forest, you may be closer to it than you think — treat it as a library that has been preserved by accident. The next person who walks through it should see at least as much as you did. Map it (carefully, without publicising the exact location of any rare individual tree small enough to be poached). Photograph it. Learn to identify the species and the indicators. Show it to a child. Defend it when someone proposes its removal in exchange for a quarter’s worth of timber revenue. The decision about whether your region still has old growth in 2200 is being made now, in this generation, mostly by people who are not paying attention.

A library is a good metaphor and not just a metaphor. Most of what we know about how forests work was learned from old growth, because old growth is the only place where forest function can be observed without the noise of recent disturbance. Future ecological research, future indigenous knowledge keeping, future climate-adaptation strategy, all rely on having reference forests left to learn from. When the last old growth in a region is cleared, the reference is gone. The young forests that come after it cannot tell anyone what they were before they were cut.