Why old growth matters

If we strip every emotional argument out and ask only: what concrete work does an old-growth forest do that a younger one cannot, four answers come back consistently from the ecological literature. Each is testable. Each has been replicated across multiple biomes. And each fails or weakens dramatically when the structural features described in the previous primer are absent.

1. Carbon

The most-cited assumption about old growth is also the one most often stated wrongly. The naive version goes: young, fast-growing forests pull carbon out of the air; old forests don't. The careful version, which has been the consensus in forest carbon research since at least 2008, is the opposite.

Luyssaert and colleagues, working from forest inventory data across the Northern Hemisphere, found that old-growth temperate and boreal stands continue to accumulate carbon for centuries — they do not reach a steady state in any human time frame^[1]. Stephenson and a 38-co-author group later showed, using a global tree-ring dataset of over 670,000 trees from 403 species, that the rate of mass gain in individual trees increases with size: a single 100 cm-DBH tree adds carbon faster than a 50 cm one, which adds carbon faster than a 25 cm one^[2]. This is the opposite of the slow-down most people expect.

Mildrexler and colleagues quantified what this means at the stand level in a real working landscape: in eastern-Cascades Oregon and Washington forests, the small fraction of trees over 53 cm DBH (about 3% of stems) holds 42% of the above-ground carbon^[3]. A handful of giants does an enormous share of the carbon work. Cut them, and the stand's carbon stock drops by an amount that takes the surrounding small trees decades to make back.

The IPCC's Sixth Assessment chapter on forestry says it plainly: protecting standing biomass in primary forests is among the most cost-effective near-term mitigation options, and the carbon released by clearing those forests cannot be made up on a relevant time scale by replanting^[4]. This is not a controversial finding. It is in the consensus document of the world's climate-science assessment.

2. Biodiversity, especially the species you can't see

Old growth is structurally complex in ways that map directly onto species niches. Walk through plantation forest and count vertical layers: usually one. Walk through old growth and count: five or six, plus standing dead, plus downed wood at multiple decay stages, plus exposed soil, plus canopy gaps of various sizes. Each of those is a habitat. Each habitat is the only one in which some species can complete its life cycle.

The marbled murrelet — a small seabird that nests on the moss-covered horizontal limbs of large old conifers — is the textbook example. It has no second-best option. Spotted owls, fishers, Pacific giant salamanders, several species of cavity-nesting woodpeckers, and a long list of mosses, lichens, and wood-boring beetles are similar: they exist mostly or only in stands that carry the structural indicators of old growth^[5]. Lose the structure and the species follow within decades.

The Convention on Biological Diversity catalogues this at global scale: forests host roughly 80% of the world's terrestrial biodiversity, and primary (old-growth) forests host more of that biodiversity per hectare than any other forest class^[6]. The "and the species we don't have names for yet" footnote is real: a substantial fraction of the fungi, the soil arthropods, and the nematodes in any old-growth stand are scientifically undescribed. We learn what an ecosystem does, in part, by losing it. Old-growth is the last baseline we have for "what an unbroken temperate or tropical forest does on its own."

3. Watershed regulation

Forests filter water on the way down and release it slowly. The deeper, more porous, more biologically-active soils under old growth do this better than the compacted, often-eroded, often-thin soils under recently-disturbed forest. The U.S. Forest Service's water-science programme is unusually direct on this: the agency manages 193 million acres of national forest, and about 18% of the surface water supply of the United States flows from those lands^[7]. The downstream cost of replacing that filtration with treatment plants when forest is converted is in the billions of dollars per year, before counting the water itself.

Old growth does this work at higher capacity than younger forest because the things that do the filtering — the soil column, the coarse woody debris, the moss layer, the persistent canopy — accumulate over centuries. A young plantation has very thin organic soil, almost no understorey moss, no decaying logs to retain moisture. The same rainfall produces a flashier hydrograph: more peak runoff, more sediment, less recharge. Cities downstream of mature watersheds learn this distinction the hard way after the upstream forest is cleared.

4. The underground forest

Most of an old-growth forest, by mass, is underground. The mycorrhizal fungal networks that connect tree root systems are now believed to mediate substantial carbon and nutrient transfer between trees, including between trees of different species. Their density and connectivity build up over many decades. We treat this in detail in the next primer; for now it's enough to know that "just plant more trees" almost always produces a forest with shallow, recently-disturbed, low-diversity fungal soil — structurally not the same thing as the soil in old growth.

This is the piece that most often surprises people. The above-ground forest you see is roughly a third of the system. The other two-thirds is fungal hyphae, root mass, organic-rich soil, and the decay community working through it. Centuries of relative stability are what build that. A clearfell undoes it in days. A replant doesn't restore it.

The reversibility argument

Most environmental problems are reversible on human time scales. Polluted rivers can be cleaned. Cleared fields can be re-meadowed. Degraded grasslands can recover with grazing pressure removed. Old growth cannot be repaired. Once cleared, the four things above are gone for the lifetime of every adult alive when the chainsaw arrived, and probably for the lifetime of every grandchild they will have. Replanted forest may, eventually, after several centuries of being left alone, become old growth again — but only if no further interruption happens. In a world that is interrupting forests faster than ever, "we'll regrow it" is rarely a real plan.

This asymmetry is what makes old growth a special case. Other forest types can be reasonably managed, harvested, and regenerated. Old growth has to be subtracted from that conversation. It is the part of the world's forest stock that we should treat as the baseline reference, the carbon library, the species archive, and the watershed core — and which the rest of forestry should be built around, not at the expense of.

The science here is settled enough. The political application is where the action lies, which is what the rest of this library and the map are for.