An old forest is more than old trees
People often picture old-growth forest as a stand of giant trees, and that is part of the picture. But the term describes a forest that has developed over a long time without major disturbance by people, and that has therefore acquired a particular structure, mix of species and set of processes. Age matters, but what counts is the condition that age produces.
There is no single, universal definition. Different countries, regions and agencies use different criteria, and the same forest may qualify under one definition and not another. Related terms include primary forest, ancient woodland and virgin forest, and these do not all mean exactly the same thing. The general problem of definitions is covered in the article on forest definition. Treat the features below as typical rather than as a checklist every old forest must meet.
Large old trees
The most obvious feature is the presence of very large, old trees. How old “old” is depends on the species and the region. A tree that is old for a short-lived pioneer species may be young for an oak, and trees on poor, cold sites grow so slowly that small trunks can be very old. In many forests, the biggest trees are among the oldest, but size alone is a rough guide.
Old trees often look different from younger ones. They may have broad, irregular crowns, thick and deeply furrowed bark, large limbs, hollow trunks, broken tops and patches of decay. These features, which a forester growing timber might consider defects, are valuable habitat.
Many ages and sizes
Where a younger stand is often even-aged, an old-growth forest typically has a range of ages and sizes. Seedlings, saplings, mid-sized trees and ancient giants may all be present. This follows from the long history of small-scale disturbance and renewal, as in the process described under canopy gaps, rather than one event that started all the trees at once.
The result is a forest with a complex vertical structure: multiple canopy layers, a patchwork of gaps and dense areas, and plenty of variation from one spot to the next.
Dead wood, standing and fallen
Dead wood is a defining feature. Old forests usually contain much more of it than managed ones: standing dead trees (snags), large fallen logs in all stages of decay, and decaying stumps. Over time, a fallen log can take decades or even centuries to break down, depending on the species, size and climate.
Dead wood supports a huge range of life. Woodpeckers and other cavity nesters use snags, small mammals shelter in logs, beetles and other insects bore into wood, and a great variety of fungi, mosses and lichens grow on it. Logs also act as nurseries; in some forests, seedlings of certain species establish mainly on rotting logs. More detail is in the article on deadwood habitat.
Soils and the forest floor
The soils of old forests have often been undisturbed for a long time. They may have deep layers of organic matter, an intact litter layer and a rich network of fungi and soil animals. The ground tends to have a pit-and-mound surface from past tree falls, and it has not been compacted by machinery or plowing.
Underground fungal networks tie trees together. The role of mycorrhizal fungi is central in old forests, where they have had centuries to develop. Their detailed workings are still an active area of research, and popular claims about trees “talking” through fungi should be treated with some caution.
Species that depend on old forest
Some plants, lichens, fungi, insects and animals are found mainly or only in old forests, usually because they need large trees, deep shade, stable humidity, dead wood or continuity of habitat over very long periods. Some lichens, for example, spread slowly and need old bark. Certain birds and mammals need large cavities or large trees for nesting or denning. Which species depend on old forest varies by region.
Because many of these species colonize slowly, they may take a very long time to return to a site, if they return at all, once old forest has been lost. That is one reason conservationists place a high value on remaining old stands, and a reason many are included in protected areas.
Carbon and old forests
Old forests store large amounts of carbon in big trees, dead wood and soil. A long-standing question is whether they keep absorbing carbon or reach a balance. Evidence suggests that large old trees can keep adding wood, and the soil and dead wood hold carbon for a long time, though the net balance varies by forest and year. The matter is discussed in the article on old forest carbon. What is clear is that cutting an old forest releases carbon that takes a long time to recover, a point relevant to carbon accounting and wider talk of climate mitigation.
How they differ from younger stands
Compared with a young or managed stand of the same type, an old-growth forest usually has:
- Larger and more varied trees, including very old individuals.
- More dead wood, both standing and fallen.
- A more complex structure, with multiple layers and gaps.
- Richer, less disturbed soils.
- A greater share of species that need old, stable habitat.
A young forest can grow quickly and store carbon rapidly, and it supports wildlife of its own, so the contrast is not about one being good and one being bad. Different stages of forest succession offer different habitat, and a landscape generally benefits from a mix.
Rarity and recovery
In many temperate regions, little old-growth is left because of centuries of logging and clearing for farmland, while larger areas remain in parts of the boreal zone and in some mountain and remote regions. Exact amounts are hard to state, partly because of the definition problem.
Can forests regain old-growth qualities? Over very long periods, yes, and some management approaches, such as leaving dead wood, retaining large trees and allowing a mix of ages, can speed up the development of old-forest features. But full recovery usually takes centuries, which is why protecting existing old forest is often seen as more reliable than hoping to recreate it.