A large store built over time
Old forests are often described as carbon “banks”. The comparison is imperfect, but it captures something real: over many decades or centuries, trees, dead wood and soil can accumulate a large stock of carbon. A stand that has been growing for a very long time usually holds more carbon per hectare than a young stand of the same type, because it has had time to build big trunks, thick root systems and deep layers of organic matter.
The exact amounts vary widely by forest type, climate and soil, so there is no single figure that fits all old forests.
Where the carbon sits
In a mature forest the carbon is spread across several pools:
- Large living trees. Wood accumulates as trees grow in height and diameter. A few very large trees can account for a disproportionate share of the stand’s living biomass, because wood volume increases quickly with trunk size.
- Dead wood. Standing dead trees and fallen logs may hold carbon for years or decades as they slowly decay. Old stands usually have more of it than managed or recently disturbed ones, and it doubles as habitat.
- Litter and soil. Layers of leaf litter and soil organic matter hold carbon too. In many cold regions the soil holds more than the trees do.
- Roots and fungi. Below ground, roots and associated mycorrhizal fungi move carbon into the soil.
Do old forests keep taking up carbon?
An older idea held that mature forests are “carbon neutral”, with uptake balanced by decay. Research on many sites has found that this is often not the case: many old forests continue to add carbon, though usually more slowly than rapidly growing young stands. Growth is slower, but the trees are large and the stock is already big, so even a modest rate adds up.
It is a mistake to push this too far. Some old forests are roughly in balance, and others lose carbon in bad years because of drought, fire or insect outbreaks. Whether a particular stand is gaining carbon depends on its species, its climate and its recent history.
Young versus old forests
Fast growth does not mean a young forest holds more carbon. A rapidly growing stand adds carbon at a high rate but starts from a small stock. An old stand adds more slowly but holds far more in total. Both statements are true at once.
This distinction matters for the choice between protecting and replacing. If an old forest is cleared and a new one grows back, the new stand can take up carbon quickly but may need a very long time to rebuild what was lost, as forest succession from clearing to mature stand shows. Some of the carbon, especially in large dead wood and deep soil layers, can take a long time to return.
Not the same as old-growth
“Old forest” and old-growth forests overlap but are not identical. Old-growth usually implies a stand with little history of major human disturbance and with structural features such as large trees and a mix of ages. A forest can be old and still have been logged long ago. Definitions vary by country and agency, so it is worth checking how a source uses the term.
Risks to the store
A large store of carbon is only as secure as the forest holding it. The main threats include:
- clearing for farming, pasture or development;
- logging, particularly of the biggest trees, which holds much of the biomass;
- severe fire and drought, whose likelihood is influenced by a warming climate;
- insect outbreaks and disease.
Because old forests take so long to recover, a loss is not quickly reversible on human timescales. That asymmetry is the core of the argument for protecting them, and it is why protected areas are often central in conservation planning.
Old forests and management
None of this means that managed forests are irrelevant to carbon. Managed stands can grow fast and supply wood that substitutes for other materials, and some of that wood ends up in long-lived products. Comparing the two approaches requires looking at time frames, what the wood replaces and how the land would otherwise be used. Reasonable people and researchers weigh these differently, and results depend on local conditions.
What to keep in mind
- Old forests typically hold large stocks of carbon in trees, dead wood and soil.
- Many continue to take up carbon, though not all do, and not every year.
- Their stocks take a very long time to rebuild if lost.
- Estimates of how much carbon a specific forest holds usually come from field plots and models, so they carry uncertainty.
In short, the strongest case for old forests as carbon stores rests on what they already hold, not just on what they add each year.