Cut down a two-hundred-year-old forest and plant seedlings in its place, and for a while the accounting looks almost reassuring. The young trees shoot up fast, pulling carbon dioxide out of the air at a brisk clip. On the ledger that tracks how much carbon the land is absorbing each year, that patch can even look like it is doing more work than the ancient forest it replaced. A new global analysis argues that this reassurance is an illusion, and it now has the maps to show why.
A team led by Simon Besnard at the GFZ Helmholtz Centre for Geosciences in Potsdam tracked how the world's forests changed age between 2010 and 2020. They combined a new dataset of forest age with satellite estimates of how much carbon is locked in wood and leaves, plus atmospheric measurements of where carbon dioxide is being taken up or released. The result, published in Nature Ecology & Evolution, separates two things that usually get blended together: the carbon a forest holds in its trunks, and the carbon it happens to be absorbing in a given year.
A decade of forests getting younger in the wrong places
The picture that emerged is lopsided. Across the Amazon, the Congo Basin, Southeast Asia and parts of Siberia, forests got younger over the decade. That sounds gentle, but the mechanism behind it is not. Older stands were being wiped out by fire and logging and replaced by young regrowth. In China, Europe and North America the trend ran the other way, with forests on average aging as earlier plantings matured and some land was left alone.
The carbon math of that swap is stark. An old stand, more than two centuries old, holds roughly 98 tonnes of carbon per hectare in its aboveground wood. Convert it to a young stand under twenty years old and that figure drops to about 43 tonnes. Every hectare that flips from old to young sheds more than half its stored carbon, most of it heading into the atmosphere as the felled or burned wood decays. Summed across the planet, the researchers put the loss at around 0.14 petagrams of carbon a year. A petagram is a billion tonnes, so this is real weight, not a rounding error.
Why the annual sink can lie
Here is the twist that makes the paper worth reading closely. In regions where young stands were rapidly replacing old forests, the yearly carbon sink actually got stronger for a time. Fast-growing young trees are hungry, and a landscape full of them can pull down carbon dioxide faster than a mature forest sitting near its ceiling. So if you only watch the annual flux, the number that many climate reports lean on, a freshly cleared and replanted region can briefly look like good news.
The stock tells the opposite story. The forest is holding far less carbon than it did before, and the temporary surge in uptake does not come close to refilling that deficit. Besnard and colleagues make the point plainly: the young forests do not compensate for the long-term storage that old forests provide. A strong sink this decade is not the same as a full vault, and treating the two as interchangeable flatters young regrowth while quietly writing off the loss of the old trees.
What the study can't say yet
This is a global synthesis, and it inherits the blur that comes with that scale. Forest age here is inferred from models and satellites rather than counted ring by ring, and the oldest categories are the hardest to pin down because so little truly ancient forest remains to calibrate against. The carbon fluxes come partly from atmospheric inversions, which are good at continents and shaky at the level of a single valley. The work also stops at aboveground carbon and the decade it measured, so it does not fully settle what happens to soil carbon, which can hold enormous stores and responds on its own slow schedule.
None of that undercuts the central message. It just means the exact tonnage will get revised as the data sharpen. The direction of the finding, that swapping old forests for young ones drains carbon even when the yearly sink looks healthy, rests on the physical fact that big old trees store more than small new ones.
The practical payoff is a warning against a comforting shortcut. Planting trees is often sold as a clean way to bank carbon, and young forests really do grow fast. But a policy that treats a vigorous annual sink as proof of success can end up rewarding exactly the churn that empties the vault. The carbon that matters for the climate over the next century is the carbon sitting in wood right now, and the surest way to keep it there is to leave the oldest forests standing.
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