Old-growth forests remain Earth’s strongest carbon vaults

Age matters in forests. A new study tracks how the world’s trees shifted in age from 2010 to 2020 and what that means for the carbon they hold.

Forests help pull carbon dioxide from the air every year, but that help depends on how many stands are young, middle-aged, or old. A global carbon assessment found land ecosystems absorbed roughly 3.5 petagrams of carbon per year in the 2010s, with wide swings from place to place.

The research was led by Simon Besnard at the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany.

Old trees hold more carbon

Young trees grow fast, yet older stands hold far more carbon per acre. In the new analysis, old forests above 200 years averaged about 98 megagrams of carbon per hectare, while stands under 20 years held about 43.5.

Old stands are not carbon idle, either. Decades of research show many old growth forests keep accumulating carbon – just at steadier rates – and they store large amounts in wood and soils.

Stock and flow differ. A young stand can add carbon quickly year by year, but a large, old stand already holds a deep reservoir that took centuries to build.

Measuring the age of forests

The team used a high-resolution forest age map and paired it with satellite-based estimates of aboveground biomass to estimate aboveground carbon. They relied on the European Space Agency’s Biomass CCI v4 dataset for 2010 through 2020.

The researchers also tracked stand replacement – cases where fire or harvest removed most trees and a new cohort took over. Satellite maps of forest loss and regrowth were combined with machine-learning age estimates to pinpoint when and where these events occurred.

To link age-related forest changes with the atmosphere, the team compared shifts in forest dynamics with atmospheric inversion estimates of net ecosystem exchange.

This method uses measured CO₂ concentrations to infer how much carbon regional landscapes absorb or release, providing a way to connect local changes with broader carbon flux patterns.

Where trees are maturing

Several regions added older forests over the decade. Europe, China, and parts of North America showed net aging, which reflects long-running reforestation and managed regrowth.

Large areas moved the other way. The Amazon, the Congo Basin, Southeast Asia, and parts of Siberia saw more young stands as disturbances replaced older cohorts.

Patterns were patchy inside countries and biomes. Some grids showed strong aging while neighbors trended younger, a sign that local fire history and logging shaped the mosaic.

Younger forests, less carbon

“We estimate a net global loss of 140 million metric tons of carbon each year from aboveground biomass,” said Besnard. The shift from older to younger stands reduced the carbon stored above ground at a planetary scale. 

When old forests were replaced by young ones, those areas accounted for roughly 0.10 to 0.16 petagrams of the annual global loss in aboveground carbon. That was a small slice of forest area, but a large share of the stock change.

The sink can still look stronger for a few years after a big replacement event. Young stands often pull in carbon quickly, yet they start from a smaller store and do not rebuild centuries of stock on policy relevant timelines.

Old trees are irreplaceable

Secondary forests can accumulate biomass at impressive rates. A large synthesis paper in 2016 found tropical secondary forests recover a big fraction of their biomass within decades, though exact speeds vary by climate and land-use history.

High growth does not equal high storage right away. Many services linked to older, complex canopies, including deep carbon pools, take far longer to return.

Fast growing plantations can help with uptake, but uniform stands usually store less carbon and support less biodiversity than naturally regenerating, diverse forests. That tradeoff matters when the goal is both climate mitigation and durable ecosystem function.

Shifting forest ages worldwide

The study’s age maps indicate a clear global pattern. In the tropics, stand replacement shifted age classes toward younger forests, while large temperate zones nudged toward maturity.

In boreal regions, vast stretches kept their older structure, yet fire hotspots created younger pockets. Those pockets may grow fast, but they also highlight vulnerability to repeated burns.

Mature forests, between 81 and 200 years old, expanded in area over the decade. This expansion points to regrowth catching up in places where disturbance slowed or management favored retention.

Age alters carbon balance

Net ecosystem exchange can strengthen in regions where old stands were recently replaced by young ones.

The analysis reported that for every one-percent annual increase in old to young replacement, the carbon sink trend deepened by about 34 grams of carbon per square meter per year.

That correlation does not mean replacement is a climate strategy. The apparent boost leans on legacies old stands leave behind, like fertile soils, seed banks, and structural remnants.

As inversion tools improve, we will get cleaner signals that separate mixed-age patches. For now, the link between age transitions and regional flux changes is strong enough to inform policy.

Protecting forests, preventing carbon loss

The researchers tested two simple futures out to 2050. Under business as usual, total aboveground carbon stayed roughly flat due to ongoing turnover, with losses in some regions balanced by gains in others.

Under a conservation scenario that halts stand replacement after 2030, total aboveground carbon rose by roughly 0.55 to 0.63 petagrams per year relative to 2020 levels. That gain came mostly from forests aging into mature and maturing classes.

“Forests are among our most important natural climate regulators,” said Besnard. “Protecting old forests is essential to prevent further carbon losses, while careful management of younger forests can help maximize carbon uptake.”

The study is published in Nature Ecology & Evolution.

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