Forests around the world are undergoing significant changes in their age structure. In many tropical regions, including Amazonia, the Congo Basin and Southeast Asia, extensive regeneration of forests can be observed due to disturbances such as fires and deforestation. This is the conclusion reached by an international team of researchers led by Simon Besnard from the GFZ Helmholtz Center for Geosciences. The study was published in the journal Nature Ecology and Evolution.

Age structure plays an important role for forests. Although young forests grow quickly and can absorb CO2 from the atmosphere, they store much less carbon than older forests. Thus, the regeneration of old, carbon-rich forests is already reducing the global carbon storage of forests.

"We estimate the global net loss of carbon from above-ground biomass at 140 million tons per year," says first author Simon Besnard from the GFZ Remote Sensing and Geoinformatics Section. He adds: "Although young forests are also valuable for the climate, they cannot completely replace the long-term carbon storage of old stands."

For their study, the team developed a new high-resolution global data set on forest age (GAMI v2.0). The researchers combined this dataset with satellite-based measurements of carbon stocks and observations of CO2 levels in the atmosphere. "By analyzing the changes between 2010 and 2020, we were able to determine exactly where forests are aging naturally and where disturbances are pushing them into younger age classes, and what this means for carbon storage," says Simon Besnard. Some regions in Europe, China and parts of North America show a net ageing trend of forests, while large forest areas in the tropics and Siberia are getting younger.

The results underline the central role of forests for the Earth's climate. "Forests are among our most important natural climate regulators," says Simon Besnard. "Protecting old forests is essential to prevent further carbon loss, while careful management of younger forests can help maximize carbon uptake. The right balance between disturbance, reforestation and protection will determine how forests contribute to climate change mitigation in the future."

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