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Forests across the globe are experiencing significant changes in their age structure. Many tropical regions, including the Amazon, the Congo Basin, and Southeast Asia, are seeing widespread shifts towards younger forests, driven by disturbances such as fire and logging. This is the result of a study from an international team of scientists led by Simon Besnard from GFZ Helmholtz Centre for Geosciences. The study is in the journal Nature Ecology & Evolution.

Age matters when it comes to forests. While young forests grow rapidly and can absorb COâ‚‚ from the atmosphere, they store much less carbon than older forests. As a result, the replacement of mature, carbon-rich forests with younger stands is already reducing the world's forest carbon stocks.

"We estimate a net global loss of 140 million tons of carbon each year from aboveground biomass," says first author Simon Besnard from GFZ's section Remote Sensing and Geoinformatics. "Young forests provide valuable climate benefits, but they cannot fully replace the long-term carbon storage of old-growth stands."

To reach these conclusions, the team developed a new high-resolution global forest age dataset (GAMI v2.0). They combined this dataset with satellite-based carbon stock measurements and atmospheric COâ‚‚ observations.

"By analyzing changes between 2010 and 2020, we could pinpoint where forests are aging naturally and where disturbances are pushing them towards younger age classes, and what that means for carbon storage," says Simon Besnard. Regions such as Europe, China, and parts of North America show a net aging trend, while large areas of the tropics and Siberia are getting younger.

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