The planet’s undisturbed old-growth boreal forests may be far more important in the fight against climate change than previously realized, according to a new study, which finds that primary forests in Sweden store over 70% more carbon than managed secondary forests. Boreal forests, the world’s largest forest biome, play a crucial role in absorbing roughly 30% of human-caused carbon dioxide (CO₂) emissions. Yet they are increasingly exploited to supply timber and bioenergy. In European boreal forests, intensive management practices, such as clear-cutting, thinning, planting fast-growing trees, fertilization, drainage, and soil preparation, aim to maximize harvestable wood and timber quality, but their long-term impacts on carbon storage remain uncertain. Limited observations from undisturbed primary forests make it difficult to quantify how converting these forests into managed secondary forests affects carbon sequestration. With models projecting increased reliance on northern forests for bioenergy, understanding the effects of transforming primary boreal forests and their impact on carbon storage is essential.

Didac Pascual and colleagues combined data from the Swedish National Forest Inventory (NFI), the Swedish National Forest Soil Inventory, and targeted field surveys to estimate carbon storage in Sweden’s primary forests and to quantify how it differs from the region’s managed secondary forests. They measured carbon across multiple components, including vegetation, dead wood, soils, and harvested wood products, and applied multiple analytical methods to estimate total carbon storage. Pascual et al. found that primary forests stored about 72% more carbon than managed secondary forests when considering all carbon pools combined. Soils contained the largest carbon store and accounted for much of the difference between the forest types. Overall, across Sweden, primary forests store 9.9 kg carbon per square meter more than managed secondary forests – 2.7 to 8 times higher than earlier estimates. This sugegsts that the climate impact of converting primary boreal forests to managed forests may be much greater than previously thought.