A forest floor can store lots of atmospheric carbon, helping to limit global warming that results from carbon dioxide emissions. Most of that storage, scientists have thought, is found in tree leaves and branches that absorb carbon, eventually fall to the ground and slowly decay into soil. A new study in Sweden, however, indicates that 50 to 70 percent of the carbon bound in soil is actually from tree roots and the fungi that grow on them.

This surprising insight comes from Karina Clemmensen at the Swedish University of Agricultural Sciences and colleagues who studied boreal forests on 30 islands in northern Swedish lakes. The forests were consumed by different numbers of fires over the past 5,000 years, providing a broad mix of soil compositions on different forest floors. The comparison revealed that the amount of carbon stored in soil was linked to mycorrhizal fungi that grow along tree-root systems and help to keep them healthy.

“These fungi live in symbiosis with plant roots and transport carbon from plant photosynthesis directly into the soil,” Clemmensen wrote in response to e-mail questions. “The prevailing dogma had been that aboveground plant litter (dead needles and wood) is the principal source of carbon storage in boreal forest soils,” she explained. But her results show that “a large proportion of the carbon stored in boreal forests instead enters the soil from beneath, via roots and their associated mycorrhizal fungi.”

Boreal forest soils are a major sink, holding 16 percent of all carbon sequestered in soils worldwide, according to a paper by Clemmensen’s team published March 29 in Science. The most immediate implication of the finding is that climate models should be revised to take into account the role that the fungi play. Revised models, Clemmensen wrote, would give more precise predictions of how forest management practices (such as thinning of trees) and environmental changes could influence carbon storage.

More research is also needed to determine if more older trees (so-called old-growth forests) worldwide would mean increased storage. As trees age, they allocate less carbon to root fungi, yet residues from old, dead fungi hang on to carbon more tightly than do dead needles and wood in the soil. Other studies, however, suggest that mycorrhizal fungi decompose organic matter in the soil, thereby releasing carbon. How these factors interact to form stable soil “is a very interesting and intriguing question that we do not yet have the answer to,” Clemmensen wrote. What is clear is that mycorrhizal fungi are much more important to carbon sequestration than anyone had realized.