A ‘shocking’ carbon discovery in Sweden’s forests

The world’s northern forests act as massive carbon vaults, locking away greenhouse gases in spruce, pines, and needle-covered soils. But industrial logging is quickly eroding their ability to mitigate climate change, according to a major new study led by scientists at Lund University and Stanford University. The biggest losses are happening in soils beneath the forest floor. 

Working in Sweden, researchers mapped old-growth forests across the country and then measured carbon at more than 200 forest plots over the course of three years. They then combined the field data with decades of national forest and soil carbon inventory data and statistical models. The result is a first-of-its-kind analysis quantifying carbon in vegetation, dead wood, soil, and harvested timber. 

Their findings, published March 19 in Science, reveal a stark gap. Undisturbed primary forests store 72% more carbon per acre than the managed forests that are replacing them, typically with single-species plantations. That figure gives managed forests credit for all the carbon stored in goods made from harvested wood, including bioenergy, paper, and building materials. Excluding harvested wood products, primary forests store 83% more carbon per acre.

The difference is 2.7 to 8 times larger than current official estimates. To put that in perspective, restoring carbon storage in Sweden’s managed forests to the level maintained by primary forests would mean keeping nearly 8 billion tons of carbon dioxide out of the atmosphere. That’s equivalent to Sweden’s cumulative emissions from burning fossil fuels over the last two centuries and hundreds of times larger than the country’s current annual fossil CO₂ emissions. 

Boreal forests under threat

The researchers were surprised to find the biggest difference lies in the soil. “There’s far more carbon in the soil than in the trees in these old-growth boreal forests,” said Rob Jackson, a senior author of the study and professor of Earth system science at the Stanford Doerr School of Sustainability. The carbon storage capacity that an old-growth forest loses once it’s been heavily furrow-cut and logged can’t easily be recovered, he said. “The loss of soil carbon through industrial management is persistent and shocking.”

In lowland primary forests, the researchers found the top meter of soils contained roughly 64% of total carbon in an average plot, compared to about 30% in live trees and 6% in dead wood. 

Previous research has shown that between 2003 and 2019, Sweden lost unprotected old-growth forests to clear-cutting at a rate of 1.4% per year, or six times the current rate of loss of primary forests in the Brazilian Amazon. 

Similarly rapid loss of primary forests may be widespread across the planet’s boreal regions, but it is often harder to track than tropical deforestation. While satellite sensors can easily distinguish oil palm plantations from primary rainforest, native spruce, pine, and birch dominate both old-growth and managed stands in the north, making them appear similar from space. 

“Unfortunately, the logging of primary forests in Sweden continues,” said study author Anders Ahlström, who spearheaded work on the research over the past eight years – first as a postdoctoral scholar in Jackson’s lab at Stanford and then as a senior lecturer at Lund University in Sweden. “Our results show that protecting the few primary forests that remain has a much larger potential to slow climate change than previously thought. Restoring sites degraded by industrial forestry could also boost biodiversity and store even more carbon.”

Overlooked value of old-growth forests

Around the world, nations are counting on forests to help them meet climate commitments. Most models used to construct pathways that stabilize global temperatures assume increased use of northern forests, especially for bioenergy production. 

But if managed forests and plantations store less than half as much carbon as the old-growth boreal forests they replace, as the new study suggests, those projections may overestimate climate benefits from forest-derived biofuels, especially if slowly growing boreal forests take centuries to accrue those benefits. They may also miss the value of protecting old-growth forests or improving forest management. 

“Some of the changes we found are intuitive – that the primary forests have larger trees and hold more dead wood. But we weren’t sure what to expect from the soils,” said lead study author Didac Pascual, a postdoctoral scholar at Lund University. “We learned that primary forests stored more carbon in their soil alone than managed forests do in trees, dead wood, and soils combined.”

Understanding what drives carbon storage

Major questions remain, including how much specific forest management practices may contribute to carbon storage capacity. Drainage ditches, plowing, and prescribed burns may all play a role, as could the loss of beneficial fungi in soil that can help trees take up nutrients. The researchers will need those answers before extrapolating the results from Sweden to other boreal regions across Canada, Russia, and Alaska.

Based on the results published in Science, Jackson and Ahlström are now working with Stanford biologist Kabir Peay to understand what drives carbon storage in old-growth forest relics in Sweden and across Scandinavia. One possibility is that old-growth forests harbor a greater variety of microbes in tree roots and soil. The team wants to know whether microbe-driven mechanisms could be transplanted elsewhere. 

“Our goal is to understand what makes the fungi and bacteria in these old-growth forests unique,” Peay said. This knowledge could potentially then pave the way for using microbes to speed up the transition of managed forest soils to a state where they lock away more carbon, “without having to wait centuries for old-growth forests to develop naturally.” 

Related: Learn about the Natural Climate Solutions Initiative at the Stanford Woods Institute for the Environment.