The results have been published in Nature Climate Change.
The taiga stretches from Alaska through Canada and Scandinavia to Siberia and forms the largest forest area on Earth. In areas where these forests grow on permanently frozen ground (permafrost), the canopy acts as a natural insulating layer. Due to the shade provided by the trees, the soil remains cooler and thaws less deeply in the summer. As a result, the carbon that has been stored in the permafrost for thousands of years remains safely frozen.
Much larger climate role than thought
Until now, the climate value of boreal forests was primarily determined by the carbon stored in the trees themselves. The new study shows that their greatest contribution is actually located underground. The researchers estimate that the cooling effect of the canopy keeps approximately 59 petagrams of carbon in a frozen state. That is many times more than the 7 to 19 petagrams of carbon stored in the biomass of the forests. Without this natural protection, a large portion of that soil carbon could gradually thaw and become available for degradation by microorganisms, releasing additional greenhouse gases into the atmosphere.
In addition, the results show that the soil beneath boreal forests thaws on average about a meter less deeply during the summer than in open terrain.
Contribution from VU Amsterdam
Researchers from Humboldt University in Berlin, the Alfred Wegener Institute, the University of Oslo, and Vrije Universiteit Amsterdam collaborated on the study.
Earth scientist Moritz Langer of Vrije Universiteit Amsterdam helped develop the central idea of the study together with first author Simone Stünzi, who led the international research team. “We have identified a major gap in our understanding and quantification of the role of boreal forests in the global carbon cycle,” explains Langer. “This is particularly important because these forests are coming under increasing pressure due to rapid climate warming leading to increasing droughts and wildfires, which could fundamentally alter their ability to store carbon.” Stünzi and Langer jointly developed the study concept and modeling approach and worked closely together throughout the analysis and interpretation of the results.
The results underscore that the protection of boreal forests is essential for limiting climate change. Forest loss due to deforestation, forest fires, or other disturbances not only affects the carbon stored in trees but can also lead to the release of much larger carbon stocks from the underlying permafrost.
That carbon has accumulated over thousands of years since the last ice age. When the protective effect of the forest disappears and the permafrost thaws further, this stock cannot be restored on human timescales. With this, the researchers make clear that boreal forests play a crucial, but until now underestimated, role in the global carbon cycle and constitute an important focus for future climate and environmental policy.
Photo: Guido Grosse; Boreal black - and white spruce forest - Interior Alaska