An unmanaged forest (left) borders a clear-cut area that is now regenerating (right). Marcus Larsson studied how such changes affect carbon in forests. Photo: Jenny Dahl, 2017
Boreal forests store about one third of the world's forest carbon. But how they should be managed to store more carbon and help mitigate climate change is debated. In his PhD thesis, Marcus Larsson shows that focusing only on trees is not enough, carbon in the soil should also be taken into account.
A significant proportion of carbon in boreal forests is stored in soils, and these forests play a key role in the global carbon cycle. In Sweden, most boreal forests are managed using even-aged forestry, based on clear-cutting followed by regeneration. However, these practices are mainly designed to optimise tree growth and timber production, and therefore primarily affect carbon stored above ground. Processes in the soil are often overlooked.
Marcus Larsson studied the effect of forest management practices on soil carbon.
"I wanted to get a more complete picture of how carbon is stored and moves within the forest ecosystem by also including soil carbon. When we better understand how carbon is stored in the soil, we can optimise current forest management practices to improve how forests store carbon and thereby strengthen their potential to help tackle climate change," says Marcus Larsson.
Marcus Larsson examined how fertilisation and tree species choice, two forest management practices commonly applied in Sweden, affect carbon in the ecosystem, with a particular focus on the soil. He also looked at what happens when a previously unmanaged forest is subjected to even-aged management.
Soil carbon responds more subtly to forest management
The thesis shows how forest management affects carbon across the forest ecosystem. Most changes occur in the tree biomass, while effects in the soil are smaller and mainly limited to the organic layer. The deeper mineral soil layer remains largely unchanged.
Marcus Larsson and his colleagues revisited an older fertilisation experiment, where the original forest had been harvested and new stands established. Ten years after regeneration, fertilisation still increased tree growth by up to 20 percent, but 23 years later, that effect had disappeared.
"Fertilisation increases tree growth and can boost carbon storage. This effect persists into the next forest rotation, but it gradually diminishes over time," says Marcus Larsson. "We were able to estimate how long these effects last and show that they can extend beyond a single forest rotation, which can be of benefit for forest management."
A broader view is needed to assess forest carbon
Larsson's work also shows that while soil carbon may appear relatively stable, it is influenced by many factors. While fast-growing tree species initially store more carbon in biomass than slow-growing species, the total amount of carbon in the soil is largely determined by site conditions rather than species. This means that choosing tree species becomes more complex when considering the total carbon stored in the ecosystem rather than tree biomass alone.
"A more integrated view including both above- and belowground carbon can help us better understand how forests function and how they store carbon over time," says Larsson. “If we base management decisions on more complete information, we can potentially make better decisions in the long term. Such an approach is particularly important in boreal forests, where changes unfold over decades or even centuries”.
Left: One of the forests Marcus Larsson studied was a planted birch forest near Vindeln in northern Sweden; middle: Soil sampling in the forest during fieldwork - Marcus Larsson carried out extensive field studies as part of his PhD project; right: Marcus Larsson’s dog often accompanied him during fieldwork. Photos: Marcus Larsson
About the public defence
Marcus Larsson, Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, defended his PhD thesis on 10 April 2026. The faculty opponent was David Paré from Natural Resources Canada, Canadian Forest Service in Quebec. The thesis was supervised by Michael Gundale and Annica Nordin.
Title of the thesis: Forest management and soil carbon stocks in Swedish boreal forests: Impacts of harvesting, fertilisation and tree species choice
For questions, please contact:
Marcus Larsson
Department of Forest Genetics and Plant Physiology
Swedish University of Agricultural Sciences