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Winter climate change is affecting the carbon exchange of northern coniferous forests, but the response depends upon reindeer grazing, according to a new study from the University of Oulu, Finland. , published in Science of The Total Environment, compared areas with ongoing reindeer grazing to areas where grazing has been excluded for 25 years at Oulanka and as long as 55 years at Kevo in Finland.

Northern forests store one third of the global carbon and act as sinks for atmospheric carbon dioxide. In addition to trees, understory vegetation and soil play a significant role in regulating carbon sinks. Climate change-induced alterations in snow depth and duration affect the carbon cycle not only in winter but also during the following growing season in summer. At the same time, reindeer grazing affects the carbon cycle, for example, by reducing the abundance of lichens in the understory.

Researchers at the University of Oulu have measured the impacts of snow cover and reindeer grazing on the carbon cycle in northern Finland's coniferous forests during the growing seasons of 2019–2023. The experiments included areas with ongoing reindeer grazing as well as in areas where grazing has been excluded for 25 years at Oulanka in eastern Finland and for 55 years at Kevo in northernmost Finland. In addition, snow experiments of increased and decreased snow depth were carried out at each of these areas.

In the area where reindeer grazing had been excluded for 55 years, shallow snow increased carbon release from the understory and soil. In contrast, deeper snow decreased carbon release at this same location. In areas with ongoing grazing and in the 25-year-old exclosure, carbon release remained stable regardless of changes in snow cover.

It is notable that snow depth did not affect carbon exchange in areas with ongoing reindeer grazing. "Altogether, this could indicate that northern coniferous forests may be relatively well resistant to short-term changes in winter climate," says Doctoral Researcher Noora Kantola.

According to the researchers, it is important to investigate why changes in snow depth affected carbon exchange only in the 55 year absence of reindeer grazing at Kevo. "It is possible that at Kevo, the substantially recovered lichen cover has influenced soil temperature and moisture conditions. Together with changing snow depth, these factors may affect soil decomposers and thereby the amount of carbon released," Kantola continues.

"Our results show that reindeer grazing can buffer ecosystem functions, such as carbon exchange, under changing climate conditions," explains Postdoctoral Researcher Maria Väisänen.

The new study from the University of Oulu provides, for the first time, evidence of the role of reindeer grazing has in shaping winter climate change impacts on the carbon cycle. The results provide perspective on the role of reindeer in sustaining the biodiversity of northern ecosystems. The lichen-mediated impacts of grazing on the understory carbon exchange of winter pastures can be significant," summarizes Professor Jeffrey Welker.

In addition to altering understory vegetation and soil, reindeer grazing can also affect tree carbon sequestration and overall forest carbon balance. At the University of Oulu, ongoing projects are investigating the combined impacts of grazing and long-term climate change trends on tree ring growth and ecophysiology.

The findings of the changes in carbon cycle help estimate ecosystem functions under changing conditions and can also inform land-use planning in coniferous forests where lichen cover has been altered by different land-use practices, such as reindeer grazing and forestry.

The experimental sites are part of EcoClimate system, which investigates the impacts of changing snow conditions on the functioning of northern ecosystems. Previously, reindeer grazing has been found to mitigate the impacts of summer climate change on the carbon cycle in tundra ecosystems.