Boreal Trees Are Spreading Into the Arctic Tundra

In an increasingly warmer world, the Arctic tundra is becoming more forestlike. This process, known as borealization, is particularly widespread in Eurasia and in Arctic mountain regions, where the distance to the boreal (northern) forest is shorter. Many species of grasses and shrubs that can live in both the tundra and the forest are gaining ground in the tundra. This is shown by a new large-scale study of 1,100 sampling sites across the entire Arctic biome.

Consequences

“If this trend continues, it could have a number of consequences. For example, it could accelerate the melting of the permafrost or change the seasonal migration of reindeer. It could also affect the traditional livelihoods of the indigenous peoples of the Arctic, who rely on hunting, reindeer herding, and gathering plants as a part of their subsistence and cultural practice,” says Robert Björk, a researcher on Arctic ecosystems at the University of Gothenburg.

When shrubs and other tree-like species expand, they trap more snow in winter and cover the ground in summer. This changes the ground temperature and can accelerate the thawing of the permafrost. As a result, large amounts of carbon that have been frozen for thousands of years could be released, contributing to global warming. 

Near the tree line

However, the study shows that the effects of climate change are complex and difficult to predict.

“We see that borealization is strongest near the tree line, in warm and humid places, and in areas with limited climate change, which suggests that borealization does not necessarily occur where warming is greatest, but where conditions are most favorable for plant establishment,” says Anne Bjorkman, researcher in plant ecology at the University of Gothenburg.

Most evident in Dalarna

In Sweden, the borealization trend is stronger in the Dalarna mountains in the south than in the northern Abisko mountains, which is a good example of how borealization is also generally progressing. In addition, the ongoing colonization of forest species into the mountain world may have several consequences: 

• Lichens can disappear in the shade of bushes, reducing the food supply for reindeer in the mountains. This can affect the lifestyle of reindeer herders by changing the migration routes of reindeer and reducing access to traditional plants.
• Certain animals such as moose, red foxes, beavers and forest voles may spread to new habitats and thus affect ecosystems. 

Lingonberries spread

The study also analyzed the characteristics that enable certain plants to spread more successfully than others. It showed that short boreal species find it easier to spread in the tundra than taller species. In addition, grasses and shrubs colonized more areas than flowers, partly due to more efficient uptake of nutrients in the soil. Some examples of boreal species that managed to reach a large number of areas are the Bigelow's sedge and lingonberries.

This suggests that a certain degree of adaptation to the tundra habitat – in the form of short-growing and already established species – is still required in order to initially exploit the new habitat created by climate change.

Reference: Criado MG, Barrio IC, Speed JDM, et al. Borealisation of plant communities in the Arctic is driven by boreal-tundra species. Ecol Lett. 2025. doi: 10.1111/ele.70209

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