Climate change will accelerate forest damage in Finland.

“Our forests have long benefited from sheltered conditions. Unfortunately, this means that most changes will be for the worse, and the risk of forest damage will increase,” says Juha Honkaniemi, a researcher at Natural Resources Institute Finland.

Climate change is expected to increase overall rainfall in Finland. Although trees need water, the additional drops will come during the wrong season – in winter. If the soil is watery, trees fall over more easily in winter storms. Frost has traditionally bound tree roots to the ground in winter, but a warmer climate will weaken this protective element.

“Winds and storms are not expected to change significantly, but changes in the soil mean that trees will be less resistant to winter winds.”

Instead of harsh freezing temperatures, future winters will be characterised by temperatures oscillating around zero. This means heavy snow or sleet that accumulates and freezes on the branches. Trees will be damaged and may fall over.

“Snow damage is already the most significant observed cause of forest damage, and it will become more serious in the next few decades. Of course, if global warming continues, the amount of snow will ultimately decrease.”

Everything affects everything

“In forest damage, things are indirectly linked. Everything affects everything,” says Honkaniemi.

More dry spells are expected in summer, which will restrict tree growth. Drought also causes stress to trees, making them less resistant to pest insects. In turn, a warmer climate will increase the number and activity of all insects.

According to Honkaniemi, fungal diseases will not automatically benefit from climate change. While some fungi will win, others will lose.

Our main fungal disease, root rot, produces spores and spreads through its root system when the temperature is above zero. Although climate change benefits it, this does not really help it spread to new areas.

“The spores of root rot propagate around the forest through stump surfaces. Fungi are most frequently found in areas where forestry has the longest history, meaning Southern Finland and the west coast up to Vaasa. They propagate slowly, because spores travel only a few hundred metres at a time,” says Honkaniemi.

Forest structure prevents fires

Climate warming makes it easier for non-native species and invasive species to find their way to Finnish forests, but Honkaniemi believes they will have a minor impact on our main tree species.

Roe deer and white-tailed deer can cause great damage to trees, but their populations in Finland are the result of our hunting permit policy rather than climate conditions.

In 2018, forest fires caused widespread damage in Sweden. Finland has so far been spared from similar incidents.

“In Finnish forests, fires are slowed down by the dense network of forest roads, efficient monitoring and the mosaic structure of forests – we don’t have as large continuous patches of forest as Sweden,” says Honkaniemi.

“Drought will probably cause more forest fires here in the future, at least to some extent.”

Forest growth to accelerate in many places

Climate change is not solely detrimental to forest owners. In fact, the basic mechanisms of climate change are favourable to growth. A warmer climate means a longer growing season, and an increased amount of carbon dioxide in the atmosphere accelerates photosynthesis.

“However, in northern coniferous forests, the amount of nitrogen available in the soil often becomes the bottleneck. This may prevent trees from benefiting from the increased carbon dioxide,” Honkaniemi points out.

It is difficult to estimate the net impact of disadvantages and benefits. Natural Resources Institute Finland is running several projects, trying to model forest damage caused by climate change while considering the general accelerating effect that climate change has on forest growth.

Honkaniemi points out that the models involve generalisations and Finland as a whole.

“From an individual forest owner’s perspective, much may depend on sheer luck. Many may see their forest grow better than ever, while those who suffer damage, will be hit hard.”

Text Antti Kivimäki
Photo Vastavalo/Vesa Greis

This text is an abbreviated version of an article published in Metsä Group’s Viesti 1/2023.