Five years ago, record-breaking heat and drought led to the yellowing of aspen leaves in July and the thirsting of spruce trees. Dehydrated firs also found it difficult to produce resin and defense substances to fight off attack, making them vulnerable to spruce bark beetles. While it may require up to ten thousand of these insects to kill a healthy spruce, a drought-stressed or weakened one can be defeated by far fewer. The resulting outbreak is now known to be the largest that Sweden has ever experienced, with 31 million cubic meters of spruce killed so far, which is equivalent to 74 percent of the total volume killed during the six outbreaks that have occurred in the country since 1961.
Climate change is partly to blame for the scale of the outbreak, as rising temperatures cause insects to develop more quickly. Some spruce bark beetles in southern Sweden were even able to produce two generations before winter arrived in 2018 due to the extreme heat and drought. This led to more adult bark beetles overwintering until the following year and an increase in attacks at the end of the summer. Another factor is the practice of clear cutting, which favors moose because it creates large areas of leaves and needles at a suitable height for grazing. In response, many forest owners have planted spruce on their clearings, leaving them vulnerable to bark borer infestations.
Protected forests with a continuity of old spruce and other tree species are less likely to suffer from bark beetle outbreaks as they have a more species-rich fauna of natural enemies for the beetles. Hyggeskanter or cleared edge zones can also become gateways for bark borers, with spruce trees in dense forests being particularly susceptible to stress when suddenly exposed to sunlight. Harvesting infested trees during winter months can remove natural enemies, so it is ideal to catch the infestation before it advances. However, spruce bark beetle-killed trees can also be seen as an ecosystem engineer, much like the beaver, with a significant impact on the ecosystem that can benefit many other species, particularly those dependent on dead wood.
It started the record summer five years ago. The drought and heat caused the aspen leaves to turn yellow in July. Spruce trees thirsted for water.
Dehydrated firs find it harder to produce resin and defense substances to ward off attack.
It may take up to ten thousand spruce bark beetles to kill a healthy spruce, but far fewer are needed to defeat a spruce that is drought-stressed or weakened for other reasons.
– We have known for a long time that heat and drought can trigger spruce bark beetle outbreaks. But the magnitude of this outbreak was completely exceptional compared to what we have previously experienced, says Martin Schroeder, professor of forest entomology at SLU.
The outbreak that began five years ago is the largest that has been registered in Sweden. In Götaland and Svealand, according to SLU, 31 million cubic meters of spruce have been killed so far, which corresponds to 74 percent of the total volume killed during the six outbreaks that have affected Sweden since 1961.
How could the eruption be so large?
One factor is climate change. Rising heat causes insects to develop faster. In the summer of 2018, some of the spruce bark beetles in southern Sweden managed to have two generations before winter came. Attacks at the end of summer increased and more adult bark beetles were able to overwinter until the next season.
– The extreme heat and drought in the summer of 2018 was the triggering factor, but that is not the whole explanation, says Gunnar Isacsson, ecologist at the Norwegian Forestry Agency.
Photo: Kjell Mohlin
Since the 1980s, moose populations have increased sharply in southern Sweden. Clear cutting benefits the moose, because it creates large areas with leaves and needles at a suitable height for grazing, he explains.
– The moose eats pine seedlings. At the same time, hardwoods are considered unprofitable. The consequence has been that most forest owners in southern Sweden have only planted spruce on their clearings. 30-40 years after planting, these spruce stands are now very attractive to the spruce bark borer, says Gunnar Isacsson.
In large natural forests the spruce bark borer normally kills only a few trees, or groups of five to ten trees every few years. In spruce plantations of the same age, on the other hand – without elements of pine or hardwood – the spruce bark borer can increase greatly.
In 2021, the timber group AB Karl Hedin sued the state because spruce bark beetles had spread from the Hälleskogsbrännan nature reserve and damaged the company’s adjacent production forest. But Karl Hedin lost in the Land and Environment Court.
Photo: Roger Turesson
Today, it is known that spruce bark beetles in protected nature do not affect outbreaks in general.
In addition to more tree species, protected forests typically have more dead wood – nesting sites for the bark borer’s natural enemies.
– Most people agree that the fir bark borer is mainly produced in production forests. Forests that instead have a continuity of old spruce have a more species-rich enemy fauna for the spruce bark borer, says Lars-Ove Wikars, member of SLU’s expert committee for beetles and works as a nature conservation consultant for the management of protected areas.
Photo: Jeppe Gustafsson/Shutterstock
Hyggeskanter can too become gateways for the spruce bark borer. Spruce trees in an edge zone against a clear-cut risk partly being felled by storms, partly being exposed to sunlight.
This is explained by Bengt Ehnström, founder of SLU’s expert committee for beetles and one of the initiators of the Artdata Bank.
– Spruce trees in dense forests have a coniferous crown with so-called shadow needles. When they are suddenly exposed to sunlight, they are subjected to severe stress. For 50-60 years, I have been able to see how spruce bark beetles easily burrow into clearing edges, and then, in the worst case, eat into spruce stands from there, he says.
Photo: Lukas Jonaitis/Alamy
Many forest owners cut down infested trees when they discover an infestation in the fall. But then it is usually too late, say several experts.
– By then, most of the spruce bark beetles have already left the trees, but their natural enemies remain as larvae. Many species of predators and parasitic beetles live on the spruce bark beetle. If you harvest during the winter months, you remove these enemies, says Bengt Ehnström.
Photo: Markus Varesvuo/TT
Since the outbreak in 2018, it is estimated that the spruce bark borer has destroyed forests worth SEK 14 billion.
Cumulative volume of spruce forest killed by spruce bark beetle
Millions of cubic meters. 1971–2021.
Source: Schroeder & Kärvemo, SLU, 2023. Graphics: DN.
But the spruce bark beetle is not only destructive. In a new project, researchers from SLU will investigate the significance of spruce bark beetle-killed trees for biodiversity.
Research leader Thomas Ranius, professor of conservation biology, believes that the spruce bark borer can be considered an ecosystem engineer – that is, a species with a large impact on an entire ecosystem.
Photo: Bengt Ekman/TT
In that sense, the spruce bark borer is reminiscent of another wood rodent: the beaver.
The beaver can fell a tree in a few hours. Trees killed by bark beetles usually remain standing for 3-6 years after their death – so-called skeletal forests – until they are loosened by decaying fungi and felled by the wind.
– Like the beaver, the spruce bark beetle affects the habitat in a way that is important for many other species. In managed forests, there is a lack of dead wood, which many red-listed species depend on. The question is what significance the skeletal forests have for nature conservation, says Thomas Ranius.
Bengt Ehnström, who among other things wrote the book “I roten på en old stubbe”, believes they can play a big role.
– Often the trees are dead for 5-6 years before they are broken down at the bottom by sticky ticks and blow over. Then they deliver partly a tall stump, partly a lying, dead log, both of which become a smorgasbord for hundreds of species of beetles, parasitic beetles and fungi. It can take 50-60 years before a dead spruce has supplied the last species associated with this decomposition.
Read more:
The record heat causes an attack of the spruce bark beetle
The scientists’ warning: The forest goes from a carbon sink to a source of emissions