A recent article published in DN writes that after an extremely hot and dry summer in 2018, the dreaded spruce bark beetle has caused the largest outbreak in Swedish forests ever. The spruce bark beetle gnaws away billions of euros worth of values by killing spruce trees. But how did the outbreak get so big?
Climate change is part of the explanation, but there are also other factors. One of them is forestry. It started with the summer of 2018, a record summer. The drought and heat caused aspen leaves to yellow in July. Spruces thirsted for water.
When spruce trees are dehydrated, they find it harder to produce resin and defences to ward off attacks. It can 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 quite 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 ever recorded in Sweden. According to SLU, 31 million cubic metres of spruce have been killed in Götaland and Svealand so far, which corresponds to 74 percent of the total volume killed during the six outbreaks that have affected Sweden since 1961.
Deciduous trees are unprofitable and the result is that most forest owners in southern Sweden have planted only spruce in their clearings. 30-40 years after planting, these spruce stands are now very attractive to the spruce bark beetle," says Gunnar Isacsson, ecologist at the Swedish Forest Agency.
In large natural forests, the spruce bark beetle normally kills only a few trees, or groups of five to ten trees every few years. However, in even-aged spruce plantations - with no pine or deciduous trees - the spruce bark beetle can increase dramatically.
Another factor contributing to the outbreak is forestry strategies. Traditionally, dense planting of spruce in clear-cuts has been favoured, leading to monocultures of individual trees of the same age and species. This has created ideal conditions for the spruce bark beetle to spread and multiply rapidly. Instead of having a diverse forest with several tree species and different ages, homogeneous and vulnerable forest environments have been created.
In addition, clear-cutting has been used, which means that all the trees in an area are cut down at once. This leaves the forest full of leaves and needles that fall down and are not removed, creating a favourable climate for the spruce bark beetle to survive and multiply. At the same time, the natural enemies of the spruce bark beetle, such as woodpeckers and fungi that thrive in dead wood, also disappear.
All these factors have contributed to the spread of the spruce bark beetle, which has caused so much damage in Swedish forests. Preventing future outbreaks requires a change in forestry strategies, with a focus on a diversified forest with several tree species and different ages, and a move away from clear-cutting and towards customised methods.
