Researcher Verna Vālmane (Werna Wahlman) of the University of Helsinki explains the signs that point to a risk of damage by the European spruce bark beetle (hereafter also referred to as the bark beetle).
“Heterobasidion parviporum (the fungus that causes root rot in spruce) appears to be one of the most important factors making trees susceptible to European spruce bark beetle damage as well, particularly before the beetles multiply.” V. Vālmane observed that three out of four spruces infected with Heterobasidion had also suffered from the bark beetle. “If you take appropriate action to protect trees from Heterobasidion, you can indirectly help to avoid spruce bark beetle damage too,” she adds.
The facts relating to European spruce bark beetle damage are set out in V. Vālmane's master's thesis, “The effect of Heterobasidion-induced root rot on the risk of Ips typographus infestation in Norway spruce” (Effect of Heterobasidion root rot on Ips typographus infestation risk on Norway spruce). The thesis was submitted to the University of Helsinki in March.
According to the author, an indirect link (a correlation) between Heterobasidion and bark beetles has been established. The thesis is the first field study to investigate the link between these two species directly. Previously there had been only “educated guesses” (educated guesses) on the question. “The fungus that causes the rot weakens the tree's roots, leaving it more vulnerable to wind damage, and wind-thrown trees attract spruce bark beetles.”
Members of the Heterobasidion genus are fungi that are highly dangerous to trees, damaging conifers and causing considerable financial losses every year both in Finland and elsewhere in Europe. They also weaken forest growth and the forest's capacity to sequester carbon dioxide. Another threat to tree health is insect damage, which in 2022 made it necessary to harvest more than 6,000 hectares of forest in Finland. The volume of timber felled because of pest damage has increased.
As is well known, in recent years the European spruce bark beetle has destroyed vast areas of forest in Germany and the Czech Republic.
Insects attack weakened trees
Trees invaded by Heterobasidium are also more vulnerable to insects. “As root rot progresses, the tree becomes less able to take up water, which in turn reduces hydrostatic pressure. This impedes the flow of water from the roots up through the trunk.” Hydrostatic pressure is generated by the water in plant cells. This pressure maintains cell structure and drives the flow of water and nutrients within plants.
Heterobasidium also inhibits resin production. “Trees weakened by stress cannot defend themselves against spruce bark beetles by exuding resin.”
During the study, living spruces and spruces only lightly affected by bark beetles showed lower levels of Heterobasidion infection than dead or heavily infested spruces.
“The observations indicate that the influence of Heterobasidion matters more at the start of a European spruce bark beetle infestation than later on. Once the beetle population has reached sufficient numbers, they can also attack trees that appear healthy.” The study examined 442 beetle-infested spruces across nine different felling sites. A large number of beetle-infested trees without Heterobasidion were also recorded.
“So spruce bark beetle infestation can be explained by other factors as well.” Verna Vālmane explains that, besides Heterobasidion, the study identified two other, albeit weaker, “predictors” of potential damage: less growth in tree diameter over the previous five years, and a larger trunk diameter at breast height.
“The study found a positive correlation between tree diameter and the presence of the spruce bark beetle. The larger the diameter, the greater the likelihood that the tree will be attacked by beetles. This has also been observed in several other studies,” Verna Vālmane explains.
Other factors identified as predictors of potential damage were higher tree density in the stand, drought-induced stress, and increased light levels as a result of thinning the stand.
Mixed forests are better protected
V. Vālmane concluded that Heterobasidion can be countered by scheduling harvesting for the winter and by treating stumps during the warm season, when the fungus releases its spores.
Immediately after a tree is felled, the stump needs to be treated with urea or with spores of the fungus Phlebiopsis gigantea, a competitor of Heterobasidion. The researcher notes that combating Heterobasidion alone is not enough to protect a forest from the bark beetle; other forest management measures are needed as well. “Overall, mixed forests are better protected against extensive damage caused by the European spruce bark beetle,” she stresses.
Seen from another angle, thinning a stand can expose the remaining trees to Heterobasidion. If thinning is too heavy, there will be less shade in the forest. A sudden increase in the amount of light can also cause stress to trees. Forest management always calls for sensible compromises. Appropriate thinning can improve access to water and reduce competition within the stand. Even so, it is not that simple.