According to a new report from NIBIO - Norwegian Institute of Bioeconomy Research, the European spruce bark beetle has killed several hundred million spruce trees in Sweden and Central Europe over the past five years. A warmer climate will ultimately lead to more damage in Norwegian spruce forests as well. Fewer spruce trees and softer edges can prevent and mitigate future bark beetle outbreaks.
Ecosystems all over the world are threatened by climate change - and this is particularly true for the vast coniferous forests of the Northern Hemisphere. Spruce forests make up as much as a quarter of Europe's productive forests, i.e. forests where harvesting can be economically viable, and 45% of northern boreal forests. These forests are important for biodiversity, carbon storage and raw materials for industry.
However, in recent years, spruce forests in southern Sweden and central Europe have suffered catastrophic bark beetle outbreaks. Areas with a lot of planted spruce are particularly hard hit by the ravages of the European spruce bark beetle.
In just one year (2019), 118 million cubic metres of spruce were killed by the spruce bark beetle. This is 10 times more than Norway's annual harvest. The infestation was triggered by the extremely dry summer of 2018.
In addition to damaging timber production, spruce bark beetle infestations also reduce the forest's uptake of carbon dioxide. When many trees die, it can lead to more flooding, erosion and a change in the nitrogen balance in soils and water bodies. In the Czech Republic, several water treatment plants have run into problems because the forest, which used to help purify the water, is gone.
From harmless to aggressive
The European spruce bark beetle does not usually attack healthy trees. Most of the time, it lays eggs and breeds in dying spruces or fresh timber. But when the beetles become numerous - or the trees are weakened by drought or wind - the system reaches a tipping point where the bark beetle changes its behaviour; it goes from being relatively harmless to aggressive and starts attacking healthy trees.
The first beetles to attack a tree attract other spruce bark beetles using chemical signals called pheromones. This behaviour can lead to colossal outbreaks, killing hundreds of thousands of trees.
Norway has not experienced a major bark beetle outbreak since the 1970s, but there is growing concern - among scientists and forest managers alike - about new outbreaks as the climate warms.
The calm before the storm?
NIBIO researcher Jostein Gohli is studying fluctuations in spruce bark beetle populations. He says we can now experience the "calm before the storm" in Norwegian forests.
"The European spruce bark beetle is probably the most destructive insect species in European forests today. It is an aggressive tree killer that attacks spruce-dominated forests in outbreaks that can last for years."
"As climate change accelerates, even the large spruce forests of northern Europe could be hit by devastating bark beetle outbreaks," he says.
Together with colleagues at NIBIO, Gohli recently published a scientific paper identifying factors that increase the incidence of spruce bark beetles.
The study was based on 17 years of bark beetle trap data from a period without outbreaks, information from the SR16 forest resource map and climate data.
The results show that drought, higher temperatures, the amount of spruce forest and clear-cut edges were the most important factors. Trees left along the edge of a clear-cut are likely to be vulnerable to beetle attack because they are weakened by sudden exposure to sun and wind.
"The spruce bark beetle doesn't usually attack healthy trees, but it does when they become numerous enough. The more spruce bark beetles there are in the landscape, the more likely they are to kill healthy trees and start year-long outbreaks," says Gohli.
Both the amount of spruce forest and the shape and size of clearings can be affected by changes in forest management. Gohli points out that such changes can be crucial to creating more robust spruce forests that can cope with climate change.
Living with bark beetles
Insect infestations can contribute to more diverse forests, providing more light and more dead wood - which is important for biodiversity. The gaps created in the forest canopy also provide opportunities for new trees to grow up in place of fallen giants. However, the huge scale of the ongoing bark beetle infestation in central Europe has forest scientists sounding the alarm.
"Many people still talk about the need for even more forestry in the same form as today. The truth is that even Norway's boreal forests probably need to be directed towards increased stability to avoid huge future losses of bothCO2, biodiversity and money", says insect expert Bjørn Økland, senior scientist at NIBIO and one of the co-authors of the study.
"Other research from NIBIO shows that there is already an increase in CO2 emissions due to damaged, weakened and dead trees in Norwegian forests."
The increasing spruce population in European forests has favoured beetles
So what should a forest owner do?
"Specifically, we recommend planting less spruce in vulnerable areas and focusing more on mixing in other tree species, such as pine and deciduous trees," says Mr Gohli.
"Locally, drought-prone areas are problematic for spruce. But by vulnerable areas we also mean the southern parts of Eastern Norway. Here, there are large volumes of spruce that will face climate warming early."
Gohli points out that there is now more than twice as much spruce in Norwegian forests as there was 60 years ago. In 1960 there were about 200 million cubic metres of spruce, now it is approaching half a billion cubic metres.
More spruce forests have also been the trend in many other countries, but further south in Europe, forest owners are now reducing the amount of spruce as a result of bark beetle infestations. In areas of the Czech Republic where spruce forests have been wiped out, research is now underway to identify other tree species that can be used to establish new forests.
Hedges are important for beetle reproduction
Gohli explains that an alternative to today's clear-cutting is more closed felling, where clear-cuts are small or where more trees of varying heights are left, so that the edge zones are not so sharp. Such forms of felling are collectively known as continuity forestry.
"But closed felling is not suitable everywhere and can also lead to other problems, such as more root rot," he says.
"Continuing with high-density spruce monocultures is a strategy that can yield high returns, but it also carries a high risk of extensive bark beetle damage and long-term economic losses."
To achieve forests that are more resilient to climate change and bark beetles, researchers suggest changing forest structure - that is, the age and species distribution of the forest.
"Natural forests, and other forests that have not yet been clear-cut, usually have a favourable structure. If we replace such forests with single-species spruce plantations, we can ease more bark beetle problems in the future," says Gohli.
Changing the current spruce monocultures and establishing a more multi-layered, multi-species forest will necessarily take time.
"Given that it takes decades from planting to harvesting, we should start changing our forests as soon as possible. Not least because the forests we plant today will grow up to be harvestable in a climate that is likely to be very different from today's," concludes Mr Gohli.
Detecting Outbreaks in a Timely Manner
With the help of drones can quickly detect spruce bark beetle infestations. The main advantage is the time saved, as the drone can find already established infestations and thus predict new infestations nearby.
This saves time that would otherwise have been spent surveying the forest on foot and allows you to focus on areas where damage is likely to be found.
When a tree is infested with the spruce bark beetle, its timber quality may be downgraded due to the marks left by the beetle. In some cases, the tree may be so severely damaged that it is only suitable for pulpwood.
For smaller properties, it is possible to survey the area on foot relatively quickly, but when the property is larger than 100 hectares, it is much more efficient to use drones.
