article by the Science for Environment Policy

An early-warning system developed by researchers could help forest managers in Europe predict which trees are at greatest risk of barkbeetle infestation. The study looked at the probability of bark-beetle outbreaks on two important conifer-tree species in Slovenia: the Norway spruce (Picea abies) and silver fir (Abies alba). It found that high temperatures, and extreme weather linked to climate change - including droughts and ice storms - weakened trees, making them more vulnerable to attack by bark beetles.

Bark beetles are tiny insects that live and reproduce under the bark of trees, weakening and even killing the trees in the process. While bark beetles have existed for
a long time, the intensity of infestations has increased notably over recent years, due in part to climate change.

For example, since the year 2000, bark beetles have ravaged 85 000 square miles of forest in the western United States - an area the size of Utah. In Europe, the Czech Republic, Poland, Germany, and Slovakia are all experiencing intense beetle attacks on Norway spruce.

In Slovenia - the location of this study - bark-beetle infestations have decimated spruce forests in the majority of Slovenia since 2014. The outbreaks are thought to be linked to climate change and extreme weather events in the region, including droughts, wind and ice storms. The epidemic has left a deep scar on Slovenian forests, leading to almost 10 000 hectares of deforested area.

The impact on the economy and environment is severe. Forests conserve water, capture carbon, and help cool down the landscape. However, central Europe’s spruce forests are also a key source of income for the timber industry. The Czech Republic, one of the countries most affected, predicts that the pest has cost it $1,70 (over €1,57) billion in damages this year.

Although felling of infected trees can limit the spread of the disease, forest managers can be slow to react, leading to costly delays. In this study, however, researchers developed a short-term-forecast model that can be used to predict the rate of bark-beetle outbreaks on Norwegian-spruce and silver-fir forests in Slovenia. To do this, they collected 20 years of climate data, as well as data on the location and geography of trees, local soil conditions, and the rate of tree felling due to infestation.

They found that for both spruce and silver fir, outbreaks of beetle infestation were closely linked to extreme weather events. For instance, large-scale droughts in 2003, 2012 and 2013, and an ice storm in 2014 were all closely followed by outbreaks of bark beetle. There was also a correlation between the number of trees weakened or felled in the previous year - either due to adverse weather or beetle infestation - and beetle outbreaks in the current year.

Another factor associated with bark-beetle outbreaks was high temperatures, with trees lying at lower altitudes, where the climate is traditionally warmer, at higher risk. Trees at higher altitudes and on steeper slopes, on the other hand, were protected.

Finally, the density of forests and soil characteristics also influenced the probability of bark-beetle outbreaks, with trees in dense woodland at greater risk, while those planted in deeper soils with increased availability of minerals were less likely to succumb to infection.

Despite the similarities between the two species of tree, the model revealed key differences in how each will be affected by bark-beetle outbreaks in the future. For example, the forecast predicts that Norway spruce - found throughout Slovenia - will be at a much greater risk of infection in the future than silver fir trees.

This could be due, in part, to the role of biodiversity in protecting trees from infestation. Silver firs are only found in biodiverse forests, and are not planted outside of their range, reducing the probability of attack. The Norway spruce, on the other hand, is planted in large monocultures in the lowlands, reducing its vigour and making it more susceptible to infection; however, outbreaks also occur in the higher mountains, where it is an indigeneous species.

Whatever the individual case, the findings of the study suggest that the dynamics of bark-beetle outbreaks will be strongly affected by climate change in the future, especially in areas where more frequent and severe droughts are predicted. This is because the increased prevalence of dry and hot summers will weaken the trees’ natural defences.

Overall, the study has clear policy implications, as an early-warning system for detecting trees most at risk could be rolled out across Europe, potentially saving millions of euros in lost revenue. While the model focuses only on Slovenia, it is likely that when calibrated, the approach could be also used in other countries with similar environmental conditions. There may be possible pitfalls preventing the application of the warning system, however; much of the meteorological data needed for the model to work is only released in February/March, just a couple of months before the onset of the beetle swarming season, leaving little time to monitor the at-risk trees. Policymakers could help by encouraging increased monitoring and digitising of data on forest dynamics.