In Finland, forest stands damaged by the European spruce bark beetle (Ips typographus) tend to concentrate in groups and form clusters, especially in mature Norway spruce forests close to previous infestations, a new study from the University of Eastern Finland shows.

The European spruce bark beetle is the most damaging insect pest of Norway spruce (Picea abies). With warmer and drier conditions becoming more frequent in Finland, bark beetle outbreaks are expected to intensify across the country, threatening one its most economically important tree species.

The study assessed the impacts of forest stand structure and landscape on the formation of European bark beetle damage clusters in Finland and analysed the role of forest and landscape attributes in shaping bark beetle damage patterns across the southern part of the country. The researchers examined 11.4 million hectares of forestry land using data on salvage logging operations conducted between 2012 and 2020.

The researchers identified bark beetle damage clusters using spatial statistics and analysed which forest characteristics increased the likelihood of a stand becoming part of a cluster, or of a recurrent cluster in the same area over time.

During the study period, clusters of damaged stands first emerged in southeastern Finland and later in the south and southwest. Clusters were more abundant in southeastern Finland, but their recurrence were rare. Stand age and proximity to previous damage increased the risk of a stand to form these concentrations.

“The conditions needed for sustained bark beetle concentration were not consistently met, but when they did occur, they were tied to mature stands and previous outbreaks nearby,” says Doctoral Researcher Alexander Pulgarín Díaz of the University of Eastern Finland. “Timely detection and removal of infested trees remain crucial to prevent the formation of clusters.”

Understanding how forest structure and landscape influence bark beetle damage helps inform risk management and supports the development of more resilient forest management practices under a changing climate.

The study was supported by the Research Council of Finland (UNITE Flagship); the European Union – NextGenerationEU instrument through the Research Council of Finland (Multirisk project); LUMETO Doctoral Programme in Science, Forestry and Technology at the University of Eastern Finland; by EU funding under the Horizon Europe project Precilience; and by Corporación Colombiana de Investigación Agropecuaria - AGROSAVIA.