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Bark beetle infestation found to slow down tree growth early on

Bark beetle-infested spruce trees begin to dry out already before any visible signs of tree mortality appear, according to a recent study from the University of Eastern Finland in Trees, Forests and People.

Led by Dr. Samuli Junttila, associate professor of forest health and Earth observation, a team of researchers conducted dendrometer measurements of 26 bark beetle-infested and 31 healthy spruce trees between 2020 and 2022. The trees studied were from two different sites in Southern Finland, all of them of approximately the same age.

A dendrometer is an instrument used to measure the diameter of tree stems. In this study, dendrometers were used to measure stem diameters at micrometer-level precision every 15 minutes.

The researchers wanted to compare stem diameter variation between healthy trees and those infested by bark beetles. There is natural diurnal variation in stem diameter: during the day, trees transpire more water and are thinner. At night, they are thicker because they retain more water.

"We wanted to capture this diurnal variation because it is important for trees. Tree growth mainly occurs at night when more water is available," Junttila notes.

The researchers found that tree growth slows down once bark beetles infest the tree, i.e., long before growth stops, and the tree dies. Infested trees showed sharp drops in diameter as they began to dry out.

The European spruce bark beetle (Ips typographus) is a bark beetle that primarily causes damage to spruce trees. While the European bark beetle is an important part of the ecosystem and contributes to biodiversity, it can cause significant damage in monoculture spruce forests. Due to the recent hot and dry summers, this has been particularly visible in Central Europe and Sweden. In Finland, warmer temperatures have expanded the European bark beetle's habitat, accelerated their reproduction, and consequently increased forest damage caused by bark beetles.

Since commercial spruce plantations are abundant in Central and Northern Europe, researching, monitoring and understanding the behavior of the European spruce bark beetle is crucial as climate change progresses.

"However, there was plenty of variation between individual trees. This may be influenced by the number of bark beetles living in the tree, or by the amount of other stress the tree experiences," Junttila concludes.

Led by Associate Professor Junttila, the Global Ecosystem Health Observatory, GEHO, at the University of Eastern Finland is developing advanced remote sensing methods for the detection and monitoring of bark beetles and other forest damages. According to Junttila, the study shows that remote sensing methods alone aren't enough to detect the effects of bark beetles on trees sufficiently early on for pre-emptive actions.

"On-site monitoring is needed as well, supported by research exploring which areas are particularly susceptible to bark beetles," Junttila says.