How Bark Beetle Attack Changes the Tensile and Compressive Strength of Spruce Wood (Picea abies (L.) H. Karst.)

Abstract

Since 2014, forestry in the Czech Republic has been significantly affected by a bark beetle outbreak. The volume of infested trees has exceeded processing capacity and dead standing spruce (Picea abies) remain in the forest stands, even for several years. What should be done with this bark beetle wood? Is it necessary to harvest it in order to preserve the basic mechanical and physical properties? Is it possible to store it under standard conditions, or what happens to it when it is “stored” upright in the forest? These are issues that interested forest owners when wood prices were falling to a minimum (i.e., in 2018–2019) but also today, when the prices of quality wood in Central European conditions are rising sharply. To answer these questions, we found out how some of the mechanical properties of wood change in dead, bark beetle-infested trees. Five groups of spruce wood were harvested. Each of these groups was left upright in the forest for a specified period of time after bark beetle infestation, and one group was classified as a reference group (uninfested trees). Subsequently, we discovered what changes occurred in tensile and compressive strength depending on the time left in the stand and the distance from the center of the trunk. When selecting samples, we eliminated differences between individual trees using a CT scanning technique, which allowed us to separate samples, especially with different widths of annual rings and other variations that were not caused by bark beetle. The results showed the effect of log age and radial position in the trunk on tensile and compressive strength. The values for tensile strength in 3-year infested trees decreased compared to uninfested trees by 14% (from 93.815 MPa to 80.709 MPa); the values for compressive strength then decreased between the same samples by up to 25.6% (from 46.144 MPa to 34.318 MPa). A significant decrease in values for compressive strength was observed in the edges of the trunks, with 44.332 MPa measured in uninfested trees and only 29.750 MPa in 3-year infested trees (a decrease of 32.9%). The results suggest that the use of central timber from bark beetle-infested trees without the presence of moulds and fungi should not be problematic for construction purposes.