Mechanical properties of Norway spruce (Picea abies) infested by the bark beetle (Ips typographus)

ABSTRACT

This study considered the mechanical properties of primary blue-stained sapwood of Norway spruce infested by the bark beetle Ips typographus. Nineteen stems from Stammerberg (ZH, Switzerland) were harvested in three different seasons and after different periods of bark beetle outbreaks. Density, Brinell hardness including elastic recovery after load, and key strength properties were tested on blue-stained wood and non-infected controls. Differences in mechanical properties were analysed according to the stand and storage time of the stems after the bark beetle attack. Spruce harvested at a late stage after the bark beetle attack (2–3 months) showed a clear reduction in impact bending strength, a significant loss in surface elastic recovery after indentation during Brinell hardness test, and a striking number of specimens with brittle fracture behaviour during Three-Point-Bending. Light microscopy and the results of the mechanical tests give clear evidence to suggest that wood-degrading fungi are already active in addition to the sap-staining fungi. Especially after larger outbreaks of bark beetles, it must be assumed that sapwood of Norway spruce does not only show the typical blue stain, but also incipient stages of wood degradation by wood-degrading fungi.

KEYWORDS:

• Bark beetle
• blue stain
• mechanical properties
• fracture behaviour
• wood decay

Acknowledgements

The Forest and Wood Research Founding Switzerland (FOWO-CH), the Swiss Conference on Forests, Wildlife and Landscape (KWL), and Empa financially supported this work. We thank the Forest District Stammheim and the Sawmill Konrad Keller AG in Switzerland for providing and processing the wood. We also thank the following Associations of Swiss forest and wood industry for their content-related support: Holzindustrie Schweiz, VSH, and Wald Schweiz.

Disclosure statement

No potential conflict of interest was reported by the authors.