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

Abstract

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.