Degradation of Norway spruce (Picea abies) heartwood and sapwood during 5.5 years’ above-ground exposure

Abstract

Differences in durability between heartwood and sapwood of Norway spruce [Picea abies (L.) Karst.] were investigated to determine wood qualities most favourable for use in outdoor constructions above ground. Trees grown on sites with either good or poor access to water were used. Seventy-eight specimens measuring 20 × 50 × 300 mm³ separated into heartwood and sapwood, half untreated, half painted, were exposed horizontally outdoors above ground for 5.5 years with the pith side up and the bark side down. Crack length and crack number were measured. Fungus growth and surface changes were visually estimated. Fungus type was determined by microscopic analysis. The main finding was that spruce heartwood had fewer and shorter cracks and less surface-discolouring fungus growth than sapwood. This was valid for both painted and untreated wood. After 2 years’ exposure, the cracks in sapwood (upper surface) were more than three times longer and about five times more numerous than in heartwood for both painted and untreated boards. Microscopic study showed that surface discoloration was due mainly to Aureobasidium pullulans, together with a few other discolouring fungi. After 5.5 years, initial decay was established on the surface and in the end grain of four untreated test objects.

Keywords:

• Cracks
• decay
• heartwood
• outdoor exposure
• Picea abies Karst
• sapwood
• spruce

Acknowledgements

I would like to thank Norrskogs Forskningsstiftelse, The Swedish Agency for Innovation Systems—VINNOVA and the Swedish Forest Industries Federation—Skogsindustrierna for supporting this work. I gratefully acknowledge the following people: Thomas Nilsson, the Swedish University of Agricultural Science (SLU), Department of Wood Science, Uppsala, who evaluated the occurrence of fungi on the test pieces, Tomas Lundmark and personnel at Vindeln's Experimental Forest for helping to select the trees and for help with the fieldwork, and Kerstin Vännman, Department of Mathematics, Luleå University of Technology, for statistical advice.