© 1979 by Institute of Chartered Foresters
Fomes annosus Butt-Rot of Sitka Spruce
II. Loss of Strength of Wood in Various Categories of Rot
Forestry Commission Research and Development Division Alice Holt Lodge, Surrey
The results of static bending tests on small (150mm × 10mm × 10mm) green samples of sound, stained, incipiently decayed and decayed Sitka spruce wood from 18 naturally infected and 8 healthy trees showed that there were marked differences in the Modulus of Rupture (M of R) and Modulus of Elasticity (M of E) of wood in the various categories of rot, but that differences in density were slight. However, when the underlying systematic distribution of strength properties within stems of healthy Sitka spruce of this age (45 and 47 years old) were taken into account by a method involving multiple regression analysis, estimated losses in strength due to the decaying action of Fomes annosus were not so marked. Thus, stained wood was not significantly reduced in strength compared to healthy wood. Incipiently-decayed wood, which was not readily distinguishable from sound wood, sustained an average 3 per cent loss in M of R and 11 per cent loss in M of E. In contrast, the average M of R of wood in the decay stage was reduced by 26%, and the average M of E by 39%. There were no significant reductions in density in wood in any of the various categories of rot. Factors affecting loss of strength of wood in the various rot stages are considered, and the uses to which infected wood may be put are briefly discussed.
The method used to estimate strength loss by taking into account the systematic variation in strength properties of Sitka spruce wood is, as far as is known, new and is therefore described in some detail.