Edinburgh Napier University

  Concerns about changes in the quality of the maturing British spruce resource (principally stiffness) have raised doubts about maintaining strength grading pass-rates. Acoustic (or stress wave) instruments provide a non-destructive measurement of stiffness and are increasingly used to classify/segregate forests, trees and logs. A fundamental assumption in the use of acoustic instruments is that of constant density, yet there is little understanding of density variation within the British spruce resource. Understanding this variation is essential for understanding the accuracy of acoustic instruments, which can affect how and when they should be used. The extent of variation in density was determined experimentally from a range of sites with contrasting silviculture and environments, and trees within-site were chosen to reflect the extremes of growth rate. Variation within tree (important for log resonance measurement) was found to depend both on dominance class (i.e. relative diameter) and height within the tree. The density in the outer part of a tree (important for standing tree time of flight measurement) was found to vary with dominance class, distance in from the bark and season. Mean green density profiles of the outer part of the tree show that density ceases to be constant between dominance classes after 10 mm
in from the bark. The effect of this variation could not be quantified because the propagation behaviour of the stress wave within a tree is not fully understood. An examination of wave propagation showed that it did not conform to
behaviour as described in the literature and interaction with both density variation within the tree and with the tree’s boundaries is likely to affect the accuracy of this technique. The utilisation of these instruments was also examined within a sawmill simulation study, to provide an alternative to simply
diverting low stiffness logs to lower value, non-structural products. Exploiting the predictable within-tree stiffness variation and adjusting cutting patterns to avoid the lower stiffness core of lower stiffness logs allowed production of
consistently higher stiffness battens. A reduction in twist and knot severity was also observed, but not at significant levels until the juvenile zone (first ten years) was completely excluded. All batten properties improved with increasing
cambial age (number of growth rings from the pith). Alternative cutting patterns had no effect on the recoverable batten volume from within a log and showed considerable potential to improve value to sawmills.