Effects of genetics on the wood properties of Sitka spruce growing in the UK: bending strength and stiffness of structural timber

Abstract

Summary Mechanical tests were conducted on structural timber from a 37-year-old Sitka spruce ( Picea sitchensis (Bong.) Carr), progeny trial located in Kershope Forest, Cumbria, UK. Values of modulus of rupture (MOR) and global modulus of elasticity (MOE G ) in bending and density were compared between timber cut from four of the eight different seed lots which made up the experiment. Three of these seed lots were open-pollinated progeny of selected plus trees, while the fourth consisted of trees grown from an unimproved collection imported from the Queen Charlotte Islands (QCI) in British Columbia, Canada. The progenies from the plus trees were selected for their contrasting growth rates, stem form and wood density relative to the QCI control. Overall, the timber had characteristic values for density, MOR and MOE G consistent with the requirements for the C16 strength class. A signifi cant difference in timber basic density was observed between two of the seed lots; however, there was no difference in MOR or MOE G between any of the seed lots. Most of the variation in strength properties in the study was attributable to differences between individual trees ( » 40 per cent) and individual pieces of timber from within a tree ( » 50 per cent), with only a small amount ( 5 per cent) due to treatment differences. Results indicate that gains in merchantable log volume that have been achieved due to tree breeding do not appear to have been offset by a reduction in the mechanical properties of timber.