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

JOHN R. MOORE1*, SHAUN J. MOCHAN2, FRANKA BRÜCHERT3, ADRIAN I. HAPCA1, DANIEL J. RIDLEY-ELLIS1, BARRY A. GARDINER2 and STEPHEN J. LEE2 1 Centre for Timber Engineering, Edinburgh Napier University, 10 Colinton Road, Edinburgh EH10 5DT, Scotland 2 Forest Research, Northern Research Station, Roslin, Midlothian, Scotland 3 FVA Forest Research Institute Baden-Württemberg, Freiburg, Germany *Corresponding author. E-mail: 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 (MOEG) 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 MOEG consistent with the requirements for the C16 strength class. A significant difference in timber basic density was observed between two of the seed lots; however, there was no difference in MOR or MOEG 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. Introduction Due to its ability to grow on a wide range of sites, coupled with a growth rate exceeding that of other conifers in this country, Sitka spruce (Picea © Institute of Chartered Foresters, 2009. All rights reserved. For Permissions, please email: sitchensis (Bong.) Carr) is the most widely planted commercial conifer species in the UK (Brazier, 1970; Joyce and OCarroll, 2002). Currently, the total area planted in this species is ~730 000 ha (Forestry Commission, 2007). Some 80 per cent Forestry, Vol. 82, No. 5, 2009. doi:10.1093/forestry/cpp018 Advance Access publication date 23 June 2009 Effects of genetics on the wood properties of Sitka spruce growing in the UK: bending strength and stiffness of structural timber 492 FORESTRY These values will then be combined, with appropriate weightings, to generate a multi-trait index value for each progeny, and plus trees will then be ranked according to the index value of their progeny (Lee, 2001). This approach will be used to develop the General Breeding Population consisting of the best 240 plus trees, while the best 40 of these plus trees will be reselected to form the production population. In order to produce trees that yield a greater proportion of structural timber capable of meeting the requirements for the C16 and possibly the C24 strength classes as defined in EN338 (CEN, 2003a), tree breeders have proposed two additional production populations: (1) a high wood density production population and (2) a high stem straightness production population. While tree breeders have assumed that increasing the wood density and reducing branch size will increase the proportion of timber satisfying the requirements for the C16 (and possibly the C24) strength class, there have not been any studies that have compared the mechanical properties of timber from genetically improved Sitka spruce with those of timber cut from unimproved QCI trees. Timber properties have only been compared between four different Sitka spruce provenances growing in Ireland (Treacy et al., 2000). This study found that the MOE of timber did not differ between the selected provenances but that the MOR of timber from the Californian provenance was significantly lower than from the Washington provenance. In 2004, the opportunity arose to fell a mature tree breeding trial and to make detailed measurements on a number of characteristics on the standing trees and logs and then to measure the density, microfibril angle and the mechanical properties of structural timber and small defect-free wood samples cut from the trees. This trial was chosen because it consisted of large plots, each containing sufficient mature trees to represent all the individual treatments including the QCI control, which can often suffer badly later in the rotation due to competition from faster growing trees in adjacent plots (Lee, 1992). The primary purpose of this study was to compare the properties of timber cut from the progeny of selected plus trees with those of timber from an unimproved QCI seed lot and to gain a better understanding of the distribution of these properties within and between trees. This of the predicted doubling in the UK’s wood production by 2020 will be due to an increased outturn of Sitka spruce (SFIC, 2004). Therefore, it is important for the profitability of the UK’s forest industry that home-grown Sitka spruce can gain increased acceptance as a structural timber, as the markets for non-structural end uses such as pallets, packaging and fencing are close to saturation (McIntosh, 1997). For construction uses, three of the key timber quality criteria are dimensional stability (i.e. low distortion), strength (modulus of rupture (MOR)) and stiffness (modulus of elasticity (MOE)) (Kliger et al., 1994). These properties, particularly strength and stiffness, are in turn related to other properties including wood density, knot size, spiral grain and microfibril angle (Cave 1968; Cave and Walker, 1994; Walker, 2006), which have varying degrees of heritability (Rozenberg and Cahalan, 1997). The need to produce good quality sawn timber has been recognized by Sitka spruce tree breeders whose main objective has been ‘to develop breeding populations well adapted to a range of site types, with improved stem form and growth potential and wood qualities satisfactory for the sawn timber market’ (Fletcher and Faulkner, 1972). The UK’s tree improvement programme for Sitka spruce began in 1963 with the selection of ‘plus trees’ that had a combination of superior height, diameter, stem straightness, branching quality and low external grain angle. Approximately 1800 such trees were selected over the following 20 years from stands consisting of material grown from seed that was believed to have originated from the Queen Charlotte Islands (QCI), which are located off the coast of British Columbia, Canada (Lee, 1999). This origin of seed was commonly planted in the UK because early results from provenance tests indicated that it was the most suitable for the bulk of Sitka spruce (...truncated)