As Europe moves towards more sustainable construction, there is an increasing demand for structural timber. The purpose of this research is to explore new forestry management strategies, which will produce an optimum balance of the quantity and quality of structural timber over a minimum period of time. The focus is on timber from Sitka spruce, which is the most important Irish commercial forest tree species. Planting density is an important factor affecting both structural timber quantity and quality. Trees with wider spacing get more light and grow faster, which affects annual ring width and knot size. These, in turn, affect the bending strength, modulus of elasticity and density of timber, which are the grade determining properties according to EN 338:2003. The current standard planting density in Ireland is 2500 stems/ha and the timber produced meets the requirements for the C16 strength class. For Irish-grown Sitka spruce timber, modulus of elasticity is the critical grade determining property, which dictates the strength class. This study has two aims, first to explore the possibilities of improving the strength class by increasing the planting density and second, to explore the possibility of keeping the same class while increasing the quantity of structural timber by reducing the planting density. The effects of different planting densities, ranging from 1550 to 3700 stems per hectare, on the structural properties of Sitka spruce timber originated from a forest in County Leitrim are examined. Overall, 72 trees were felled and cut into logs. Logs were non-destructively tested using acoustic tools and subsequently processed into structural timber. The timber boards were then tested using three different types of strength graders in order to examine the difference in timber modulus of elasticity between different planting densities. The results showed statistically significant effects of different planting densities on the variation in timber modulus of elasticity.
Irish grown sitka spruce, planting density, structural timber performance
 Wardle, P.A., Spacing in plantations. Forestry, 40, pp. 47–69, 1967. https://doi.org/10.1093/forestry/40.1.47
 ROI Forest Sevice, Ireland’s forest - Annual Statistics, 2014.
 Macdonald, E. & Hubert, J., A review of the effects of silviculture on timber quality of Sitka spruce. Forestry, 75(2), pp. 107–138, 2002. https://doi.org/10.1093/forestry/75.2.107
 Ward, D. & Gardiner, J.J., The influence of spacing on tracheid length and density in sitka spruce. Irish Forestry, 33(1), pp. 37–56, 1976.
 Gardiner, J.J. & O’Sullivan, P., The effect of wide espacement on wood density in sitka spruce. Irish Forestry, 35(1), pp. 45–51, 1978.
 Javadi, Z., MacSiurtain, M. & Gardiner, J.J., The effect of tree espacement upon wood density in sitka spruce. Irish Forestry, 40(2), pp. 92–97, 1983.
 Joyce, P.M. & OCarroll, N., Sitka Spruce in Ireland. Dublin: Coford, 2002.
 Moore, J., Achim, A., Lyon, A., Mochan, S. & Gardiner, B., Effects of early re-spacing on the physical and mechanical properties of Sitka spruce structural timber. Forest Ecology and Management, 258(7), pp. 1174–1180, 2009.
 Hendrick, E., The establishment, growth and stability of Sitka spruce (Picea sitchensis (Bong.) Carr.) in Ireland with particular reference to wet mineral soils, NUI Dublin, 1999.
 Brazier, J.D. & Mobbs, I.D., The influence of planting distance on the structural wood yields of unthinned Sitka spruce. Forestry, 66(4), pp. 332–352, 1993. https://doi.org/10.1093/forestry/66.4.333
 Ni Dhubháin, A.N., Magner, D. & Nieuwenhuis, M., Juvenile wood in Irish grown Sitka spruce and the impact of rotation length. Irish Forestry, 63(1–2), pp. 26–36, 2006.
 Schaible, R. & Gawn, L.J., Variation of timber strength of Fast Grown Unthinned Sitka Spruce in Northern Ireland. Irish Forestry, 46(1), pp. 43–50, 1989.
 CEN, EN 338:2003 - Structural timber - Strength classes. 2003.
 Ross, R.J., Nondestructive evaluation of wood: second edition,” General Technical Report FPL-GTR-238. Department of Agriculture, Forest Service, Forest Products Laboratory., Madison, WI, U.S., p. 169, 2015.
 Macdonald, E., Mochan, S., Connolly, T. & Forest, O.F., Protocol for stem straightness assessment in sitka spruce. Information Note-Forestry Commission, pp. 1–8, 2000.
 COFORD and IFER, TreeModel - Irish Stem Profile & Single Tree Volume Equations, 2013.
 Unterwieser, H. & Schickhofer, G., Influence of moisture content of wood on sound velocity and dynamic MOE of natural frequency- and ultrasonic runtime measurement. European Journal of Wood and Wood Products, 69(2), pp. 171–181, 2011. https://doi.org/10.1007/s00107-010-0417-y
 CEN, EN 14081-4:2009 - Timber structures - Strength graded structural timber with regular cross section - Part 4: Machine grading - Grading machine settings for machine controlled systems, 2009.
 Hoaglin, D.C. & Iglewicz, B., Fine tuning some resistant rules for outlier labeling. Journal of American Statistical Association, 82, pp. 1147–1149, 1987. https://doi.org/10.1080/01621459.1987.10478551
 Simpson, H. & Denne, M., Variation of ring width and specific gravity within trees from unthinned Sitka spruce spacing trial in Clocaenog, North Wales. Forestry, 1997.
 Rais, A., Poschenrieder, W., Pretzsch, H. & van de Kuilen, J.W.G., Influence of initial plant density on sawn timber properties for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Annals of Forest Science, 71, pp. 617–626, 2014. https://doi.org/10.1007/s13595-014-0362-8
 Hendrick, E. & Murphy, G., The effect of fast growth on structural yields of SS - a study on a yield class 30 stand - Report No. 8/91. Coillte Research and Development, Bray, p. 20, 1991.