The effects of planting density on the structural timber properties of 23-year-old Irish-grown Sitka spruce

In Europe, with the move 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... [ view full abstract ]

In Europe, with the move 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 the EN 338:2003 standard. The current standard planting density in Ireland is 2500 stems/ha and the timber produced meets the requirements for the C16 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 possibilities 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 two different types of strength graders in order to examine the difference in timber quality between different planting densities. The results showed statistically significant effects of different planting densities on the variation in timber modulus of elasticity and density.