LAUSR

The British forest industry is limited to a few species, and particularly reliant on Sitka spruce (Picea sitchensis (Bong.) Carr). Recent outbreaks of pests and diseases, and uncertainties about species suitability with climate change have raised concerns of the reliance on those few species to provide timber in G.B., leading tree growers and the timber industry to consider planting a wider range of species. This Ph.D. studied the properties of four minor species that for a long time have garnered the attention of foresters to diversify the forest resource in G.B. The study can be considered as the first of its kind on homegrown timber of: noble fir (Abies procera Rehd.), Norway spruce (Picea abies (L.) H. Karst.), western red cedar (Thuja plicata Don ex D.Don) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Material was obtained from even-aged pure species plantations at three growing regions. Different standing tree measurements were taken, and 108 trees were felled for production of structural timber and small clear pieces. The three key properties for timber grading were investigated. The characteristic value for stiffness (mean) per forest ranged from about 6.5 to 9.5 kN/ mm. The characteristic value for bending strength and density (5 percentile) ranged from 14 to 22 N/mm and from 315 to 410 kg/m, respectively. To avoid the likely influence of age in the quality, the material was restricted to the common 45-year rotation length of British-grown Sitka. The four species produced high yields of C16, the most widely used strength class for construction in G.B. The performance of Norway spruce and western hemlock was comparable to Sitka spruce and less prone to twist due to drying. Although species had an influence on the variation of properties, most of it was attributed to the radial differences from the pith outwards. The use of clears allowed modelling variation of properties with age without the complicating influence of defects, and with a higher detail compared to structural timber. Stiffness showed an exponential increase with age in the four species. Bending strength showed a linear pattern within the range of years studied, whereas density decreased from the pith to minimum values around ring 11, followed by a gradual increase in the following rings. For prediction of wood quality, the thesis also assessed the use of non-destructive techniques in clears, sawn timber, logs and standing trees, mainly for stiffness prediction. Besides confirming the reliable use of portable acoustic tools in sawn timber, the thesis looked at the use of these devices in standing trees addressing the variation of wood properties within a tree. Specially interesting was to found that measuring distances of two or three metres, rather than the commonly used one metre, offered more reliable results, most likely due to a change in the wave propagation. The full picture of the potential of the four species for timber production was completed by studying the tree architecture. Straightness, slenderness and crown ratio were investigated, but particular attention was given to the taper and branch characteristics. Taper functions, that allow estimations of the volume in a tree, and between any specified heights, were developed using the diameter at breast height, distance along the stem and total tree height. The taper profile for noble fir produced the largest volume for sawlogs, whereas western red cedar described the smallest. Norway spruce and western hemlock described an almost identical taper profile. Branches cause the presence of knots, and therefore have an influence on the wood properties. Models were built for number, diameter and angle of branches based on the large variation found with height in the stem. The number of branches was inversely related to the branch diameter, with Norway spruce and noble fir having more branches for the same stem length than western red cedar and western hemlock. This thesis concluded that the four species investigated can contribute to diversify the timber industry in Great Britain. Future work should produce grading machine settings and address the variation of timber quality and merchantability under different silvicultural regimes.