Fourteen Norway spruces [Picea abies (L.) Karst.], randomly sampled as 7 fast-grown and 7 slow-grown trees, were used to test whether an increased tree growth rate in circumference affects the intra-ring and inter-ring trajectories of the microfibril angle in the S2 layer of the tracheid wall. Those trajectories describe the fluctuations of the S2 microfibril angle, respectively, from earlywood to latewood within rings and from pith to bark among rings. Using the cross-field pit apertures, intra-ring measures of microfibril angle were made at 11 equally-spaced sampling sites over each of 8 growth rings, following an 11 × 8 doubly repeated measures design with the tree as the 'subject' on which repeated measures were made. All the intra-ring trajectories of microfibril angle decreased linearly from earlywood to latewood, whereas the inter-ring trajectories showed significant year effects. Both types of trajectories are significantly affected by the growth rate after first thinning, as the fast-grown spruces showed a systematically larger microfibril angle and a mean microfibril angle of 290 compared to 21 0 for the slow-grown spruces. Thus, lower tensile and tear strengths of tracheids as well as some modifications of the mechanical properties of solid wood and paper can be expected from Norway spruces growing faster than 2.2 cm/year in circumference.
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