Variation of ring width and specific gravity within trees from unthinned Sitka spruce spacing trial in Clocaenog, North Wales

The aim of this work was to quantify patterns of change in ring width and specific gravity within trees of Picea sitchensis with ring number across juvenile and mature wood, in relation to height in tree, original spacing, and crown dimensions. Five trees were sampled from each of three plots on a 52-year-old unthinned spacing trial at Clocaenog, North Wales. The ring width of wood produced in early decades of the plantation was most strongly correlated with original spacing, while that produced in later decades was more strongly correlated with branch diameters of the upper crown. In later decades, trees originally at the widest spacing had higher specific gravity than those originally at closer spacing, presumably associated with self-thinning of the narrower-spaced unthinned plots. Differences in amount of juvenile wood between spacing plots were less marked than those reported from the same plots when harvested 7 years earlier; it is suggested that as trees surviving after self-thinning are likely to be those with a competitive advantage from an early stage, differences between plots in amounts of juvenile wood may become less apparent in trees harvested later in the rotation than in those sampled earlier. Underlying patterns of variation in ring width and specific gravity across the tree were found to be modified by a progressive drift with height in the tree. Linear regressions between specific gravity and ring width also varied in a systematic way; the intercept and slope of these regressions tended to increase with ring number from the pith, and with height in tree at a specified ring number. Equations are given as a basis for quantifying these trends, but more data are needed from other sites to determine the extent to which these equations represent trends inherent to cambial and apical ageing, as distinct from influence of changing environment around the trees.