Above and below ground responses to competition, and wood property variation in juvenile Sitka spruce clones

Stem growth, biomass partitioning and wood properties vary between genotypes, so information on how these traits interact with each other and are influenced by environment conditions is required. To this end, above and below ground growth, morphology and wood properties of a selection of elite juvenile Sitka spruce (Picea sitchensis (Bong.) Carr.) clones grown in Ireland were investigated in this study.The effects of intra- and inter-clonal competition were assessed for up to three years after planting in a field trial containing ramets (i.e. an independent individual of a clone produced by tissue cultures) planted in multi-clone (mixed) and mono-clonal (pure) plots. Stem height, diameter and biomass partitioning were found to be significant reduced for some clones when planted in mixed plots than in pure plots. Height and diameter results were more heterogeneous and competition was more asymmetrical in mixed than in pure plots. Plot type significantly increased fine root surface area in mixed plots and fine root length in pure plots for two fine root diameter classes for some clones. However, intra- or inter-clonal competition did not result in any significant changes in specific root length, specific root area, root:shoot ratio or root chemical composition.Increased height growth in the most vigorous clone was associated with significantly thinner cell walls and wider tracheids than the less vigorous clones, reducing mean wood density and other indicators of wood quality. The width of the early- and latewood band in each annual ring was a more important determinant of juvenile wood quality than the characteristics of the cells within each band. Radial tracheid width was significantly and positively correlated with ring width and stem height but was negatively correlated with wood density. Wood properties differed greatly between clones, suggesting that there is potential to improve juvenile wood properties through selective breeding while achieving modest gains in height.The results of this study demonstrated that competition-induced morphological changes did not occur either above or below ground for the majority of the studied Sitka spruce clones, but greater size heterogeneity was observed for clones planted in mixed plots than in pure stands. Additionally, the most vigorous clones had less-desirable wood properties than the less vigorous clones, which would reduce their wood quality.