Above- and below-ground biomass partitioning and fine root morphology in juvenile Sitka spruce clones in monoclonal and polyclonal mixtures

•Biomass partitioning differed significantly between the two most productive clones.•Deployment type significantly influenced fine root morphology.•Above- and below-ground relationships differed significantly by deployment type.•Relationships between root variables were weaker and less significant in mixed plots. Six juvenile Sitka spruce (Picea sitchensis (Bong.) Carr) clones were grown in monoclonal (pure) and polyclonal mixtures (mixed) in a field experiment to assess effects of clonal diversity on above- and below-ground growth, biomass partitioning and fine root morphology. Shoot height and root collar diameter were measured twice annually and after the second year 48 clones were destructively harvested. Fine roots were divided into 10 diameter classes, for each of which morphological characteristics were calculated from digitized images. Competitive interactions arising from deployment type were found to significantly increase fine root surface area in mixed plots and fine root length in pure plots for two fine root diameter classes for some clones. Competitive interactions were also found to significantly alter relationships between above- and below-ground variables and reduce the strength and significance of relationships among root traits in mixed plots. Significant inter-clonal variation for many above- and below-ground parameters was also observed. The two most productive clones, in terms of height and diameter growth, differed significantly in biomass partitioning suggesting that a high degree of variation occurs among Sitka spruce clones, which may affect stand productivity and uniformity.