The specific gravity of wood (WSG), a variable property, has a strong influence on the quality of the wood. Eccentric growth contributes to the variation of WSG within a tree but is poorly understood. To clarify the effects of eccentric growth on WSG variation, two opposite sides of the same tree, representing a differential growth rate, were used for the bivariate modelling of the long-short-axis system. The WSG values on the long and short axes were considered two distinct traits, and their common biological architecture was examined. By jointly modelling the long-short axis, correlations between aspect (spatial correlation) and contemporaneous correlation (within aspect) were examined. The variances of WSG in the long-short axis were different and non-zero. The spatial correlation was -0.03 in the outer wood modules and 0.52 in the core wood modules, averaging 0.25 for the entire WSG profile between the long-short axis. The contemporaneous correlation ranged from 0.79 to 0.86. The results suggest a spatial correlation between the long-short axis and this spatial correlation change with the developmental stage of the tree. The bivariate mixed model method provides the underlying physiological, genetic, and environmental mechanisms related to the long-short axis to explain, clarify, and predict the behaviour of complex wood formation associated with eccentric growth. When young, wood formed on opposite sides may be controlled by the same gene set, but in the outer wood shell, the cambium can be controlled by different and unrelated gene sets that drive WSG variation between aspects. It may be that cambium evolves to meet the needs of a changing crown architecture or local environment over time.