Growth-vigour differentiation in whole-stem axial form and released growth strain of plantation Eucalyptus

Background Growth-vigour classes in even-aged plantations are usually distinguished by attained size, yet size alone does not show whether social differentiation extends to whole-stem construction and xylem mechanical state. We tested whether breast-height form represents the stem as a whole, whether axial geometry and released growth strain differ among vigour classes, and whether axial descriptors add information beyond tree size and local form. Results Twenty-four trees representing four growth-vigour classes in an 8-year-old plantation of Eucalyptus urophylla × Eucalyptus grandis were measured repeatedly along the stem, and released growth strain was measured in four cardinal directions at breast height. Breast-height form showed little agreement with the mean state at the other sampled heights (Lin's concordance correlation coefficients = 0.025–0.114). Cross sections became more regular upwards, while normalised major- and minor-diameter trajectories differed among vigour classes. Mean absolute released growth strain decreased from class A to D, whereas its within-tree standard deviation, range and coefficient of variation increased. In hierarchical models, adjusted R² for mean absolute strain increased from 0.225 with tree size to 0.585 after breast-height form was added and to 0.890 after axial form was included; leave-one-tree-out Q² increased from − 0.063 to 0.407 and 0.860. Axial form did not improve models of strain dispersion. In six purposively selected trees, higher strain tended to coincide with lower microfibril angle, higher cellulose crystallinity and variation in fibre-wall and relative FTIR traits. Conclusions Growth-vigour differentiation was expressed not only in attained size but also in whole-stem axial organisation and released-strain pattern. Repeated axial measurements captured biological variation that was poorly represented by a single breast-height section and added information specifically about mean strain magnitude rather than circumferential heterogeneity.