This dissertation examined the impact of species, physiological age and spacing on tree form and branching at a Nelder experiment located near Rolleston, Canterbury. Two species were compared, Pinus radiata and Eucalyptus nitens, at a range of stockings from 271 stems/ha to 40,466 stems/ha. Within the P. radiata, two different physiological ages were compared. Stocking and species significantly affected (p-value <0.05) tree height, diameter at breast height (DBH), crown depth, branch mortality, branch angle, branch size and internode length. Only stocking was statistically significant for crown width, and height from the ground was also statistically significant for branch angle and branch mortality. DBH, crown width, crown depth, branch size and branch survival decreased with increasing stocking for both species. Branch angle and average internode length increased as stocking increased for both species, and branch angle and average internode length also increased as you moved away from the base of the tree. DBH, average internode length and branch size were significantly larger for P. radiata across all stockings, however branch mortality and branch angle were significantly larger for E. nitens. Physiological age was not statistically significant for any aspects of tree form or branching examined in this study.

The influence of initial stocking and physiologically aged cuttings (taken from 1-year- old and 5-year-old parents) on corewood modulus of elasticity (MOE) in 6-year-old Pinus radiata D. Don was studied in a Nelder-design experiment in Rolleston, Canterbury. In the same experiment, the influence of initial stocking on MOE in 5-year- old Eucalyptus nitens was also investigated. The study incorporated 19 different stocking levels ranging from 207 to 40,446 stems/ha. Green dynamic modulus of elasticity was assessed in standing trees using the TreeTap stress-wave method over the lower part of the stem (0.3 – 1.9m) for 151 P. radiata trees and 115 E. nitens trees. The interaction between species and stocking significantly influenced MOE (P<0.001). MOE of P. radiata increased by 55% (or 3.9 GPa) between 271 and 40,466 stems/ha, and by 41% (or 2.2 GPa) between 271 and 4370 stems/ha. MOE of E. nitens was also influenced by stocking but the slope was significantly lower indicating that the effect of stocking was less pronounced than for P. radiata. Over the usual range of stockings for E. nitens there was an insignificant relationship between stocking and stiffness (P=0.335). Trees were also assessed for DBH, height, and stem slenderness (height/ DBH). None of these latter variables had a significant influence on MOE after the effects of stocking and species were accounted for. No effects of physiological age of cuttings were detected in this study. The findings of this study highlight the importance of stocking as a tool that forest managers can utilize to regulate corewood stiffness in P. radiata trees. These results also suggest that for E. nitens, where wood stiffness is a priority, forest managers could reduce establishment costs by planting at much lower initial stockings. This study also highlights the superior stiffness of E. nitens in direct comparison with P. radiata, with many trees in the experiment already exceeding stiffness thresholds for structural timber in New Zealand.