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.