Pinus radiata timber is inherently hindered by low stiffness due to high microfibril angle in the corewood zone. Determining how foresters can manipulate microfibril angle in plantation forests to increase stiffness is of high economic and silvicultural importance. A Nelder systematic spacing design in Canterbury was used to assess the stiffness and tree dimensions of 16-year-old P. radiata (n = 344) and 15-year-old Eucalyptus nitens (n = 211) at stocking levels ranging from 271 stems per hectare to 40,466 stems per ha. Using regression modelling independent variables species, aging and stocking were used to predict response variables outerwood stiffness, diameter at breast height (DBH) and tree height. Stocking, species and physiological aging had a significant effect on modulus of elasticity (MOE). Outerwood MOE significantly increased with increasing stocking for P. radiata up to 17,564 stems per hectare and up to 1,023 stems per hectare for E. nitens (P < 0.001). There was little stiffness gain in planting E. nitens at a greater stocking than 1,023 stems per hectare. By planting P. radiata at 2,505 rather than 823 stems per hectare, stiffness can increase by 14%. Stiffness was 41% greater for E. nitens however, P. radiata stiffness can be significantly (P < 0.001) increased by up to 1.2 GPa by planting physiologically aged clones. Stocking had significant effects on tree dimensions (P < 0.001) for both species: DBH decreased in an exponential trend, whereas tree height decreased more linearly. Physiological aging significantly affected DBH (P < 0.0067) but not tree height (P = 0.31). Wind direction was a significant predictor of MOE and as such standing tree stiffness should be measured on the windward and leeward sides of the tree to account for compression wood. At present, the potential of E. nitens as an alternative structural timber species is limited by its poor sawing and machinability due to growth stresses. However, the increasing trend of MOE seen with increasing stocking demonstrates that foresters have a lot of control over the stiffness of a tree crop through the choice and manipulation of stocking, seed stock and species.