Thinning and fertilization treatment effects on forest stand growth are usually evaluated in terms of volume increment. Measurements of growth response can be biased and misleading if treatment effects on stem form are not considered. A statistical approach, which enables detection of stem form changes and accounting for them in a single volume equation, is demonstrated. This approach is applied to data obtained from a black spruce (Picea mariana (Mill.) B.S.P.) experiment and reveals statistically significant (at 5% level of significance) changes in stem form following heavy thinning and fertilization.