Increasing stocking and competition with weeds significantly increased Hitman estimates of stiffness at the significance level α=0.05. Accuracy of models predicting Hitman from TreeTap measurements can be improved by customizing them for particular silvicultural regimes and diameter at 1.4m (DBH). Controlled factors: genetics, wind sway and fertilizer use, did not significantly influence Hitman estimates of stiffness. Tree height did not significantly influence stiffness estimates, but including DBH in prediction models improved models of stiffness estimates.

Stiffness in 10 year old Pinus radiata stems was studied in an experiment with the following factors: genetics, herbicide/fertilizer use, stocking and wind sway. Acoustic velocity was used as an estimate of modulus of elasticity (MOE) and was estimated using 2 different tools: Hitman, a resonance based tool used on 2m log sections, and TreeTap, a time-of-flight based tool used on 1.2m outer-wood sections of standing trees. DBH and tree height were also recorded for each tree. Green density was measured using submersion in order to use the formula: MOE = green density∗ acoustic velocity²

Stiffness estimates from TreeTap were strongly correlated with Hitman estimates, but were about 30% higher on average. The relationship between stiffness estimates from these tools changed with weed competition and with stocking. No significant difference in stiffness was found between the northwest and the southeast sides of the stems when using the TreeTap tool, and an average value for each tree was used for subsequent analyses.

These findings are similar to those from other studies carried out on different sites, and to a previous destructive sample at the same site. There were a few major outliers, but despite these the final model relating TreeTap and Hitman estimates was significant (P<0.0001).
Weed competition and stocking significantly affected the intercept (P=5.71e-05 and P=1.08e 05 respectively) of a model predicting Hitman values from TreeTap estimates of stiffness.