To identify hybrid-specific differences in developmental response to mechanical perturbation (MP), we compared the effects of stem flexure on several morphological and mechanical properties of two Populus trichocarpa Torr. & A. Gray x P. deltoides Bartr. ex Marsh. hybrids, 47-174 and 11-11. In response to the MP treatment, both hybrids exhibited a significant increase in radial growth, especially in the direction of the MP (47-174, P = 0.0001; 11-11, P = 0.002), and a significant decrease in height to diameter growth ratio (P = 0.0001 for both hybrids), suggesting that MP-treated stems are more tapered than control stems. A direct consequence of the MP-induced increase in radial growth was a significant increase in flexural rigidity (EI, N mm(2)) in stems of both hybrids (47-174, P = 0.0001; 11-11, P = 0.009). Both control and MP-treated stems of Hybrid 47-174 had significantly greater height to diameter ratios and EI values than the corresponding stems of Hybrid 11-11 (11-11 stem ratios and EI values were 85 and 76%, respectively, of those of 47-174). In Hybrid 47-174, Young's modulus of elasticity (E, N mm(-2)), a measure of stem flexibility, for MP-treated stems was only 80% of the control value (P = 0.0034), whereas MP had no significant effect on E of stems of Hybrid 11-11 (P = 0.2720). Differences in flexure response between the hybrids suggest that Hybrid 47-174 can produce a stem that is more tolerant of wind-induced flexure by altering both stem allometry and material properties, whereas Hybrid 11-11 relies solely on changes in stem allometry for enhanced stability under MP conditions.