Within-stem variations in the mechanical properties of 17–19-year-old Melia azedarach planted in two sites in northern Vietnam were examined by destructive and nondestructive methods. Wood samples were collected from 10, 50, and 90% of the radial length from pith on both sides (North and South) at 0.3, 1.3, 3.3, 5.3, and 7.3 m heights above the ground. The mean values in whole trees of wood density (WD), modulus of rupture (MOR), modulus of elasticity (MOE), and dynamic modulus of elasticity (Ed) at 12% moisture content were 0.51 g/cm3, 78.58 MPa, 9.26 GPa, and 10.93 GPa, respectively. Within the stem, the radial position was a highly (p < 0.001) significant source of variation in mechanical properties. MOR, MOE, and Ed increased from pith to bark. WD had a strong positive linear relationship with both MOR (r = 0.85, p < 0.001) and MOE (r = 0.73, p < 0.001). This suggests that it is potentially possible to improve mechanical properties through controlling WD. MOR had also a strong linear relationship with Ed (r = 0.84, p < 0.001). This indicates that Ed is a good indicator to predicting the strength of wood if the density of measured element is known. Besides, the stress wave method used in this study provides relatively accurate information for determining the stiffness of Melia azedarach planted in northern Vietnam.
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