Radial and axial variations in the longitudinal ultrasonic wave velocity within stem of 17–19-year-old Melia azedarach planted in two different sites in northern Vietnam were experimentally investigated. Wood samples were collected from 10, 50, and 90% of the radial length from pith at 0.3, 1.3, 3.3, 5.3, and 7.3 m heights above the ground to measure ultrasonic velocity (Vu), fiber length (FL), air-dry density (AD), and compressive strength (CS) at moisture content approximately 12%. The average wave velocity for two sites exhibited minimum value near the pith about 3700 m/s. It kept increasing to 4200 m/s at the middle position before remaining constant value forward to outside. In axial direction, the variation of Vu with height was very small and without statistical significance. Vu had a strong positive linear relationship with both FL (r = 0.69, p < 0.001) and AD (r = 0.67, p < 0.001). These findings reveal that the FL and AD greatly influence the within-tree variations in the ultrasonic wave velocity in M. azedarach. Besides, the significant positive linear correlation between CS and Vu suggests that Vu was a useful predictor of the CS for M. azedarach planted in northern Vietnam.
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