Growth, specific gravity, and wood fiber length of Acacia mangium, Acacia auriculiformis, artificial acacia hybrid clones, and combinations, which were planted in a trial forest in Bavi, Vietnam, in July 2001, were examined. The radial variations from pith to bark were investigated to clarify the effect of genetic factors on these traits. Superiority of hybrids over their parents ranged from 36.3% to 41.6% for diameter, from 20.0% to 25.3% for height, from 6.9% to 20.7% for specific gravity, and from 6.1% to 12.8% for wood fiber length. The hybrid possessed heterosis in diameter, height, specific gravity, and wood fiber length regardless of whether the female parent was A. mangium or A. auriculiformis. The profiles of wood fiber length and specific gravity in the radial direction were similar for all the trees investigated. Wood fiber length was initially 0.5-0.6 mm near the pith and then increased slowly, finally reaching 1.0-1.2 mm near the bark. The specific gravity of acacia increased from 0.49-0.58 near the pith to 0.63-0.74 near the bark. From a relative distance of 30% from the pith, the specific gravity increased slightly and seemed to be stable. The relations among tree diameter, specific gravity, and wood fiber length were fair and could be represented by positive linear regression formulas. Hybrids for which A. auriculiformis was the female parent and A. mangium was the male parent had a faster growth rate and longer wood fibers than the inverse hybrids.
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