Is the branch of Viburnum odoratissimum var. awabuki reaction wood? Unusual eccentric growth and various distributions of growth strain

For Viburnum odoratissimum var. awabuki, a pronounced growth promotion was observed on the lower side of the branches, which is a special case in angiosperms. To study the biomechanics of V. odoratissimum branch, we measured the growth strain (GS) in the standing tree based on a cumulative method and a direct method. GSs showed various distribution patterns: for some branches, the values on both sides corresponded to a tensile growth stress typical of normal wood, sometimes a high tensile GS was found in the upper side similar to tension wood, and for other branches, the values on the lower side corresponded to a compressive growth stress. At most measuring positions, a relatively larger growth stress was observed on the upper side of the branches opposite to eccentric growth, which is unusual in angiosperms. To investigate the effect of eccentric growth on the mechanical properties of the branch wood, we measured basic density (BD), microfibril angle (MFA), elastic modulus (EL) and the creep deformation of the samples cut from the upper and lower sides of the branch. BD of the lower-side wood was relatively larger, but the influence of BD on the creep deformation was weak. Although MFA was different between the trees, EL of the lower-side wood was larger that that of the upper-side wood, meaning that MFA does not play an important role in the mechanical properties of the branch wood. Moreover, the creep compliance curves showed that the upper-side wood had a low elasticity and a high fluidity whereas the lower-side wood had a large elasticity and a low fluidity. These results mean that the stable growth of the branches in V. odoratissimum should has a unique mechanism different from that in reaction wood.