Strain distribution, growth eccentricity, and tension wood distribution in the plagiotropic and orthotropic branches of Koelreuteria henryi Dummer

Key message With the diversified distribution of growth strain and tension wood, Koelreuteria henryi branches function in a different way from the trunk in maintaining the tree architecture. The tree branches, as well as trunks, function in keeping tree biomechanical equilibrium; however, researches regarding the reorientation of branches have received less attention than those of trunks. The presented paper aims to discriminate the biomechanical behavior of branches from leaning trunks. We thus investigated the development of growth strains, distribution of tension wood, and eccentricity on the branchwood of Koelreuteria henryi. The results revealed the unusual distribution of released growth strain and tension wood as well as growth eccentricity. The growth strain parameter showed seasonal changes, possibly due to the maturation of the secondary cell wall. Both the upper and the lower sides of the plagiotropic branches exhibited either contractive or extensive growth strains, whereas the orthotropic branches exhibited mostly contractive strains on the both sides, which implied different physiological functions of the two branch types. The tension wood arcs may occur in any direction of the branchwood which is different from the inclined trunk with tension wood on the upper side, suggesting dynamic adjustment in branch reorientation. In contrast to trunks, the hypotrophic eccentric growth in branches functioned in obstructing upward movement and even facilitates downward movement, probably because of the dissociation between tension wood and eccentric growth. Diversified growth strain and tension wood distribution on the branches may reflect the individual biomechanical requirements for each branch depending on the environmental factors, possibly gravitropic and phototropic stimuli.