Biomechanical features of eccentric cambial growth and reaction wood formation in broadleaf tree branches.

摘要

Tree branches and stems have different physiological functions that work collaboratively to maximize light interception. Light penetration in tree crowns is controlled by the orientation of the branches. However, mechanisms of branch bending have not received the attention they deserve. This study approached the problem by investigating the growth strain distribution in the upper and lower sides of branches of broadleaf trees, estimating the bending tendency of branches, and observing the branch eccentricity and the distribution of gelatinous fibers. The strain distribution was compared between the branches of 11 species (including 8 examined species and 3 referenced species) and tilted stems of 37 species from both our data and previous reports. Compressive strain was generally observed on the lower side of branches, but little was measured in tilted stems. The pith eccentricity of branches was in a reverse pattern to the corresponding strain distribution of stems. The radial growth of branches was hypotropic in contrast to the epitropic eccentric growth in inclined trunks. Furthermore, on the upper side of branches, G-fibers within the fiber arcs formed in an intermittent manner rather than in the continual manner found in artificially inclined stems. The resultant upward bending moment might not suffice to counteract the branch's own weight; therefore, most of the measured branches, differing from tilted stems, tended to bend downward. In conclusion, by comparing the biological and mechanical aspects of the strain distribution, bending tendency, and eccentricity, our experiments could discriminate the bending dynamics and role of G-fibers in tree branches from that of main stems.