How softwood tree branches are attached to stems: hierarchical extension of Shigo’s stem–branch model

Abstract

The main achievement of this study is an extension of the existing model of Shigo of the branch–stem junction of coniferous trees by introducing the concept of a sacrificial tissue. This tissue is acting as a predetermined breaking point between branch and stem, and limits fracture and damage to a small and isolated zone within the tree. Shigo developed a macroscopic model of the fibre structure in the vicinity of a branch–stem junction of coniferous trees to explain special physiological functions of the junction. However, abrupt changes in the cell orientations at the vertex of the branch observed on fracture surfaces and micro-cuts of the branch–stem junction interphase demand an extension of the existing model. The recent introduction of the concept of a sacrificial tissue, formed in the upper region of the branch–stem interface, brings more insights into the hierarchical junction microstructure and its mechanical and biological functions. Beyond a critical load, the sacrificial tissue serves as a predetermined crack path of zig–zag morphology originating from the stepwise distribution of transversally loaded cells at the junction. The hierarchical branch–stem junction microstructure, however, secures the stem and branch physiological functions, even when the crack opening is formed along the channel of the sacrificial tissue. Moreover, after the removal of the load, complete closure of the crack can be observed, which is explained by the release of the elastic energy stored in cells of the bent branch with high microfibril angle. The self-repair mechanism of the living branch is based on covering the crack by cell division of a sound cambium in combination with resin deposition.