Branch attachment

A branch attachment is where a branch is attached to the trunk of a tree. Three types of branch attachment are recognized due to differences in the anatomical position of buds that form them. Two key components contribute to the mechanical strength and toughness of the attachment: interlocking wood grain at the top of the attachment and an embedded knot that often lies within the attachment. A common malformation of a branch attachment is the inclusion of bark within the join, which can weaken the attachment.

Initially branches are mechanically attached to the trunks of trees by forming interlocking wood grain patterns at the top of the joint (Fig. 1). The xylem tissues in this location are denser than in surrounding tissues of the tree's stem or branch, the wood grain pattern formed is tortuous and in these tissues there is typically a reduction in vessel length, diameter and frequency of occurrence (i.e. more of the xylem consists of fibre cells). This specialized xylem tissue, formed under the branch bark ridge, provides unique mechanical properties to the union of the branch to the trunk, requiring that wood fibres are stretched along their length (the axial strength of the wood) in order to rupture the attachment apart.

As the typical lateral branch of a tree and its trunk expand in diameter at different rates, the base of the smaller branch becomes occluded in the larger trunk of the tree which is producing a larger increment of growth, and this occluded part of the branch forms a knot that provides substantial additional mechanical support to the attachment as it develops (Fig. 2). This is not the case in tree forks, where the growth rate of both branches is approximately equal, and no occluded knot is formed.

The combination of the interlocking wood grain at the apex of the branch and the occluded knot embedded into the tree's trunk make mature branch attachments in trees very strong components of a tree's crown. From static testing, branch attachments have been found to be far stronger than attachments made at forks in trees.

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