Branch and stem attachment
The branch or stem attachment describes the point at which the branch or stem is anchored into the trunk or parent stem . These attachments bear the full load and lever arm of the branch and are usually formed over many growth cycles .
visible in the angle between the branch and stem . This is known as the branch-bark ridge ( BBR ) or stem-bark ridge ( SBR ) depending on the union in which it is formed .
Shigo ’ s model of branch attachment
In 1985 Alex Shigo proposed a model of branch attachment in which he described the branch as being secured into the trunk by overlapping layers of trunk and branch xylem .
A swelling is usually visible around the base of the branch , which is known as the branch collar . A raised , bumpy ridge of bark is often
Strength of branch and stem unions
A good indicator of the strength of branch unions may be the diameter of the branch compared to the diameter of the trunk . This is described as the branch : stem aspect ratio and usually expressed as a ratio ( e . g . 1:2 ) or decimal ( e . g . 0.5 ). Where the two are of a similar size the union may be weak .
For rescue technicians , acute unions with evidence of included bark should be treated with care . Rescue systems should avoid separating the two stems .
Strong unions in most species will be U- shaped , and the branch : stem aspect ratio will be 1:2 or higher ( i . e . the stem is twice the diameter of the branch or more ).
Acute unions are more V-shaped , and often weaker than U-shaped unions . An acute union may indicate bark is present in the union , preventing the xylem from interlocking . This is known as included bark .
Unions between two similar-sized stems , or where the branch is a similar size to the parent stem ( branch : stem aspect ratio 1 : < 2 ), may be weaker than unions with a branch : stem aspect ratio of 1:2 or higher . Unions with a 1:1 ratio may be particularly weak , although may still be structurally adequate .
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