What causes included bark unions?
Included bark is created when branches or stems do not flex separately enough to create the need to build additional strength between the point at which they meet. Let’s call this flexing ‘exercise’. Where branches are somehow limited in their movement, or move together, in the same direction, at the same time, they do not experience enough exercise to warrant the construction of a strong branch unions, and so, included bark forms. Examples of prevention of movement can be touching twigs as is commonly seen in Paperbarks with a dense canopy or crossed/rubbing branches. Contact with neighbouring trees or buildings may also prevent exercise. These things that typically restrict movement are termed ‘natural bracing’.
Why do bark inclusions lead to failure?
Natural bracing, when suddenly lost, can lead to the failure of branches, as the branches have not grown accustomed to the sudden freedom of movement after years of being restricted. Where natural bracing is slowly lost, such as the gradual decline of a dying branch that is making contact between two stems, the gradual increase in exercise can lead to a build-up of connective tissue as the tree reacts. Therefore, consideration should be taken when removing rubbing branches that have had time to mature. Perhaps improving the bracing between the rubbing branches may be preferred.
Are all bark included unions the same?
No. Bark inclusions can come in a variety of shapes including ‘open mouthed’, ‘cup shaped’, and ‘occluded’. In my experience, branch unions can also be half cupped, as one side of the union begins to occlude while the other is open mouthed. This may be due to one side of the tree experiencing more exercise than the other.
These shapes can also indicate differences in strength, with the open mouthed being the weakest, cup shaped being less weak, and occluded being as strong as a normal branch union. Personally, I have never seen an occluded, included bark union failure. But you must remember to put the strength of these included bark unions in their wider context. Is the natural bracing still there, has it been lost, slowly, or suddenly? I personally do not believe that natural bracing is responsible for every included bark union, but I do think it applies to the vast majority and is generally a good predictor of the likelihood of failure.
Branch:branch ratio also plays a part in indicating likelihood of failure. Where the ‘aspect ratio’ of the branches or stems is between 70-100% the size when measured just above the union, there is a greater likelihood of failure. Where the branch is less than 70% the size of the adjoining branch, there is little likelihood of failure.
Duncan Slater 2015, The level of occlusion of included bark affects the 2 strength of bifurcations in hazel (Corylus avellana L.)
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