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Why Co-Dominant Trees fail


We had another emergency call out this Sunday (29 Oct) to a tree that had partially failed and a stem had fallen against a house in Airdrie. For this reason we're doing a blog post specifically looking at Co-Dominant tree's and their potential for failure.


What does Co-Dominant mean? When we say Co-dominant we are talking about a tree that has more than one main trunk. When a tree is in its formative years i.e. a sapling, the trees genetics will determine whether it has 1 main "leader" or multiples thereof. If the tree has 2 leaders these will grow together, usually, but not always trying to out-compete each other for dominance. As the tree matures with these 2 leaders the base of the trunk will continue to grow in whats called "secondary growth" and get wider, pushing these 2 stems together.


The problem lies when the 2 stems are pushed together. Each individual stem has bark around its circumference, where these 2 sides of bark meet each other as they are pushed together is called "included bark". As the tree grows with height and girth this included bark always stays within the tree and never properly fuses to become a single trunk, causing a natural weakness and prone to leverage from the weight of the tree and more specifically wind.


A video of a failed Willow with co-dominance and included bark.


What can you do if you have a tree with co-dominant stems? The first step any tree owner should think about is formative pruning when a tree is young. Formative pruning involves pruning a tree in its early years, to avoid any unwanted genetic traits from occurring. Thats all well and said, however a lot of homeowners inherit mature trees with all the pro's and con's that this entails. Mature tree's with co-dominant stems can be crown thinned, so that the tree doesn't have such a sail effect in windy conditions, and thus less leverage on poorly anchored stems.


Depending on the tree species, co-dominant stems can be removed altogether. The downside to this is 2 fold; the stem being removed will leave a large wound or "header cut", that will not compartmentalize well, if at all, leading to Basal Rot from Fungus and Bacteria. The second problem is removing more than 25% of tree tissue can rob the tree of precious carbohydrates and Sugars and reduce its ability to Photosynthesize, sending the whole tree into shock and into a spiral of decline.


Sometimes unfortunately the whole tree may need to be removed depending on the likelihood of failure VS the target area should it fail. This is a job for an experienced and qualified Arborist. An initial quote from your local Arborist should be free, and always remember to try and get 2 opinions/quotes.


Excelsior tree Surgery are Based in Glasgow and cover the central belt of Scotland. Our quotes are 100% free and we are insured upto 5 million.

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