One of the most common reasons arborists are asked to look at trees is to try and judge whether the tree is a potential risk. When asked to assess trees we look at the tree’s flaws (every tree has them) to judge the level of risk they pose.
One of the structural things we look at is co-dominance. A tree that grows a single trunk tends to have a stronger structure than a tree that forks into 2 trunks.
For a few reasons, trees may end up with two trunks.
When this happens, the union of these two trunks can be a failure point. Not all co-dominant unions are equally bad. One of the main things an arborist will look for with codominant unions is a “V” shape or a “U” shape.
The maple in the photo has a very tight “V” shape. Every year, when the tree grows thicker, the two trunks actually push against each other. This form of co-dominant tree has a higher likelihood of failure.
This codominant sycamore has a wide “U” shape. This may be less advantageous than a single trunk, but it is a very strong union. The risk of this union failing is low.
There are steps that can be taken to lower the risk of these unions failing. Pruning and cabling may be recommended. The best (and most affordable) thing that can happen is to give a young tree structural pruning to eliminate the problem early on.
It should be noted that not every co-dominant union is destined for failure. Every year a healthy tree will grow more wood where it needs to in order to compensate for its flaws. These are just some potential issues with trees that deserve a closer look.