Urban trees and forest stand trees have a variety of different stresses impacting their growth and longevity. Forest stand trees have evolved over time to naturally address and handle these stresses, while urban trees are very much dependent upon proactive and reactive care from arborists to effectively incorporate trees into our urban environments.

Focusing on urban trees, one of the more unique differences that they have than their forest counterparts is the spaces that they grow in. Urban trees are often growing in a much more open space without as many trees surrounding them as would naturally occur in a forest stand. This results in urban trees spreading out more rather than growing straight up as you see in a forest stand. The problem that this presents for urban trees is structural integrity. There are often more branch unions within the crowns of urban trees and a higher risk of these unions not being structurally sound.

What do we mean by “not being structurally sound”? The first thing to understand is the term codominant stems. Codominant stems are essentially when two leads (i.e. limbs, branches) diverge in opposite directions from one point or branch node. What is created between these two leads is then referred to as a branch union. There are two types of branch unions, U-shaped and V-shaped. U-shaped branch unions are more structurally sound as wood grows within the union. V-shaped branch unions are structurally weak because instead of wood growing within the union you get what is referred to as “included bark”. Included bark is layers of bark growing over the top of each other rather than producing wood. Wood is the structural strength of trees and bark is the protective outer layer of a tree, thus included bark in branch unions results in a structurally weak and compromised union resulting in a hazard and risk of structural tree failure.

What can be done about hazard trees with these conditions? The answer to this question is two-fold as there are proactive and reactive measures that can be taken. Proactive measures would be training pruning while the tree is young. Training pruning is done while the tree is still growing at the nursery, but also after the tree is planted. This type of pruning focuses on encouraging a central terminal leader in the tree and eliminating potential codominant as well as intersecting stems.

Reactive measures include pruning or cable/through rod systems. Often, pruning is difficult to do when the tree is older because the limb has grown to a significant size and represents a significant portion of the tree. Cable and through rod systems are often the best option for trees with these conditions. Cables are utilized when there is a significant span between codominant stems. They are installed about two thirds above the branch union and are intended to help hold the tree together and prevent a catastrophic failure if one were to occur.  When we install the cable, it will pull the tree together, but it will not eliminate it from failing at a later point. With that said, if it does fail at a later point, the cable will help to reduce the risk of the limb falling onto a hazard below, such as a home or physical structure.

An ISA Certified Arborist is your best bet to determine the best recommendation for your tree. Proactive and reactive measures can both be utilized to protect your trees and properties. The urban environment presents many stresses for trees, but the benefits that trees bring to our communities are well worth the risks. Thanks for caring for your trees and enhancing our urban forest!

Codominant Stems- Cable BracingCodominant Stems

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