A Closer Look at Codominant Trees
You’ve likely seen us chat about tree risk assessments a fair amount, and we’ve mentioned codominant spruce a few times now, but just what makes them so hazardous?
Codominant trees are those that develop 2 or more main stems or ‘leaders’ that are about the same size in diameter and emerge from the same location on the trunk. As the tree grows, neither stem becomes dominant and both continue to grow together.
Codominant stems can be identified by their vertical nature. The main trunk will split with either a V or U - shaped pattern, and two stems will continue vertically from there. This is different from branches which tend to grow closer to horizontal, approximately perpendicular to the main stem. Branches are also smaller than a main stem and have a branch collar around them where they join the tree.
Trees with codominant stems tend to fail far more often than single leader trees, especially in storms. A tree may look healthy on the exterior with a lush canopy or plenty of new needle growth, hiding the hazards within.
The challenge with codiminant stems comes from their continued growth. As the tree adds size year after year, the stems impinge on each other and push against each other to gain room. If caught early in a tree’s growth, one stem can be selected and others structurally pruned. The tree will grow in the canopy to fill the gap and will continue to develop with one leader. Often, however, the tree may be grown beyond that stage.
When we look at codominant stems, we look at three important risk factors in making recommendations. First, the type of union is important (and we’ve posted about this one before). A V union is much tighter than a U-shaped union, and doesn’t provide room for growth. Bark becomes compressed between the stems as they grow, leaving little physical connection between the leaders.
The photo below is a tree we removed recently in the community after spotting several issues that showed the tree was an imminent hazard. You can see the bark inclusion within the middle of the base. Both leaders were removed here, and the stump cut down to expose the true hazard of this bark inclusion. The challenge here, is the bark inclusion was not complete, leaving a bowl that caught fluid and debris and led to the premature interior rotting of one stem.
This is another piece we check for when assessing codominant stems. Any signs of rotting matter between the stems or swollen ‘ears’ on either side of the union are indications that the interior or one or both stems is compromised, and removal is required.
Finally, we often catch these trees during assessments showing imminent signs of failure. Any cracks along the union itself or the rapid formation of reaction wood (wood grown in a tree’s attempt to heal itself) is a critical sign that the tree may come down on it’s own suddenly.
The second photo included here shows two stems of a codominant tree. The stem on the left has a prominent concave structure where it was near the second stem. This is visible here due to a structural failure of the union, which has allowed the stems to distance themselves from each other. This structural failure greatly increases the risk profile of this tree, making it critically hazardous if there are any targets in its path. Targets can be structures such as your home or outbuildings, play areas kids frequent, or walking paths that experience significant traffic. Unfortunately for this tree, imminent removal would be recommended.
The final photo shows what a bark inclusion looks like when one leader is removed. The bark was significantly compressed here, and there is a fair amount of evidence of rotting material caught within the bark inclusion as the tree continued it’s growth. And while the first stem we removed wasn’t overly decayed, the second stem showed significant rot. Removing a single stem from a mature codominant tree is often not recommended, as the stem left behind is now more susceptible to the invasion of insects and disease and lacks significant structural support. Decisions have to be made on a case-by-case basis dependent on a number of factors, including possible targets the tree may have.
Written by Sean Sterna - The Rocky Mountain Arborist & Used with Permission
