Understanding and Managing Co-Dominant Stems in Urban Trees
Trees are an essential part of urban environments, offering aesthetic beauty, shade, and environmental benefits such as air quality improvement and stormwater management. However, like any living system, trees can develop structural problems that, if left unmanaged, can lead to significant issues. One common structural defect seen in urban trees is the development of co-dominant stems.
What are Co-Dominant Stems?
Co-dominant stems occur when two or more vertical stems, or leaders, grow from a tree’s trunk at roughly the same height and are similar in size. These competing leaders have what’s known as a 1:1 aspect ratio, meaning their diameters are almost identical. While this might seem like a minor issue, co-dominant stems can pose serious risks as the tree matures.
The problem arises from the fact that these leaders often meet at narrow angles, creating a weak union at the point where they join. This junction, known as a “V-crotch” or “narrow union,” doesn’t allow enough space for proper growth and can result in a defect known as “included bark.” Included bark occurs when the bark from each stem is pressed tightly together, preventing the fusion of wood fibers. Over time, this leads to the formation of a seam where the two leaders act like separate entities rather than one unified structure.
The Risks of Co-Dominant Stems
The inclusion of bark at the union between co-dominant stems can create a host of issues for the tree:
1. Weak Structural Integrity: As the stems grow, they continue to increase in size, but the union remains weak because the wood cannot fuse properly. This results in a structural weakness where the tree is most vulnerable.
2. Lever Arms and Wind Load: As the tree grows taller, the co-dominant leaders become larger and longer, acting as lever arms. These arms are more susceptible to stress from wind, snow, and ice loads, leading to increased strain on the weak union.
3. Moisture Retention and Decay: The narrow union can trap moisture, encouraging rot and decay. As the area weakens further, the risk of the stems splitting apart during storms increases.
4. Increased Risk of Failure: The combination of weak unions, external forces, and internal decay makes co-dominant stems a significant risk for structural failure, especially in urban environments where trees are surrounded by people, vehicles, and buildings.
Managing Co-Dominant Stems: The Subordination Approach
To prevent these potential hazards, arborists often prescribe a technique called subordination. This method focuses on reducing the size and vigor of one or more of the competing leaders to promote the dominance of a single, stronger stem. By doing this, the tree can develop a more stable, long-term structure.
How Subordination Works:
1. Selective Pruning: The arborist identifies the leader that is most suitable to become the dominant stem based on factors such as its angle, strength, and central position within the tree. The competing leaders are then pruned back, which reduces their size and redirects the tree’s energy toward the chosen dominant leader.
2. Reducing Growth: By removing a portion of the competing leaders, the tree allocates less energy to those stems. This encourages the tree to focus on the primary leader, gradually diminishing the influence of the subordinated stems.
3. Ongoing Monitoring and Pruning: Subordination may require more than one pruning cycle, depending on the tree’s growth rate. Regular monitoring allows arborists to continue guiding the tree’s development and address any emerging issues.
Benefits of Subordination
• Improved Tree Stability: By encouraging a single dominant leader, the tree develops a more stable and structurally sound form, reducing the risk of breakage.
• Increased Longevity: Trees with strong, well-managed structures are less likely to experience catastrophic failure, leading to a longer lifespan in urban environments.
• Safety and Reduced Liability: A well-structured tree is less likely to fail, reducing the risk of damage to property and injury to people in its vicinity.
When to Address Co-Dominant Stems:
The best time to manage co-dominant stems is during the early stages of a tree’s development. Young trees are more adaptable and recover faster from pruning, making it easier to guide their structure. However, even mature trees can benefit from subordination if co-dominant stems pose a significant risk. In these cases, an experienced arborist will carefully assess the tree and create a pruning plan that minimizes the impact on the tree’s health while addressing structural concerns.
Conclusion
In urban environments, trees face unique challenges, and co-dominant stems are one of the most common structural issues. Left unaddressed, these weak unions can lead to dangerous failures that threaten property and safety. Subordination, when done by a qualified arborist, is an effective way to manage co-dominant stems and promote the best long-term structure for the tree. By taking early action, property owners and municipalities can ensure that their urban forests remain healthy, safe, and beautiful for generations to come.
