What does that refer to?

• The term "codominant stems" is used to describe 2 or more main stems (or "leaders") that are about the same diameter and emerge from the same location on the main trunk.
• As the tree grows older, the stems remain similar in size without any single one becoming dominant.

• Codominant stems tend to fail much more often than others, especially in storms.
• Though such stems may look fine to the casual observer, they may actually be dangerous.
• Early recognition of such stems allows remedial action when it does the most good.
• Many of our most common street, highway, and park trees commonly form codominant stems.
• Maples and oaks
• Conifers that have lost the terminal during development

• Classifying codominant stems into 3 risk stages can aid in their management:
• Risk Stage 1: does the union between the two stems form a "V" but there are no other symptoms?
• A "V" union is much more likely to fail than a "U"
• Stems with a "V" union compress bark between them as they grow, leaving little physical connection
• Risk Stage 2: are there symptoms of decay in the union?
• Can you see rotted matter between the stems?
• Is there any fluid flowing from the union?
• Are there woody plants growing in the union?                                                             
• Do you see wide "ears" (swelling) on either side of the union?
• Risk Stage 3: is there any sign of failure?
• Can you see any cracks in the union itself?
• Is reaction wood being formed rapidly at the base of the stems?

                                                                             White oak with "ears" at union of codominant stems

• Risk Stage 1
• If the tree is young enough, prune out one of the stems; the tree will fill in the missing canopy
• For codominant stems greater than about 4" in diameter, pruning out one stem can cause more problems than it solves
• It leaves an unbalanced crown susceptible to mechanical failure
• It creates a large open wound susceptible to decay fungi
• Risk Stage 2
• Carry out an aerial inspection, probing the union itself to estimate its depth
• Reduce the end weight of the stems through proper crown reduction techniques
• For specimen trees, cabling and pruning can help in some situations
• You need a balanced crown and sound wood in the upper leaders for attaching hardware
• Make sure any such work follows the ANSI A300 standards
• When the stem is large and you can not cable, consider removing the tree--especially when there is a significant target such as a busy road or inhabited building
• Use a drill or other tool (such as a Resistograph®) to determine the thickness of sound wood
• There are no firm published criteria, but look for at least 1" of sound wood for each 6" of attached stem diameter
• The longer the stem above the union, the greater the breaking force, so give yourself an extra margin of safety for long stems, particularly those with lots of foliage
• Risk Stage 3
• If there is a crack or other indication of incipient failure, remove the tree as soon as possible--especially if there is any kind of target

Gilman, Edward.  2002.  An illustrated guide to pruning.  2^nd edition.  Albany, NY: Delmar.  For other information, advice and help on this topic, call offices of your State Urban Forestry Coordinator or University Extension service, or visit urban forestry web sites.