(Continued from Tree Repair - 3)
Let’s go through in a little more detail the process of how a series of steeper and steeper angled branches become weaker as more bark is included between the branch and trunk. The normal close to horizontal branch is described above. The branch tissue, trunk tissue and the BBR and collar are all plain to see. As a branch grows closer to being parallel to the trunk, a pocket of bark forms in the crotch. The steeper, more parallel the branch is to its trunk, the greater the size of the included bark pocket. As the pocket size increases, the holding power of the collar decreases.
In an extreme case, we have what is called a co-dominant branch. Co-dominance occurs when two terminal buds form, side by side. They are parallel, touching, pressed together from the beginning. There is no crotch, no space in between, the BBR is invisible, deeply folded in at the bottom of the junction. This is the maximum amount of included bark possible and it is an extremely weak crotch. Neither side acts like a branch; they are equal, two trunks.
If you find one of these co-dominant attachments on a small tree, test it. Gently try to pull the two apart; this is easily done, so be careful. Focus on the area, the space between the two branches or small trunks. A gentle separation shows the top of the included bark pocket opening, and you can see into the pocket. You will see and feel how easy it would be to tear these two apart. Now find a good strong collar somewhere else, test it in a similar way to what you did with the co-dominant form. The well-defined collar is much stronger, more able to endure nature's many challenges.
When you see a tree that close to the ground quickly develops into a two-stem tree, have a closer look. The greater chance is that this is a co-dominant trunked tree, perhaps with some splitting or separation of the two trunks already existing. There are many of these trees, in gardens, parks, streets, and most of them would survive much longer if a bolt were installed to pin them tightly together, preventing splitting.
The main problem with potential co-dominant tree splitting is the great chance for a 50% leaf loss accident, the loss of one side or the other. Many times the half left standing is too weak to ignore and removal is the safe option. It is sad to see this, when early pruning in the nursery or later installation of a bolt could have easily prevented the split.
The concept of co-dominance can get more complicated when additional trunks are part of the weak included bark junction they all share. You have your tri, quadra, penta, and on up to octa-dominant, up to eight separate trunks formed at one place, all with serious bark included between the separate members. This is the way it is with elm trees. All elms deserve clear-sighted inspection, and many have a loose trunk or two that begs for structural support. Often, when a major trunk splits out of this multi-dominant attachment, the overall tree is so weakened as to need removal. Elms are a study unto themselves, and are quite different from most trees and their relationship with their multi- dominant form.
Most other species can be dealt with by early pruning.