From a Correspondent:
I have a quarter horse that cribbed on two Palo Verde trees located where I let her out. Once I noticed that she was doing this, I fenced off the area immediately, so that she would not be able to do it anymore. The trees have bloomed since this happened. Do I have to worry that this is going to kill the trees? If so what can I do to prevent this? A nursery told me to buy some pruning sealant and place it on them.
M., in Cave Creek, Ariz.
Don’t use any kind of sealing tar or pruning paint on the tree wounds. Trees have a mechanism for healing damage, and studies have consistently found that wound sealant actually interferes with this mechanism and can result in worse damage. Palo Verdes are tough trees and will probably survive minor to moderate damage from your horse chewing on them. If the damage is severe enough to kill the trees, then there is nothing you can do to prevent it.
Unfortunately, the tree blooming subsequent to the damage isn't a very good indication of whether the tree will live. Trees have two major layers of tissue just under the surface of the trunk; you can think of them as transporting water up the trunk and sugars and starches down the trunk. The outer layer, called the "phloem," moves the sugars and other substances created during photosynthesis down the trunk, to the roots and throughout the plant. The inner layer, called the "xylem," moves water and mineral nutrients up from the roots and throughout the tree.
When the outer layer, the phloem, is damaged all around the trunk, water can still move up through the xylem and keep the tree going temporarily, but sugars can no longer move down to the roots; eventually the tree roots starve to death on a water-only diet. Because the sugars and other photosynthates are "trapped" above the damage, this can sometime result in really great bloom; kind of a last blowout celebration before the tree goes to meet its maker.
Assuming that the trees aren't girdled (missing phloem all the way around their main trunks), then they should survive. If the horse took out enough of a chunk in one place, the tree may be structurally weakened, but, again, there's little you can do about it. Over time, the tree should form new tissue around and over that area, as long as you don’t put sealant on the wound.
If these trees were in an area where they might topple and damage people or property, I would recommend that you carefully assess them, and possibly remove and replace them; because they're in a field, you probably have more wiggle room in giving them a shot at recovering.
I hope this helps,