Proper pruning cuts prevent future decay. Although tree decay progresses slowly, it eventually spreads, weakening on otherwise healthy tree. The current theory in pruning is to leave a stub so that the cut can wall off the injury properly. This stub is called a "branch bark collar" and is the area where the wound closes.
Trees do not heal as people do. A wound is never replaced with healthy new tissue. Instead, trees seal off the injury with a wall at the interface of the injury and the uninjured healthy wood. Certain chemicals, such as toluene, are produced, which fill the cells at this interface and prevent decay-producing organisms from entering the healthy tissue. It takes energy and stored carbohydrates to manufacture these protective chemicals. The healthier the tree, the better it is able to manufacture them and to withstand insults.
When woodsmen had only an axe, they always left a small stub because it’s not possible to make a flush cut with an axe. The advent of power saws, however, made it quick and easy to instantly saw off anything, anywhere. For a while these nice clean cuts were in vogue.
Then arborists began to find decay spreading above and below these spots. At the University of New Hampshire, Dr. Alex Shigo spent twenty years sawing huge trees in slices to see what actually occurred inside them. No one before him had ever carefully analyzed how trees handled their wounds and how decay spread. His findings showed that trees don’t repair injuries; they wall them off.
Each year a new ring of cells grows just under the bark. The cells in this ring are the active growing cells for that year and can be seen as tree rings. As the tree grows, each ring records the history of that year. In good years the rings were wide. Drought or frost narrows them. Traces of past forest fires may show. In examining very old trees, one can follow the weather patterns of centuries past. The long history of California wild fires that can be read in the tree ring scars of the centuries old giant sequoias is remarkable.
Each of these new rings walls off the injuries of the years past. Columns of decayed wood may gradually extend op and down in one year’s ring, but they will not move outward into the newer ones.
The walling off of injuries dictates the new theory of leaving the branch bark collar stub. A pruning cut is a wound. The deeper the wound is made, the more rings of the tree are damaged and the more decay is introduced into the tree. The branch bark collar stub keeps the pruning injury out of the main trunk’s cells.
As branches grow, some of the trunk tissue grows out and the branch tissue meets it. When the branch dies or is pruned off, the tree walls off the injury at the point where the trunk and branch meet, which shows as a seam on the top of each branch and a thickening below it. The seam on the top can be easily seen on trees with thin bark such as beech or birch. The point of the trunk and branch meeting is usually at a 45-degree angle to the trunk and slants outward from the top.
When branches die naturally, they fall off at this point. Pruning cuts should be made beyond this point of meeting, which leaves the branch bark collar.
Large limbs are first cut off about a foot from the final cut. Three strokes are used. The first cut is made underneath the lib to prevent tearing the bark. The second cut removes the branch. The final cut finished the branch bark collar.
It used to be common practice to paint all wounds with tree paint or shellac. Research has conclusively shown that it does no good. It only serves to console the homeowner and cosmetically hide the the raw wound with black tree paint. Creosote or oil-based paint should never be used, because they kill the growing cells and prevent proper walling off. Work is being done to find a tree paint that actually prevents decaying fungus.
Most important to prevent decay and promote healing is to leave clean wounds that drain and don’t hold water. Torn or ripped bark should be carefully cut off, taking care not to damage the healthy wood. Pruning cuts should be at an angle, so water will run off them.
Sometimes a tree develops "wetwood," a condition in which a hole or pruning wound weeps and drips sap. These should be left alone because the weeping is not dangerous. Trying to open a hole can cause more damage than leaving it alone. Sometimes, when a depression holds standing water, a small tube may be inserted at the base to help it drain.
Often trees have carpenter ants or termites where there is wet or decaying wood. It is impossible to get rid of them completely, but they can be treated with pesticides for temporary control. They usually don’t kill trees and generally don’t attack healthy trees.
The growth pattern of trees, followed during the course of a season, is quite interesting. Because there are not enough energy and nutrients to allow parts to grow at once, different parts (leaves, stems, and roots) grow at different times. Hormones regulate the process, switching growth from one area or another. Roots grow in spring and fall. As root growth peaks in early spring, it triggers shoot growth and leaf expansion. The leaves make carbohydrates, which then encourage stem growth. As this tapers off, new buds are set for the next year. Hormones then signal the root growth to increase again in the fall. Roots actually continue to grow even when the tree is dormant in winter, as long as the ground is not frozen.
Roots initiate growth and, at season’s end, they store the carbohydrates manufactured by the leaves. Anything that inhibits root growth restricts the whole seasonal growth process. Construction damage is especially traumatic. In the end, the health of the roots is basic to total plant health.