There are a number of correctable tree faults that an arborist can use training to modify. Correcting double stems or forks is a major training concern. Within a forked stem, bark can be included or grown around. This bark-included crotch then has reduced strength and a tendency to split. Forks should be corrected early in the life of a tree before large amounts of stress and strain are concentrated around the crotch. Retain the stem that is largest, more fully crowned, more vigorous, and/or in a more direct vertical line from the top to the roots. Trees with opposite branching should have forks removed for at least the first 7-8 meters to provide a strong and safe supporting structure.
Branches with narrow crotches are similar to stem forks in potential for structural weakness. The narrower the crotch angle or the more upright the branch grows, the weaker the connection and the greater the chance of branch failure. Depending upon its length, an average branch should occur with a near 90 degree branch angle to the stem.
Tree growth control is formulated by meristem areas using growth regulators as messengers. Disruption of these patterns of message transfer can lead to many structural and stress related problems. One of the most noticeable negative impacts seen in young trees is a side branch being taller than the main axis. Training should maintain a single main stem in the most direct vertical line to the roots. Head-back side branches that attempt to take control of the tree. With a vigorous main terminal, side branches do not need to be removed, but can be reduced below the main axis height. An associated problem is rapid growth and expansion of side branches in extent and leaf volume, compared to the rest of the tree. Controlling these ``renegade'' branches by heading is essential for proper young tree development. Do not allow branch diameters to exceed 33% of stem size at its connection point.
The litany of tree faults that can be corrected with training is long and varied. But a few educated pruning cuts made early will lead to a long, healthy and structurally sound life for the tree and an improved quality of life for the tree owner. Correctable faults that can be, at least partially, trained away include: multiple leaders; tall and expanding side branches; flat tops; large side branches; forks; mechanical or chemical damage to the circumference of the stem or main roots; sprouts on the main stem; sprouts around the base of the tree; new sprouts from woundwood areas; any crotches with included bark; branches that rub, cross, or heavily shade each other; opposite branching; rapidly growing, vertical sprouts on the inside of the crown; excessively crooked, swept, or bent branches; and, excessively drooping or weeping branches.