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Structural pruning continued

arborist pruning tree

There are three basic problems with not placing shade trees on a management program. The problems are 1) codominant stems develop and they could split from the tree, 2) aggressive branches and stems develop low on the tree, they droop and require removal resulting in large pruning wounds, and 3) aggressive branches develop low on the tree, they grow too long and break.

Many trees can develop these problems. All three problems result in tree stress, could shorten tree life, and could reduce public safety. Pruning programs on young and medium-aged shade trees should be devoted to preventing and solving these three problems. In this sense, structural pruning in the landscape is an extension of nursery production pruning. The structure on mature trees is more difficult to influence because the tree is significantly committed to its form.

The strategies to help solve these problems include 1) develop or retain a dominant leader, 2) identify the lowest branch in the permanent canopy, 3) space main branches along the dominant trunk, 4) prevent or suppress formation of included bark, and 5) prevent branches below the permanent canopy from getting too large. In short, encourage growth in the portion of the tree you want to become dominant by reducing the length of or removing other live portions of the tree. Unlike cabling and bracing limbs together which is designed to treat problems, structural pruning attempts to prevent serious problems from occurring. This is preventive arboriculture.

Medium-aged shade trees can be defined as trees that are not yet mature. In practical terms they can be thought of as trees less than about 40 to 60 years old, depending on species. These trees are young enough to meaningfully divert and direct growth to desirable portions of the tree by removing other living parts of the tree. The art of pruning is to determine how much to remove (the pruning dose) and where to remove it from (the pruning treatment). Dose is the most difficult aspect of tree care to teach.

One way to conceive a plan for pruning is to decide what size removal cut and what size reduction cut you are willing to make on certain species. More problems develop following larger pruning cuts than small cuts. This decision would be based on the reaction of trees to pruning in your region and may vary by species. Once this decision has been made from your observation, research and discussions with others, you can develop strategies to keep cuts under this maximum critical diameter while accomplishing the five objectives listed above. Maximum critical diameter is the largest diameter cut you are willing to make on a certain species or size tree. This diameter should be smaller on trees that compartmentalize decay poorly.

For example, if you were to place trees on a 10-year pruning cycle (each tree is pruned every 10 years), larger cuts than your critical diameter might (e.g. 3 inches) have to be made in order to accomplish your objectives. You would conclude that the trees should be placed on a shorter pruning cycle so cuts will be less than 3 inches. In other words, they need to be pruned more often. Pruning cycles might have to be shorter in the warmer climates than in cooler climates.

There are two components to branch size: diameter relative to the trunk diameter and actual diameter of the removed branch. Branches that are smaller than about half the trunk diameter generally can form a branch protection zone in the base of the branch where it meets the trunk. Removing small branches usually causes little harm. Since old, large diameter branches could have heartwood or wood that is unable to react to injury, there is little to retard the spread of decay-causing organisms into the trunk following large branch removal. If you suspect that heartwood is present, consider shortening the branch instead of removing it.