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Pruning is the intentional removal of plant parts. Trees are complex plants that utilize all their parts to produce energy for growth and reproduction, and to form defensive chemicals that ward off pathogens. In addition to producing their own fuel and defensive compounds, tree parts also serve as essential mechanical support and conduit for infrastructure. Like plumbing, the tree's vessels in the trunk and branches enable the flow of water and essential elements to and from leaves and down to the roots. The woody limbs and trunk are naturally engineered in their formation and development to resist gravity, and withstand wind forces.

A tree needs every leaf it grows. In the tree "economy" there is no surplus or waste. The leaves form a canopy that is optimized for efficiency to maximize energy production. If a part is no longer producing, it is shed. For example, some trees have leaves with different leaf shapes and sizes on the same tree. The differences in leaf characteristics and their placement throughout the canopy are naturally engineered to maximize productivity across the seasonal variety of sunlight and temperature conditions. Each leaf produces food and energy that supports growth and survival. If a branch or leaf is not contributing, it dies and falls to the ground to be broken down into soil by microorganisms and fungi.

Pruning creates a wound that must be addressed. In nature, when a branch is torn off by a storm or other cause, the tree has a survival mechanism to limit infection by pathogens and subsequent decay. This survival mechanism is called compartmentalization, where the tree uses a variety of chemical compounds to build barriers or walls around the exposed tissues. Bark is the first line of defense against this decay; when the bark is breached, the compartmentalization defense immediately goes into effect.

Wounding impacts can be minimized by pruning with a purpose, and pruning correctly. Correct placement of a pruning cut will work in harmony with internal control systems the tree has naturally engineered. Knowing where to make the cut is as simple as locating a node or branch lateral, or the part of the tree anatomy referred to as the branch collar. The branch collar is the area of separation between branch and trunk tissues, and sometimes this area has a swelling to help easily identify it, as seen in figure 1. Figure 2 shows the process to make a correct cut.

Figure 1Figure 2

Pruning with a purpose requires a goal. Pruning goals include but are not limited to: cleaning dead or diseased branches from the canopy, raising the canopy for clearance for sidewalks, roadways, buildings, and/or line of sight visibility at road intersections. Pruning for utility line clearance is a well-known and popular pruning goal. Other pruning goals are training younger trees for structural development and improvement; this pruning practice is done in stages over several growing seasons. Pruning for size reduction is another technique that can be employed to extend the life of very old mature trees that may have hollow trunks or branches. Judicious canopy reduction can reduce the size of a tree without drastically altering its appearance.

The practice often referred to as tree topping is very prevalent in some regions but is not a professionally recognized pruning technique. It is widely regarded as tree abuse. Topping a tree may be likened to mistreating a pet or captive animal in that the poor treatment will not usually kill the animal right away, but poor treatment of the animal can significantly shorten its life span. Topping will also sometimes kill a tree, due to the extreme wounding that occurs. The reason most cited for topping is to provide a reduction in size or height, resulting from a concern that the tree is too big. Canopy reduction pruning is an alternative to topping that is less injurious and more attractive. Depending on the situation, a structural assessment can be performed by a qualified arborist to determine the relative risk a tree may pose. Sometimes complete removal and replacement with a smaller maturing tree may be a sound decision both fiscally and environmentally.

Some trees will require very little if any pruning, if they are planted in the right place. Good pruning can reduce defects and extend the life of a tree. Bad pruning can create defects and shorten the tree’s life.