Reasons for tree pruning include:

Providing clearance: Branches that grow too low over roadways, driveways, lawns or over roofs may need to be raised or reduced in length. Also, branches may need to be pruned back from structures or from security lighting.

Removing deadwood: Deadwood accumulates in trees as branches die due to weather, disease or over-canopy shading. Deadwood ultimately falls and may cause property damage or personal injury. ANSI Standards recommend removing all deadwood that is one inch in diameter and greater.

Tree Health: Some branches are energy producers and contribute to tree health; they are a source of energy. Whenever possible they should be left on the tree. Other branches use more energy than they make; they are called a sink. A branch may become a sink because it is shaded by the upper canopy and is not capturing adequate sunlight. Another reason may be that a branch has been damaged by wind, lightning, insects etc. A good arborist can recognize a sink branch. Removing branch sinks can improve overall tree health by increasing net photosynthesis.

Reduction pruning: Sometimes it is necessary to reduce the height of a tree or the length of a lateral branch. In reduction pruning a branch is removed back to a branch that is capable of supporting the remaining branch system without causing significant decay. The branch to remain should be a minimum of one-third the diameter of the branch being removed and ideally one-half the diameter of the branch being removed. The success of reduction pruning relies on the ability of the arborist performing the work to understand species characteristics and tree energy levels. Reduction pruning is an option to acclimate trees to their environment and can preserve a tree that would otherwise be removed.

Reduction pruning is an approved pruning technique that is different than topping. Topping is an improper pruning practice that can cause a tree to fail, decline or die. Topping removes energy producing leaves and in extreme cases all of the leaves are removed. After topping roots die and branches have open cuts that can be invaded by diseases and/or insects. The new branches that develop around the cut are weakly attached and will likely fail when they grow large. If the tree’s energy reserves are low, topping may kill the tree. A topped tree will typically develop root and stem decay.

Improving tree structure: A critically important aspect of pruning relates to tree structure and the pruning procedure is referred to as structural pruning. The recommended reference publication is: Structural Pruning, A Guide For The Green Industry, authors, Edward F. Gilman, Brian Kempf, Nelda Matheny, Jim Clark, published by the Urban Tree Foundation).