One of the most destructive tree pruning practices is "topping," the drastic removal or cutting back of large branches in mature trees.
Topping removes all parts of a tree above a certain height without consideration to its structure, and leaves open wounds that increase the tree's susceptibility to disease and decay. This can ultimately result in the early death of the tree.
Topping can cause significant stress in trees, as it removes more than 50 percent of the leaf-bearing crown of the tree. Leaves are the food factories of the trees, and the loss of so many leads to starvation.
The tree needs to replace the lost leaves as soon as possible and the severe pruning triggers a survival mechanism in trees. Latent buds are activated, leading to the growth of multiple shoots below each cut. This rapid growth is an attempt to replace its missing leaf area, so it can manufacture food for the trunk and roots.
A tree that has been topped is more vulnerable to insect and disease infestations. The drastic pruning may interfere with the trees' ability to chemically defend the wounded areas against invasion. Some insects are actually attracted to the chemical signals released by the trees.
Cutting a limb between lateral branches creates wounded stubs the tree may not be able to heal, causing the exposed wood tissues to decay. The leaves absorb much of the sunlight hitting the tree. However, topping removes too many leaves, exposing the remaining branches and trunk to high levels of light and heat. The tissues beneath the bark can be sunburned, leading to cankers, bark splitting and death of some branches.
Matching your tree selection with the site conditions - such as the proximity to other trees, buildings or above-ground utilities - will prevent problems before they occur, and harmful, drastic measures such as topping can be avoided.
When a tree must be reduced in height, the branches should be cut back to their point of origin. To shorten a branch, cut back to a lateral branch large enough to assume the terminal role, and is at least one-third the diameter of the limb being removed.
Do not confuse tree topping with proper tree pruning. A topped tree is very noticeable, because the tree's natural shape has been destroyed. When a tree has been properly pruned, it often appears as if no work has been done at all.
With proper pruning, an arborist will spend time carefully selecting and removing branches in pruning a tree properly to retain the tree's natural shape and beauty.
Timothy Daly is an agricultural and natural resource agent with the Gwinnett County Extension Service. He can be reached at 678-377-4010 or firstname.lastname@example.org.