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This is second in a two-part series on fertilization. Part 1 appeared in our August 2002 issue.

Ed Gilman is professor in the Environmental Horticulture Department at the University of Florida in Gainesville. He has been studying and teaching pruning since 1986. Sharon Lilly is director of educational goods and services for ISA. She is the author of many publications and training materials in the field of arboriculture.

very cut made has the potential to change the growth of a tree, so no cut should be made without considering the benefits and potential drawbacks. Removing foliage from a tree by pruning affects its physiology and future growth. Removing branches reduces a tree’s overall photosynthetic capacity and may reduce overall growth on the portions pruned or on the entire tree. Yet growth after pruning takes place on fewer shoots, which may enhance growth rate on the remaining shoots. In addition, severely pruned trees tend to initiate production and growth of many watersprouts in a response that generates new foliage and increased photosynthetic capacity.

 As Alex Shigo observed, "Pruning is one of the best things an arborist can do for a tree but one of the worst things we can do to a tree." Pruning is a double-edged sword, either helping or hurting—depending on where, when, how, and why it is applied. Arborists must understand the biology of trees and how trees respond to pruning in order to optimize the health and structure of trees through pruning.

Branch Attachment

When branches remain small relative to the trunk diameter, a swollen collar often develops around the base of the branch. The collar is formed by overlapping and deflected branch and trunk wood. The overlapping wood makes a union strong. Inside the collar on most trees is a unique chemical barrier called the branch protection zone (Figure 1). Its function is to retard the spread of decay organisms into the trunk. If the collar is removed or severely damaged, decay can more easily enter trunk wood and lead to defects.

When two stems of approximately equal size (codominant stems) arise from a union, there is little overlapping wood (Figure 2). The result is a weaker union. Decay can enter when one stem is removed because there is no branch protection zone at the base of a codominant stem. The union is even weaker when included bark is part of the condition. Included bark becomes trapped and embedded inside the union as the two stems grow and develop. This condition weakens the union, making the tree prone to failure at that point. The raised ridge of bark (branch bark ridge) typical in a strong union is deformed when included bark is present. Instead of a ridge of tissue, the bark forms a depression or valley of tissue. Branches and stems with included bark should be removed or shortened on young trees. Removal on large trees may not be a good option because of the potential for decay. Reducing the stem’s length can minimize the likelihood of the limb tearing from the tree.

Pruning Cuts

Three general types of cuts are used in arboricultural pruning: branch removal cut, reduction cut, and heading cut. Removal cuts are preferred because they leave the branch protection zone intact. The diameter (maximum, minimum, or both) of pruning cuts should be stated before the work begins. Such specifications define the size of parts to be removed and the size of pruning wounds that result from the pruning to be performed.