The practice of cutting back the upper limbs of a mature oak tree, commonly known as topping, is discouraged by certified arborists. Topping often appeals to property owners seeking a quick reduction in tree size, but it is a detrimental pruning method that compromises the long-term health and structural integrity of the tree. Oaks, as long-lived hardwood species, are sensitive to this aggressive intervention. This technique creates wounds that overwhelm the tree’s natural defense systems, leading to negative effects that extend beyond the initial cut.
The Mechanics of Tree Topping
Tree topping is the removal of large branches and stems back to a stub or a lateral branch too small to sustain the remaining limb. This technique, sometimes called “heading back” or “hatracking,” results in a uniform, unnatural, and stubby appearance. The procedure involves making heading cuts on mature wood, severing the branch without regard for adjacent lateral growth. These cuts leave large, open wounds on the remaining branch stubs, distinguishing topping from correct pruning. The objective is to reduce height and spread, but the biological consequences are significant.
Biological Impact on Tree Health
The most immediate consequence of topping is the sudden reduction of the tree’s photosynthetic surface area. A tree depends on its leaves to manufacture the sugars required for energy. Removing a large portion of the canopy effectively starves the tree, forcing it to rapidly use stored carbohydrate reserves. This energy depletion severely stresses the entire system and can lead to root dieback.
The large, open wounds left by heading cuts are too extensive for the tree to effectively seal, especially on mature wood. Trees defend against decay and infection through Compartmentalization of Decay in Trees (CODIT). While this biological defense isolates injured areas, large topping cuts overwhelm the mechanism. Decay fungi and pests easily enter the compromised wood and spread into the main trunk.
Removing the protective canopy suddenly exposes the thin bark on interior limbs and the trunk to direct sunlight. This overexposure can cause sunscald, where the bark heats up and cracks, damaging the underlying vascular tissue. These lesions serve as entry points for wood-decaying organisms and boring insects. Even in oak species, these large wounds can lead to extensive internal decay that may manifest years later.
Structural Integrity and Hazard Risk
The tree responds to the loss of its canopy by forcing out rapid, vigorous growth from dormant buds near the wound site. This new growth is known as epicormic sprouts or water sprouts. These sprouts are numerous and grow quickly to restore the necessary leaf area for photosynthesis.
The tissue connection of these sprouts to the parent branch is fundamentally weak. Unlike a naturally grown branch that develops with a strong branch collar, the new shoots are attached superficially. They lack the structural wood necessary to support the weight they gain, creating numerous points of failure.
As the sprouts lengthen, they become prone to snapping and failure, especially during high winds or storms. The collective weight of this poorly attached growth increases the crown’s wind resistance, stressing the damaged stubs. A topped tree becomes a long-term safety hazard, as these weak limbs can fail and cause property damage or injury years after the initial cut.
Recommended Pruning Alternatives
Instead of topping, professional arborists use specific techniques to reduce a tree’s height or spread while preserving its health. When height reduction is necessary, the proper method is a reduction cut. This involves selectively cutting a branch back to a suitable lateral branch, ideally one-third the diameter of the removed branch. This technique directs growth away from the cut, maintains the tree’s natural shape, and reduces stress on the wound.
For reducing the density of the canopy and increasing light and air movement, crown thinning is the preferred method. Thinning involves the selective removal of entire branches back to the main branch or trunk, making the cut just outside the branch collar. This careful placement allows the tree to properly form protective wound-wood over the cut. This defense is much more effective against decay than the large stubs left by topping.