A conifer is a cone-bearing, evergreen tree, such as pine, fir, or spruce, characterized by a single, straight central trunk that tapers toward the top. “Topping” refers to the severe and indiscriminate removal of this main central leader and the upper crown, typically done to reduce height or size. This practice is distinct from proper pruning, which involves strategic cuts to smaller branches. For nearly all conifers, topping is a detrimental action that initiates a cascade of negative biological and structural consequences.
The Biological Mechanism of Apical Dominance Loss
The characteristic conical shape of most conifers is maintained by a process called apical dominance. The highest point of the central leader contains an active growth center known as the apical meristem. This meristem is the primary source of the plant hormone auxin, which is transported downward through the trunk. The descending flow of auxin suppresses the growth of lateral buds located lower down on the stem and branches, ensuring the tree focuses its energy on vertical growth and maintaining the strong central trunk. The removal of the apical meristem through topping immediately eliminates this concentrated source of auxin.
With the inhibitory auxin signal gone, a rapid change in hormonal balance occurs within the tree. Dormant or latent buds, previously suppressed, are suddenly activated by a surge of other plant hormones, particularly cytokinin. This release from inhibition causes numerous buds just below the cut to sprout chaotically. The tree attempts to re-establish a dominant leader and restore its photosynthetic capacity.
Altered Growth Patterns and Structural Weakness
The hormonal imbalance results in a flush of growth known as epicormic sprouts or water sprouts. These new shoots emerge quickly and vertically from the remaining trunk or stubs in a dense, brush-like formation, often described as a “witches’ broom.” This growth is the tree’s attempt to replace the lost photosynthetic surface and central leader. A major consequence of this emergency growth is structural compromise because these new shoots are weakly attached.
Unlike normal branches that develop within a branch collar, epicormic shoots develop from latent buds and lack a strong connection to the main trunk. The tissue connection is fragile and often includes bark within the union, which prevents the formation of supportive wood. As these multiple new vertical shoots grow, they compete, creating multiple competing leaders that are structurally unsound. This loss of the natural central leader makes the tree highly susceptible to mechanical failure. The topped conifer is more likely to split, snap, or shed large limbs during high winds, heavy snow loads, or ice storms, creating a substantial hazard.
Increased Susceptibility to Stress and Disease
Topping inflicts a massive, open wound that compromises the conifer’s natural defense system. Trees respond to injury by a process called Compartmentalization of Decay in Trees (CODIT), where they chemically and physically wall off the injured area. However, the large diameter cuts made during topping are difficult for the tree to seal. The massive exposed area serves as a direct entry point for fungal spores, bacteria, and wood-boring insects, such as bark beetles. These pathogens penetrate the wood before the tree can fully establish its protective barrier, leading to internal decay within the trunk.
The tree is forced to expend stored energy to compartmentalize this large invasion, diverting resources away from normal growth and defense. This energy drain, combined with the sudden loss of a significant portion of its foliage, limits the tree’s ability to perform photosynthesis and produce the necessary sugars. The resulting physiological stress and weakened state make the conifer vulnerable to environmental stressors, such as drought or extreme temperatures. Ultimately, the combination of internal decay and weakened health often leads to a prolonged decline and eventual mortality.