A tree leader is the single, vertical stem that grows upward from the trunk, serving as the primary engine of a tree’s height gain. This stem is fundamental to the tree’s architecture and structural integrity. A healthy leader ensures the tree maintains a strong, upright form, which is crucial for resisting wind and ice loads as the tree matures. By directing growth upward, the leader dictates the distribution of resources and the ultimate shape of the canopy.
Defining the Central Leader and Apical Dominance
The central leader is the main axis of the tree, extending from the base of the trunk to the highest point of the crown. This stem is structurally programmed to maintain its dominant vertical position. This growth pattern is regulated by a biological mechanism known as apical dominance, which ensures the tree prioritizes height over excessive width.
This dominance is controlled by a plant hormone called auxin, which is manufactured in the actively growing cells of the apical bud, located at the very tip of the leader. Auxin travels downward through the stem tissues, and as it moves, it actively suppresses the growth of the lateral buds located below it. This hormonal signal suppresses side-branch development, allowing the central leader to grow taller without competition. The result is the characteristic conical or pyramidal shape of many healthy, young trees, which is the strongest possible structure.
Understanding Co-dominant and Competing Leaders
Problems arise when the tree loses its single central leader, either through damage or genetics, leading to the formation of co-dominant stems. Co-dominant leaders are two or more main stems of roughly equal diameter that emerge from a single point on the trunk and compete for the same vertical space. This structural defect creates a weak point in the tree’s architecture, posing a significant hazard as the tree grows larger.
The junction where co-dominant stems meet is often characterized by a narrow, V-shaped crotch, which is inherently weaker than a broad, U-shaped attachment. Inside this narrow angle, a condition called included bark frequently develops, where layers of bark from the two competing stems grow inward and press against each other. This prevents the formation of strong, interlocking wood fibers, resulting in a physically weak union.
As the tree canopy expands and the stems increase in weight, the stress on this weak junction increases exponentially. During severe weather, such as high winds or heavy ice accumulation, the included bark connection cannot withstand the strain. This increases the risk that one or both co-dominant stems will split and fail, often tearing a substantial section of the main trunk. Identifying and addressing this structural vulnerability is a preventative measure for tree safety.
Corrective Pruning Techniques for Leaders
Managing or correcting co-dominant stems requires specialized pruning techniques to restore the tree’s natural, single-leader structure. The primary method used by arborists to fix this issue is called subordination pruning. Subordination does not involve the complete removal of the competing stem; rather, it aims to reduce its height and vigor to encourage the desired stem to assume the role of the dominant leader.
The technique involves making reduction cuts on the competing stem, removing a portion of its length back to a smaller, outward-facing lateral branch or bud. By removing a portion of the competing stem’s foliage and growing tip, the tree’s resources and hormonal flow are redirected toward the chosen central leader. This action slows the growth of the subordinated stem, effectively making it a lateral branch of the now-dominant leader. This process may need to be repeated over several years to ensure the chosen leader maintains its preeminence and creates a structurally sound union.