Forest trees tend to develop a sound structure in response to competition. They grow in close proximity to other trees and the shade created by the developing forest canopy suppresses growth of lower limbs. As such, forest trees tend to maintain a single stem and narrow crowns as they grow toward light. This produces a reasonably strong structure in mature forest trees.
Conditions are radically different in the landscape. Trees are often exposed to full sun, which encourages a broader, more complex crown than one growing in the forest. Lower branches may grow very large, limbs develop in close proximity to one another and multiple stems can develop. These structural defects frequently result in a weaker structure, more prone to broken branches and tree failure. Certain species such as maple, elm, ash and dogwood are particularly prone to developing these defects.
Structural pruning focuses on maintaining a single dominant stem, except in cases where multiple stems are specifically desired, as is frequently the case in some species such as birch and crape myrtle. Branches are pruned so their size remains proportional to the stem diameter at their point of attachment. As trees grow, some branches are removed to ensure adequate spaces between permanent scaffold limbs. The shape of the tree is maintained to provide a natural open grown from typical of the species.
In young trees, structural pruning can guide proper growth of the tree and development of a stable form. Older trees that have developed defects over time can also benefit. In mature trees, this practice can improve stability of the tree, making it better able to withstand conditions like heavy rains and high winds.