By knowing something about the basic structure of a tree, you can prune more intuitively. The tree trunk supports the major limbs, which bear branches, leaves, flowers, and fruit. Inside the trunk and branches are two rings of tissue that act as pipelines to carry nutrients and water throughout the tree. The inner ring, called the xylem, is the outer layer of wood. It carries water and minerals from the roots to the branches, leaves, flowers, and fruit. The outer ring, called the phloem, lies in the inner layer of bark. It carries sugars manufactured in the leaves to the rest of the plant.
A tree trunk supports the branches, which in turn serve as a framework for the leaves. Branches have identifying names depending upon their position on the trunk.
- The leader is the central branch, seemingly a continuation of the main trunk. (Some trees develop a single main trunk for life; others begin with a single leader but develop multiple leaders as they mature.)
- Scaffold branches are the main side branches.
- Lateral branches arise horizontally from scaffold branches.
- Spurs are short twigs or branches that bear flowers and fruit.
- Water sprouts are fast-growing shoots that develop from latent buds located on the trunk or branches.
- Suckers are vigorous shoots that grow from roots or the base of the trunk.
Food is manufactured in plant leaves by the process of photosynthesis. Powered by energy from sunlight, the leaves combine carbon dioxide and water to form sugars and complex carbohydrates. This is the source of energy for all plant growth processes.
Roots have multiple functions. In addition to absorbing water and essential minerals, roots anchor the tree and store food reserves.
Deciduous trees naturally grow in dense stands in the forest. As these trees crowd each other and shade the lower branches of neighboring trees, they develop a narrow, upright growth habit. However, when a forest tree is grown in the open — such as in the middle of a lawn — unpressured by competition, its crown is no longer limited to an upright shape and can become broader and more spreading. Many species that normally form a strong central leader in a forest form multiple leaders as they mature in a home landscape. These trees may require pruning and shaping to encourage a sturdy branch structure.
Trees are subject to the control of apical dominance. The tallest and most vertical branches, in particular, concentrate hormones in the terminal bud. These hormones inhibit the growth of the current season’s lateral buds for a year or more. The varying strength of apical dominance from species to species is the reason different trees have different shapes. Those with a prominent central leader, such as pin oak, sweet gum, and tulip tree, are governed by strong apical control. The leader outpaces growth of lateral branches to produce a pyramidal form. Eventually even these trees reach maturity, a time when apical dominance diminishes and the pyramidal appearance of the young tree gives way to a round-topped shape.
Many trees, including maple, linden, oak, and crab apple, have a rounded shape with several or many co-dominant stems. Although apical dominance is strong during the first years, control over lateral shoots quickly weakens. The laterals are free to grow rapidly, often overtaking the leader and creating a multiple-trunked tree with a rounded and spreading habit.
All trees develop characteristic mature shapes or silhouettes when grown in the open. They may be vase-shaped like the American elm or roundheaded like the European linden. Still others may have wide-spreading horizontal branches like the white oak or cascading branches like the weeping willow. When pruning a tree, you should know what its natural shape is meant to be and try to maintain that form while selecting the strongest branches.