Sometimes the growth and development of trees is better understood in terms of an economic analogy. A tree’s sole source of income (food) is the sugar that is produced in photosynthesis. Limiting factors include the number and shape of the leaves, the available sunlight, water and carbon dioxide, and adequate supplies of a few essential minerals. Each branch must sustain itself, but also must pay “taxes” in the form of sugar contributions to parent branches, the trunk and roots.

Health problems such as nutrient deficiencies, drought and defoliating pests can dramatically reduce the income potential. In such cases, the tree must draw out of savings (stored starch reserves) enough “money” to support itself until more income can be generated.

When a branch is pruned, most of what is removed is the stored starch for that branch. However, each branch pruned further reduces the “tax base” for the rest of the tree. Furthermore, some reserves will be required to close and defend the wound.

Young trees have a high income-to-expense ratio and are very tolerant of pruning. In addition, wounds are relatively small and easily closed. As trees mature, they grow larger in bulk. Although they have a large savings capacity, their income-to-expense ratio decreases with each passing year.

The cavity in this elm resulted from decay that progressed rapidly from a large pruning wound. Photo courtesy of E. Thomas Smiley, Bartlett Tree Research Laboratories.

The cavity in this elm resulted from decay that progressed rapidly from a large pruning wound.
Photo courtesy of E. Thomas Smiley, Bartlett Tree Research Laboratories.

Very large, mature trees grow little in height but increase in girth, and put out approximately the same leaf volume each year. They are, at this point, on a “fixed income.” Nevertheless, their tax rates do not decrease. Health stresses can be very serious for these senior citizens, which may lack the vitality to mount strong defenses against disease and decay. Removal of branches at this point reduces the tree’s sorely needed income potential. If a large wound is created, it may never close, and may lead to significant decay if the tree lacks the defense capability to compartmentalize.

If those who care for trees have a better appreciation of how trees make a living they can make informed decisions regarding tree maintenance. All arboricultural practices have an impact on the health of the tree. Some, such as pruning, cabling and the installation of lightning protection, wound the tree. The benefits must be weighed against the injuries. Other treatments, such as fertilization, can be beneficial when required, but harmful when administered to excess. It is essential to understand tree biology before attempting to diagnose problems and recommend a course of action, even if that action is routine pruning.