Forestry Leaflet 18: Revised October 1997
Trees are vital, functional parts of our communities, our homes and yards, and our lives. Just as importantly, most of us simply like trees. We especially like large, mature trees to shade our homes and streets and beautify our communities. Large trees are also more effective than small trees in cooling urban areas, using carbon dioxide from the air, reducing water runoff and soil erosion, and generally improving our environment.
Unfortunately, trees in our cities and communities often do not grow long enough or well enough to become nearly as large or as old as they would in typical forest conditions. Forest trees may live to be hundreds of years old or more, but the average city tree lives only 32 years and inner city trees only seven years. As a result, a large portion of our urban trees die well before maturity and never provide the aesthetic, economic, and functional benefits for which they were intended.
Why do our urban trees perform so poorly? First, they are living in a very harsh, unnatural environment. Secondly, most tree-care programs are reactionary. They focus on treating tree problems or symptoms after they develop rather than promoting comprehensive plant health care (PHC). We can do very little to alter the harsh urban environment. However, we can develop proactive tree programs that favor long-term tree health, reduce maintenance and replacement costs, and enhance tree longevity.
The Tree System
Every tree has a genetic code that enables it to grow to a certain height and exhibit specific growth characteristics. Whether a tree reaches its full growth potential depends on the effects of environmental factors and pest problems that can weaken it or drain energy from its system.
Yes, a tree is a system. All of its components are part of a single, interacting plant system. What happens in one part of the plant has an effect on the overall system. By understanding how the tree system grows, how it defends itself, and how it dies, we can apply proper, long-term tree care that can improve health and longevity.
How Trees Live
Energy - The Source of Life
Energy is needed to maintain order in the tree system. Trees depend on chlorophyll molecules, primarily in their leaves, to capture energy from the sun in a process called photosynthesis. The energy is stored in the chemical bonds that hold carbon, hydrogen and oxygen together as carbohydrates. The process of using the stored energy is called respiration. During respiration, high energy-yielding bonds of carbohydrate molecules are broken and the energy is released to "run" or "fuel" the biological work of the tree system.
Energy is allocated for various functions much like we allocate funds in our personal budgets. It is used to break dormancy in the spring and to produce and maintain adequate foliage as well as woody tissue found in trunks, branches and roots. It is required in high quantities for reproduction and for responding to regularly occurring wounds. Finally, some energy is stored for emergencies just as we allocate money in a savings account. In essence, the energy supply is the basis for the growth and defense of the tree system. When energy is deficient, either growth, defense, or both suffer.
Why Trees Die
Figure 1. Typical mortality spiral for red oaks.
Certain causes of tree death are obvious. Destructive forces of nature such as high winds, lightning, fire or other catastrophic events can physically destroy a tree in a relatively short period of time. Young, newly planted trees often die from lack of water, improper planting or other acute problems related to early tree care. However, what causes older, well established trees to eventually die? Essentially, they run out of energy! All living cells in the tree system require energy to survive. As trees age and grow, their massive size and structural complexity demands more energy. They have less energy stored for emergencies. As certain environmental or pest problems occur, energy demands increase, and reserves are depleted.
Think about a city tree growing in hard, compacted soil. The tree may appear healthy for a period of time. However, its roots are not growing well due to inadequate supplies of oxygen and moisture and an inability to penetrate the hard soil. The roots fail to supply the top of the tree with adequate water and nutrients. Fewer, smaller, yellowish leaves are produced. Consequently, carbohydrate production (energy storage) is reduced. The tree is unable to grow well because of low energy reserves. The tree is under stress.
Stress can be visualized much like a coiled spring with a heavy weight hanging from it. The weight stretches the spring to its limits. When the weight is removed, the spring returns to its "normal" state. If a heavier weight is hung from the spring, it may be so heavy that it stretches the spring beyond its ability to return to "normal" after the weight is removed. The spring can no longer function!
If the stress can be removed from our city tree, it may recover. Typically, however, other problems compound the situation. Periods of drought intensify the soil and root problems. The tree is wounded by a car. Insects attack the trunk. High amounts of energy (carbohydrates) are used for "defense." Energy levels are depleted further. The tree begins to decline and, if stress continues, the tree dies. The spring has stretched beyond its limits.
Figure 1 illustrates a typical urban tree "mortality spiral." Such a spiral can begin at any age and may take several years to run full course. The objective of comprehensive, long-term tree care programs is to use our knowledge of tree systems to prevent or minimize stress and avoid mortality spirals.
Changing Energy Demands
Growth vs. Survival
As trees age, we see distinct changes in their ability to respond to stress (Figure 2). Young trees have a high ratio of photosynthetic (leaves) to non-photosynthetic (woody) tissue. Consequently, for their size, they produce a relatively high amount of energy. Healthy, young trees produce enough energy for growth and abundant storage. They tolerate environmental change and maintenance treatments. In contrast, mature trees have heavier demands on their energy supply. They utilize some energy for growth, but a large percentage is used just to maintain the massive amounts of existing tissues in the trunk, branches and roots. Additional energy is needed to seal wounds that occur from wind breakage, insect attacks and other sources and for the development of reproductive structures. Because of their tighter energy budget, mature trees have very little stored energy for responding to environmental change. In essence, mature, healthy trees are in delicate balance with their environment. The key to preserving this balance, and therefore their health, is to maintain environmental stability around mature trees.
Figure 2. Relative ability of trees to tolerate and respond to environmental stress and maintenance treatments with age.
Comprehensive tree care programs should always be considered long-term. A program should begin before planting and should continue throughout the life of the tree. Practices should be geared to mesh with the natural changes that occur in the tree system. There are three important phases in urban tree development during which practices should be modified to meet the tree’s ability to withstand change These include the planting/establishment phase, a juvenile growth phase, and maturity.
Before planting, the most critical factor for consideration is tree selection. Too often we see the wrong tree species planted on a given site. For example, oaks, maples, elms or other species exhibiting large mature growth forms are often planted under utility lines and/or between sidewalks and street curbs. Over time the trees become stressed due to limited growing space, soil compaction or other factors, and due to their size, eventually must be heavily pruned. Attempts to force a tree to fit a given site by altering the crown or root system almost always leads to a shortened life span. In order to avoid this situation, we should be sure, prior to planting, that the species fits the site.
During planting, intensive care must be applied to assure early survival and to prevent rooting problems as the tree matures. Soil structure and moisture relations at the planting site become immediately important. In order for roots to grow, the soil must provide both water and air. Trees will not grow well in compacted or poorly drained soils.
During the juvenile growth phase, maintenance practices should take advantage of the tree’s vigor and ability to adapt to site changes and respond to maintenance treatments. Tree care practices during this period can ultimately determine the form and quality of the mature tree. Mulching and fertilization will help develop a good root system and a generally healthy tree. Pruning to promote proper branching and crown form should be done while the tree is young, so that pruning wounds will be relatively small. Energy reserves are available for sealing off wounds, and early pruning can reduce the necessity for structural pruning later, when wounding becomes more severe and the tree has less energy for recovery. Finally, comprehensive pest management practices should be established to prevent severe injury from insects and disease.
As the tree matures, its ability to tolerate and adapt to change decreases. Maintaining stable environmental conditions around the tree is vital to its continued health. Measures should be taken to protect the root area. Digging, grading, soil filling and other practices should be conducted so as not to damage roots, disrupt the exchange of soil oxygen and carbon dioxide or create moisture problems. Mulch should be maintained to conserve soil moisture and avoid soil compaction problems. The crown should be protected. Heavy pruning should be avoided except to remove dead or diseased branches. Pest management should be continued to prevent insect or disease problems.
Once mature trees begin to decline, they do not respond well to remedial or corrective treatments. For that reason, continuous tree care, beginning with tree selection and planting, and maintaining a stable environment around mature trees are absolute necessities for keeping healthy trees healthy!
Many concepts discussed in the text are summarized from: