Managing Natural Changes
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
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
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
|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 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
Figure 1. Typical mortality spiral
for red oaks.
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.
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.
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 trees 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.
Figure 2. Relative ability of trees to tolerate and respond to environmental stress and
maintenance treatments with age.
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 trees 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:
Clark, J. R. and N. Matheny. 1991.
"Management of Mature Trees." Journal of Arboriculture. 17(7):173-184.
Ossenbruggen, S. H. 1989. "Tree Energy
Systems." Journal of Arboriculture. 15(3): 53-58.
Shigo, A. L. 1991. Modern Arboriculture.
Shigo and Trees, Assoc. Durham, NH. 424 pp.
Larry R. Nelson, Extension Forester
and Associate Professor
Ham, Extension Forester and Professor
Department of Forest Resources