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
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!
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
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
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.
Figure 2. Relative ability of trees to
tolerate and respond to environmental stress and maintenance treatments
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
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!
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
Donald L. Ham,
Extension Forester and Professor
Department of Forest Resources
Understanding Plants ;
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