Tree Decline -- What Is It?
Tree Decline or dieback, is the
term generally applied to tree death from unknown or complex
ecological causes. The term generally refers to progressive
dying from the tips of the twigs to larger branches, ending
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the
death of the whole tree. Trees can remain in varying degrees of decline
for a number of years.
Trees die when they no longer have the
ability to acquire or mobilize sufficient resources to support
life. Trees must fulfill several essential life functions:
reproduction, maintenance, growth, storage and defense. They
cannot meet these needs all at once. Instead, trees allocate
resources by a priority system, with maintenance typically the
highest. Trees must have the resources to maintain all their
parts. If they lack the resources to achieve this, they must
die back to a size they can support. Thus, while dieback
appears alarming, it can serve as a survival mechanism. Trees
cannot get up and move when environmental conditions change,
so their only recourse is to alter their size to conform to
the resources available.
Initial response
to tree decline or construction damage is generally seen as
dieback to the tips of the outer
branches. As decline progresses those tree
branches will eventually die all the way back to the
trunk. The
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third major symptom will be the development of
many new branches (called epicormic growth) in the canopy as well
as along the trunk and at the base of the
trunk. |
Decline often is due to a predisposing factor,
usually a long-term stress agent that reduces the tree's
defenses, that makes it more vulnerable to another stress
agent that accelerates the decline. Often, the predisposing
factor is non-living, or abiotic, while the contributing
factor may be abiotic or biotic (there can be more than one
predisposing and contributing factor). The question in many
cases is not what is the causal agent, but how many are
there. (See article " Casual Agents of
Tree Diseases " ~
http://www.800oakwilt.com/treedisease.html)
This combination of agents makes
it difficult to identify, and then manage, tree decline. Why
was the oak tree covered with dying and dead branches in the
first place? What were the predisposing factors
that made the tree vulnerable to colonization by this insect?
What resourses in the soil have been depleated or
damaged?
A natural forest
typically begins as a large group of small trees, perhaps as
many as several thousand to the acre. Over a period of many
years, the numbers are reduced to only a few large trees per
acre. Foresters, farmers and gardeners have long realized that
an acre of land can produce only a certain amount of plant
growth for either many small stems or for a few large
ones.
In general, the plants that survive are those that
are best adapted to local growing conditions and those best
able to compete effectively for sunlight, moisture and plant
nutrients. There is a tendency for the strong plants to get
stronger and the weak plants to get weaker, eventually
resulting in a natural thinning process. As the eventual
losers decline in vigor, a few will pass out of the picture
each year. Periods of drought, early and late frosts, wide
fluctuations in temperature, insect and disease epidemics,
wildfire and other forces may speed up the natural selection
process.
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Declining from construction and
compaction |
Declining from paving -
and possibly other factors
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As with humans, a decline in vigor makes plants
more susceptible to other problems. Insects and diseases,
alone or in combination, may cause tree decline and eventual
death. In unmanaged forests, natural forces determine which
trees are removed. In managed forests, professional foresters
make the decisions about which trees will be removed to
achieve management objectives.
This publication focuses
on what takes place in the process of "tree decline" and how
it is distinguished from "tree death." The latter may be
specifically attributed to particular single causes such as
oak wilt, Dutch elm disease or pine wilt disease. Whether a
tree declines as part of the forest or as a shade tree in a
yard, some of the same general principles and causes are often
involved. Understanding the reasons for decline may help you
prevent, reduce or sometimes reverse decline of your yard or
woodland trees.
Stress as a cause of tree
decline
Various stresses cause reduced tree vigor and allow
secondary diseases and insects to cause further decline or
death of branches, roots, or sometimes entire trees. One of
the major causes of stress in trees is planting them in
locations to which they are not suited. Some examples:
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Planting acid-loving
trees in alkaline soils increases the likelihood of
stress. Planting warm-weather
trees in cold climates or moisture-loving trees in droughty
soils almost guarantees a life of stress.
Planting trees that
require full sunlight in the shade of other trees will also
cause stress. A small pine tree planted in the shade of a
group of oak trees will never fully develop.
Trees that have the
potential for large size should not be planted in confined
locations. For example, oak, pecan or other trees that develop
large tops and wide-spreading root systems should never be
planted in a 4-foot by 4-foot hole in a sidewalk or street
location. Such trees are generally short lived under these
conditions.
Planting trees that are
intolerant of air pollutants where high levels of pollution
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are expected is not a good
idea. For instance, trees such as white pine, larch, sugar maple, white
ash and yellow poplar, which are sensitive to ozone, should not be planted
along freeways where traffic is heavy. Instead, trees more tolerant of
ozone such as eastern red-cedar, black locust or bur oak should be
considered.
Planting a tree that
normally grows on dry upland soils in low swampy areas is
unwise. Wet area species such as pin oak, red maple, sycamore,
sweetgum or bald cypress could be among the logical choices in
such situations. |
Another major
cause of stress in trees is allowing too many of them to
occupy an area. Crowded trees compete with each other for
nutrients, moisture and sunlight. Not all will survive when
trees increase in size. Although some trees will eventually
dominate, their growth is slowed and stress is created until
the final survival decision is made by
nature.
Many other factors also place trees in stressful
situations and eventually lead to their decline:
See
Articles:
Drought Stress http://www.800oakwilt.com/articles/heatwave.html )
Flood Stress on
Trees ~ http://www.800oakwilt.com/floodstrees.html )
Periodically,
extreme conditions such as severe drought or flooding occur.
Disease or insect epidemics may sometimes affect enough trees
that people become concerned about the situation. Heavy
defoliation can be caused by insects such as the variable oak
leaf caterpillar, green striped maple worm, the elm leaf
beetle and the gypsy moth. Repeated heavy defoliations
seriously weaken trees and may well lead to problems that can
show up several years later. In the United States, oak species
periodically decline on a massive scale. In general, these
situations have involved adverse weather factors combined with
defoliation by insects or diseases. Finally, attacks on
weakened trees by such things as root rot or trunk cankers or
tree-boring insects result in serious decline and eventual
death.
A similar situation
often occurs on a smaller scale among shade trees following
new residential construction. Soil compaction by construction
equipment, grade changes resulting in root damage from soil
removal, or root smothering by fill dirt are among many
factors that cause stress. Decline may not become evident for
several years.
Compaction or other alteration
of the soil over the roots will occur where the children's
swing set or the family dog pen is placed. Other residential
yard activities may further contribute to the problem. The
slightly acid soil that oaks prefer may be altered by applying
lime recommended for many turf-grasses. Alterations of natural
drainage to move water swiftly away from the house also may
reduce the moisture available to the shade trees.
Construction of driveways,
sidewalks and streets can drastically modify the environment
to which large shade trees have grown accustomed in early
life. Many woodland trees are simply unable to survive the
changes associated with their "urbanization."Many stress
situations such as those listed can be avoided by checking
with a tree care professional or by acquiring a knowledge of
the cultural requirements of different tree species and the
characteristics of the area in which the trees are located or
will be planted.
Factors involved
in decline
Various reasons for tree
decline have been well documented:
exposure to extremes of weather, heat,
cold and wind
old age
live stock and wildlife attack as
well a human caused injuries
insect attack
windstorms
floods or wet conditions
drought for extended
periods
Iightning
dryland or irrigation-induced salinity
and lime
mistletoe attack
fertility build-up under trees from
stock excrement
understory brush growth - small
native shrubbery and cedar
trees
soil compaction from stock or human
trampling - parking of vehicles and equipment
improved pastures associated with
fertilization or excessive landscape
fertilization
fungal attack
fire
chemicals used excessively
mechanical
injury by ranch equipment, lawn mowing and weed
eaters.
Symptoms of decline
Symptoms of decline may
develop quickly or they may not be noticeable for years Early
symptoms include premature fall coloration, late spring leaf
development, decreased twig and stem growth, leaf scorch,
death of tissues between the leaf veins, and premature leaf
drop. Later symptoms include dieback of larger limbs and
branches; sprouting from the trunk of the tree; heavy seed
crops; foliage noticeably smaller, lighter green and sometimes
produced in "tufts" or "clumps" on sprout-origin tissues. The
foliage over the entire tree may also look thinner in decline
conditions.
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Bole Sprouts developing directly from trunk
new branches (called epicormic growth) |
Clusters
of mushrooms of A.mellea form
in autumn at the base of invaded trees. |
Certain fungi that cause
stem canker or root infections are often associated with the
decline of many tree species. When trees are under stress,
physiological changes occur in the roots, allowing the fungi
to infect and kill them. In the autumn, clusters of mushrooms
may form at the base of trees that have been infected. These
growths indicate that root and trunk infections are
present.
Many wood-boring insects
are also associated with the decline of trees. The ash borer,
bronze birch borer and the flat-headed apple tree borer are
examples of insects that breed in various hardwood tree
species that have been weakened by drought, defoliation or
other adverse conditions. Scale insects are also commonly
associated with tree stress.
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Pockets of oak
mortality resulting from the interaction
of stress,
insects, diseases, and site
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Minimizing stress and tree
decline
Tree decline can often be
minimized by taking these preventive maintenance
steps:
Avoid placing trees in a
stressful situation. This includes selecting species of trees
that are suited to the conditions of the planting location or
modifying the planting site to suit the requirements of the
species.
Do not try to grow more
trees in an area than it will support. Woodland owners should
consult professional foresters to determine how many trees of
different varieties an area will support. Similar professional
advice is available to homeowners about shade tree care. Your
local University Outreach and Extension center can assist you
in getting this type of advice.
For planting in
stressful situations, select species that are the most
tolerant of the existing type of stress.
Remove weak and dying
trees to help prevent the buildup of secondary insect and
disease problems. This practice can be carried out while
making harvests for fuel wood or when thinning a group of
trees. Decisions about which trees should be removed or left
to grow should be made with some
foresight.
In caring for trees in a yard,
more intensive control measures may be justified. Get an
accurate identification of the problem by submitting samples
to a plant diagnostic clinic. Very few foliar diseases on
mature trees need to be treated with fungicides. Raking and
removing fallen leaves may reduce a source of inoculum. Dead
limbs may be pruned out, when warranted, to remove sources of
disease or insects. Care should be given to avoid spreading
disease on pruning tools by sterilizing the pruning tools with
wood alcohol, household bleach or other approved
disinfectants.
Nutrient deficiencies can be
corrected by proper fertilization. Mulches to conserve
moisture, prevent deep freezing and prevent soil crusting are
usually beneficial. But do not apply mulches too deeply around
stems. Properly timed watering will reduce moisture
problems.
Watering to reduce drought
stress
Even large, well-established
trees can be damaged by a severe drought. A few well-timed
irrigations can greatly reduce the stress encountered by trees
during a drought and may prevent the onset of
decline.
During a midsummer drought, it
is common for about 1/4 inch of water to be lost from the soil
per day. Under these conditions, most available water would be
depleted from the root zone of a tree within 14 days. If the
soil is a clay loam, to completely replenish soil moisture in
the tree root zone (to a depth of 18 inches) would require
about 3.5 inches of water.
Fortunately, it is not
necessary to wet the entire tree root zone. Irrigation applied
to turf will usually supply enough water to trees in a lawn to
greatly reduce drought stress. However, where turf and tree
roots exist together, a rapid depletion of soil moisture will
occur. These areas require more water than areas where few
tree roots are present. A large mulched area under each tree
will allow fine tree roots to grow near the soil surface
without competition from grass roots and will allow more
efficient irrigation and fertilization of both trees and
turf.
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roots can grow 3 times
beyond the drip line of the
tree
If valuable trees
are located in a non-irrigated area, a deep soaking irrigation
every two or three weeks during a drought will prevent
excessive drought stress from developing. Application of 2 to
3 inches of water over half the surface area contained within
the drip line will be of great benefit to a drought-stressed
tree. It may be necessary to irrigate on two or three
consecutive days to apply this amount of water without
runoff.
After irrigation, the soil
should be moist to a depth of at least 12 inches and
preferably 18 inches. Keep track of the number of inches
applied with a rain gauge. The depth of water
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95 % of Tree Roots in the top soil layers
12" to 18 " depth
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penetration usually can
be estimated by pushing a 3/8- or 1/2-
inch diameter steel rod into the ground. The rod will stop when it reaches
dry
soil. |
An effective way to water
smaller trees (up to 3 inches in stem diameter) is with leaky
buckets. To water a tree about 2 inches in stem diameter,
place four 5-gallon buckets 2 feet from the trunk in a square
pattern. Two or three holes should be punctured in the sides
of each bucket near the bottom so that the water will drain
out in about five hours. The buckets should be refilled and
moved to dry areas under the same tree two or three times and
then moved to another tree. To water smaller trees, use the
same procedure but with two or three buckets. If the drought
persists, the tree should receive a watering cycle every week
or 10 days.
A permanently installed drip
irrigation system will achieve the same results as the leaky
bucket technique with much less effort.
Over-watering can also
seriously damage trees. It takes 62 gallons to apply 1 inch of
water to 100 square feet. Applying this quantity of water for
several days to a soil that already has a good moisture supply
can suffocate some of the tree roots. Inexpensive timers are
available that can be attached to the outside faucet to ensure
that the desired amount of water is
applied.
Prevention of tree decline
Here are some ways to prevent the
common causes of tree decline:
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Make sure new trees get off to a good
start through proper methods of planting, fertilization,
watering and pruning.
Avoid wounding the tree with lawn
mower, weed eaters construction or other equipment, children
with axes, nails, brace wire girdling,
etc.
Do not use harmful chemicals either
directly on the tree or in the soil beneath it. Be careful
with herbicides applied to the lawn. Always follow the
directions printed on the label. Damage caused
by irrigation with high lime and
salt content is a common problem. It is often
over-looked because the high salts and lime content in
water is common place in some area waters and the appearance of
injury and the symptoms are very gradual. We
don't normally relate danger with irrigation.
Do not top trees to shorten their
height or change their shape. The effect is ugly and very
damaging to the trees overall health. Apply proper pruning
practices
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as recommended by the
International Society of Arborcultrue. Keep leaves, the tree's food manufacturers,
healthy. Do not over-fertilize or apply weed-n-feed type
fertilizers or other pesticides unnecessarily and always
follow the directions on the label.
Avoid soil compaction of the root zone
to maintain proper moisture and air penetration to the roots.
If you decide that a tree must be
subjected to changes in soil grade, seek professional advice
on modifying the impacts. Your local University Outreach, and
Extension center will have publications on how to protect
trees from grade changes or they can refer you to professional
arborist for assistance.
Evergreen trees rarely survive a
complete defoliation. However, most deciduous trees can
tolerate one defoliation. A second defoliation in the same
year or in two successive years may trigger a decline
situation. Take extra precautions to avoid repeated
defoliations.
It is helpful to know what kind of
trees you have in your yard or wood lot. Most species have
their own specific problems. Knowing the causes of tree
decline and understanding how to keep your trees thrifty and
healthy will help prevent early decline or premature death of
your favorite trees.
Tree Decline _ What Is
It?
John P. Slusher, School of Natural Resources, University
of
Missouri-Columbia
ASIAN AMBROSIA BEETLES |
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