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AN
OVERVIEW OF FOREST HEALTH
Many of us
would agree that, whatever our personal objectives when managing
a forest, the end result we seek is a healthy, sustainable forest.
In a healthy forest, the risk of present or future damage from natural
or human-caused agents has been minimized. Forest health has been
defined as the condition of a forest when it is:
- Resilient
to change
- Biologically
diverse over a large area (landscape diversity)
- Able to
provide a sustained habitat for vegetation, fish, wildlife and
humans
A healthy forest
is made up of healthy trees. Just as humans need a certain combination
of food, water and exercise to maintain physical health, trees require
certain inputs to maintain their health and growth. If one or more
of these inputs is missing or insufficient, the tree experiences
stress. Forest managers can help trees by practicing silviculture
(from Latin silva, forest) to influence these inputs. The number
one health problem in Washington forests is stress caused by having
too many trees to the acre, or overstocking. Overstocking causes
tree stress because it forces trees to compete with their neighbors
for light, water and nutrients. Many silvicultural practices are
effective because they reduce the number of trees to the acre and
hence the competition for these essential elements. The first requirement
for healthy tree growth is light. Plants manufacture their own food
by using the sun's energy to convert carbon dioxide and water to
a usable food source. Heavy shade, such as is found underneath the
closed canopy of the forest, provides insufficient energy for the
smaller, less dominant trees to grow very fast. Some species which
are called intolerant cannot grow under these circumstances at all.
These include pines and larches. Other species such as Douglas-fir,
Engelmann spruce and most hardwoods are considered moderately tolerant.
They can grow in partial shade. The tolerant species can grow under
conditions of heavy shade, although not very fast. These species
include grand fir, hemlock, and western redcedar. Silvicultural
practices that increase light include thinning and selective harvesting.
A thinning operation can cause small, slow-growing (suppressed)
trees in heavy shade to receive much more light and to grow much
more quickly. This is called a release operation. Harvesting by
clearcutting or patch cutting permits enough light to reach the
forest floor that the intolerant species such as pine are able to
seed in and grow.
The second
requirement is water. Trees species vary considerably in their need
for water and their tolerance for drought. Shade-intolerant species
commonly grow in hot, sunny areas and thus are more resistant to
drought. Shade-tolerant species, on the other hand, grow naturally
in the cool, moist forest. When drought occurs, which happens frequently
in all western states, these shade-tolerant species get more stressed
than the shade-intolerant species. A forest manager may try to alleviate
this type of stress by thinning the stand. Thinning reduces the
total number of trees competing for water and thus can relieve drought
stress. However, over-thinning (the removal of too many trees) may
increase the amount of sunlight reaching the ground and dry out
the area more rapidly. This is especially true on slopes and steep
terrain. An individual with ornamental trees
near their home may wish to water them in dry periods, especially
if they are trying to encourage rapid growth.
The third requirement
is a good nutrient supply. Trees take up minerals through their
roots and incorporate them in the developing cells. One of the basic
determinants of potential growth of a tree is the level of nutrients
available in the soil. Nutrient-poor soils such as serpentine will
never produce large trees, but rich soils can't produce large trees
if they are overstocked. A forest manager may thin a stand to reduce
competition for nutrients. Although it is not always cost-effective,
using a fertilizer on forest soils can provide needed tree nutrients.
Light, water
and nutrients are essential for tree growth, but a healthy tree
is additionally free from agents of stress. Just as an unhealthy
person may return to health through natural healing processes and
the removal of the organism or situation causing illness, these
agents of stress do not necessarily mean the death of the tree.
However, depending on the severity, they can kill a tree outright,
especially if it has been predisposed by insufficient light, water,
or nutrients.
Temperature
extremes. Either very high or very low temperatures can cause injury
to trees. The problem is worse when the temperature extreme occurs
suddenly. Late frosts in spring can kill the emerging shoots, while
early frosts in autumn can injure shoots that are still growing.
In the winter, very cold temperatures can kill trees outright, or
periods when the soil is frozen followed by sudden warming trends
can severely stress or kill trees. Additionally, heavy snow can
cause breakage of the top or limbs, as can ice storms. In the spring,
sudden hot temperatures can kill the emerging shoots. After a thinning
or pruning operation, the newly exposed southwest side of the tree
may become sunburnt; this is called sunscald. The bark may crack
or scale off as it slowly heals,
or the tree may be killed.
Flooding. Too
much water can be as much of a problem as not enough. Tree roots
need to be able to "breathe;" there are air spaces in
soils, even muddy soils. However, during flood conditions these
air spaces fill with water and the tree roots can literally drown.
Trees that are naturally adapted to riparian, or streamside, areas
are usually able to survive a lengthy period of flooding, but most
forest trees will be damaged or killed. Local flooding is not simply
restricted to storm conditions, but can occur as a result of beavers
building dams.
Fire. Fires
can be beneficial or highly damaging, depending on their intensity.
A low intensity fire can effectively thin a stand by removing younger
trees and species that are not fire resistant, such as grand fir
and western hemlock. However, trees that remain after a fire may
be stressed from charring. Pines that have been scorched are often
attacked by red turpentine or other bark beetles, which can finish
them off.
Wind. Wind
can be a problem in stands that have recently been thinned or are
adjacent to clearcuts. The trees in those stands have developed
with many trees acting as windbreaks; when nearby trees are removed
the trees left may not be strong enough to hold themselves up in
a strong wind. Rather than thinning drastically once, a forest manager
can thin regularly through the life of a stand, which will permit
the trees to grow strong. When preparing to harvest, a forest manager
can design small clearcuts that will not expose a great deal of
the adjacent forest to the effects of wind.
Insects and
diseases. Many insects and diseases can cause stress to the tree;
they are more likely to attack if the tree is already under stress
from insufficient light, water or nutrients. See other "Forest
Health Notes" in this series for descriptions of specific insects
and diseases.
Pollutants.
Airborne or waterborne pollutants can negatively impact the growth
of a tree. Ozone, so scarce in the upper atmosphere, can be in a
higher concentration near the ground. It is particularly troublesome
near large cities, where it is produced by automobiles. Other types
of air pollution include sulfur dioxide, which is serious in the
upper Columbia River Valley, and hydrogen fluoride, which has been
a problem near Spokane. All these gaseous pollutants cause foliage
damage, thus preventing normal growth.
Logging damage.
Heavy equipment in the woods can cause very serious damage. Trees
hit by passing equipment or falling trees can receive extensive
scarring, which can lead to problems with rot. Additionally, heavy
equipment compacts the soil, crushing the air spaces in the soil
that tree roots need to breathe. For this reason it is difficult
to preserve trees near construction sites, as is often tried on
new homesites developed on former forest lands. See "Animal
and Mechanical Damage," also in this series.
Animal damage.
Feeding damage caused by bears, deer, gophers, etc. can put a tree
under stress as it strives to heal the damage. Young trees often
succumb to pocket gophers, while older trees may have reduced growth
for some years as they heal over a bear scar. See "Animal and
Mechanical Damage," also in this series.
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