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Forest flooding can be both benign or damaging. [Photo by Brian Lockhart, USDA Forest
Floods are one of the most common hazards in the United States, and one of the most
destructive. Trees and water are interconnected. Floods are part of the natural cycle
in many of the most fertile forested areas of the country. Some trees even rely on
occasional flooding to flourish and grow. In return, trees mitigate some of the damaging
power of moving flood waters.
How a flood affects your forest depends on many factors. One factor is the composition
of your woodland. Trees such as bald cypress, tupelo, and green ash that are adapted
to occasional floods are more resilient when waters rise. Trees with thicker bark
fare better, as well, because the bark protects the tree from injury. Another factor
is the nature of the flood. The flood's duration, depth, and time of year are each
Root damage - If the flood waters are fast-moving, they can undermine root systems by washing away
the soil around them. Undermined trees are more likely to fall, and exposed roots
are more susceptible to drying out, freezing or disease.
Injury - Debris in fast-moving waters can wound trees, leaving them open to disease and decay.
Suffocation - Sluggish waters, deposited mud, sediment and waterlogged soils deplete oxygen and
suffocate trees. The higher the water line, the greater the damage. A tree's chance
of surviving goes down if it's completely submerged during the growing season.
New growth - Flooding isn't always a bad thing for a forest. Floods can clear weaker trees and
competing plants from the forest floor, bring in seeds, and stimulate growth of seedlings
and surviving trees.
Flood injury symptoms include the following:
1) Yellowing or browning and wilting leaves,2) Premature fall color and defoliation,3) Twig and branch dieback,4) Sprouting from the base of the tree, and5) Tree death with severe flooding injury.
These symptoms may occur during or after flooding. Some trees may decline for several
years and ultimately die after flooding injury while others may slowly recover.
The response to flooding depends upon the season, water depth, flood duration, water
speed, and the age, health, and species of the tree.
Season of flooding - The most potentially damaging flood time is late spring, just after the leaves have
fully expanded. At this stage of growth, food reserves are low and the stress from
flooding injury may kill a tree. After spring, flood tolerance increases with the
season. During the winter or early spring, before tree growth begins, most deciduous
trees can tolerate several weeks of flooding.
Water depth - There are three flood levels:
1) Saturated soil, but no standing water,2) Water covering the ground and lower tree, and3) Water covering the leaves.
Flood intolerant trees can be injured by saturated soil conditions. Trees that are
tolerant or somewhat tolerant of flooding usually are injured only if water covers
the ground during the growing season or for extended periods during the winter. Almost
all trees are injured if water covers the leaves. If only the lower branches are submerged,
these may die while the remainder of the tree survives.
Flood duration - As flooding and soil saturation continue, oxygen in the soil is quickly depleted.
Roots can survive brief periods of oxygen deprivation but will eventually die. The
length of oxygen deprivation that the roots can survive depends on the tree species.
Water speed - When water stand, tree roots deplete the oxygen quickly. Flowing water continues
to supply some oxygen for the roots, so that flooding impacts are less severe.
Tree species - Not all tree species react the same way to flooding. Some species, those typically
found along rivers, can survive saturated soils and flooding for a time, although
growth will suffer.
An individual tree's tolerance depends on its age and health. Older and healthier
trees can withstand a longer duration of flooding than younger or weak trees.
Damage - After the flood waters have receded, inspect the tree for mechanical injury. During
the flood, tree trunks or branches may have been damaged by floating debris. Prune
broken branches to reduce the chance of disease infection. With a sharp knife, carefully
remove broken or torn bark. Do not apply pruning paint to the pruning wounds or exposed
trunk wood. Pruning paint will not speed the recovery and can increase, not reduce,
the chance of infection. The only exception to this rule is American elm. This tree
should receive a very light coating of paint on wounds to reduce the possibility of
elm bark beetle infestation.
Sediment - If the flood has deposited sediment around the tree, remove this material and restore
the original grade as far out as possible. Even three inches of new soil can smother
intolerant tree roots. This must be done carefully. Some flood tolerant trees survive
by quickly growing roots into the new layer of soil. If the tree has already grown
roots into the new soil, do not try to restore the original grade. This activity may
kill the tree.
Pests - It may take several years for a mature tree to recover from a single year of flooding.
During this recovery time, the tree is stressed and vulnerable to attacks from insects
and diseases. Inspect your tree several times during the growing season and identify
and control any pest problems. In addition, speed the recovery of the tree by fertilizing
them. Apply fertilizer in mid-fall at a rate not exceeding I pound of nitrogen per
1000 square feet of ground area. Higher rates of nitrogen fertilizer may further weaken
the stressed tree.
Trees differ in their ability to tolerate flooding. Flood tolerance ratings of some
of the trees common to our area are listed in the table below.
There isn't much an individual can do to prevent flooding. When rain is heavy and
drainage is slow, flooding will occur. There are a few things landowners can do to
minimize damage from flooding:
Whether you're dealing with flood damage or preparing for the possibility of a flood,
professional assistance can make your life easier. Some of the places you can go for
assistance with your flood planning or recovery include: