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TREES and DROUGHT
Article #98, August, 2005
By Bill Cook
Drought affects
trees in a variety of ways depending on many conditions. Water stress may kill
a tree or, more commonly, predispose it to a wide variety of ailments. Some
of these ailments may not become visible for several years.
Root systems
draw water from the soil. That water moves throughout the tree to maintain chemical
reactions in the living cells. As trees respire, water is expelled and "leaks
out" through small pores in the leaves, twigs, branches, and trunk.
During
dry periods, the larger humidity difference inside and outside the tree causes
increased water loss. Higher temperatures accelerate cell metabolism, which
requires additional amounts of water. The response to water stress involves
closing the many small pores and drawing more water from the soil.
As soil moisture
becomes increasingly low, maintaining an adequate water balance in living tissues
becomes more difficult. Sandy or coarse soils dry out quicker. Loamy soils hold
available water best. Heavy soils hold more water longer, but much of it becomes
unavailable to small tree roots due to the physical chemistry of water and very
tiny soil particles.
The inability
of a tree to maintain an adequate water balance, for whatever set of reasons,
is called water stress. A moderate amount of stress may slow growth or cause
premature autumn color change. More stress might first result in death of leaf
tissue. Browning leaf edges due to water stress is called leaf scorch. Conifers
may drop older, less efficient needles. Excessive water stress can kill trees,
especially those of certain species and very young trees that have less developed
root systems. This year's plantings and transplants will likely suffer high
mortality rates.
After a drought
breaks, the impacts of drought are not over. More commonly, the weakened condition
of trees allows a number of pests and pathogens to more successfully attack
tree tissues. Trees whose leaves are eaten by gypsy moths or budworms, in combination
with a drought, are at higher risk of damage or death. Water stressed pines
attract bark beetles, which can kill trees, especially red pine. Mature oaks
in sandy soils may die from two-lined chestnut borer attacks over several growing
seasons.
The introduction
of fungal pathogens during these weak periods can have a longer lasting impact.
For example, an Armillaria fungus might enter water stressed roots. Over several
years, that fungus might grow and eventually kill the tree. On the other hand,
with good growing conditions the natural defenses of the tree might defeat the
fungus. Some tree species are better at this than others.
Droughts
make trees more susceptible to cold winter weather. By adding a light application
of phosphorus and potassium fertilizer, a tree will be better "winterized".
Be careful to not add nitrogen.
The drought
impact on trees can be very localized. Sometimes, a single tree within a group
will die. This is common in plantations. Death might be in the year of the drought
or it might be a couple years later. Balsam fir commonly displays this behavior.
Soil structure
can be highly variable resulting in other local impacts. A tree growing above
a large boulder or over a hard pan might die, while its neighbors may recover.
Small clay pockets, fissures in bedrock, microtopography, and sandy pockets
contribute to local variability that can spell survival or death for a water
stressed tree.
In 2004,
much of the Upper Peninsula experienced a cool, wet summer. Higher water saturation
levels in the soil forced tree roots to grow closer to the surface in order
to obtain enough oxygen, and sometimes killed roots further down. That new root
growth near the surface became more vulnerable to soils dried out by this year's
drought. Many tamaracks and other shallow-rooted tree species have died because
of this extreme wet-dry cycle.
Competition
for scarce soil water becomes intense during dry periods. Most trees simply
cannot out-compete grasses and herbs for water. We normally think of competition
for light, but the underground battle for water and nutrients can be more important
to the long term survival of trees.
Any time
drought is combined with other health issues, trees are at higher risk of damage
and death. Drought can tip the scales for trees simultaneously facing an insect
attack, a disease, or damage from home construction, laid cable, paving, lawn
mowing, etc. Conversely, trees weakened by drought may fall victim to opportunistic
insects and diseases several years ahead.
Drought
impacts are usually longer term affairs, in addition to current year effects.
Doubtless, we will see the complex effects of the drought of 2005 for several
years to come.
- 30 -
Trailer
Bill Cook is an MSU
Extension forester providing educational programming for the entire Upper Peninsula.
His office is located at the MSU Upper Peninsula Tree Improvement Center near
Escanaba. The Center is the headquarters for three MSU Forestry properties in
the U.P., with a combined area of about 8,000 acres. He can be reached at cookwi@msu.edu
or 906-786-1575.
Prepared
by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005
J Road, Escanaba, MI 49829
906-786-1575 (voice), 906-786-9370 (fax), e-mail: cookwi@msu.edu
Use
/ reprinting
of these articles is encouraged. Please notify Bill Cook.
By-line should read "Bill Cook, MSU Extension" Please use the article
trailer whenever possible.
Michigan State University is an affirmative action equal opportunity institution. The U.S. Department of Agriculture prohibits discrimination on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital status or family status. (Not all prohibited bases apply to all programs.)
This website is maintained by Bill Cook, Michigan State University Extension
Forest in the Upper Peninsula. Comments, questions,
and suggestions are gratefully accepted.
Last
update of this page was
22 September, 2005
This site is hosted by School of Forest Resources and Environmental Science at Michigan Technological University.
