Long-term
Drought Effects on Trees and Shrubs
Recurring and prolonged periods of drought seem to have become
commonplace in many regions of the country in recent years. The
effects on trees and shrubs can often be seen in both natural and
man-made landscapes during the severest of droughts as leaves wilt,
show marginal scorch, or prematurely drop from the plant. However,
the long-term effects of drought on the health and survivability
of woody plants are less obvious.
Furthermore, the perception of drought can be misleading. In 1999,
New England experienced one of the worst droughts of the century.
Yet, the total rainfall for the year was at or just above normal
levels. The "normal" annual rainfall figure could be accounted
for by a significant amount of rain that fell in just one event.
That was a mid-September storm, associated with fallout from Hurricane
Floyd, that deposited up to 9 inches of rain on some areas.
With respect to influence on plant growth and health, it is the
amount and frequency of rainfall received during the growing season
that is of greater significance than the total amount of precipitation
in a given year. When defining a drought year, the pattern and frequency
of rainfall are clearly more important than the total amount of
rain.
Water relations in plants
To understand the impact of drought on plants, a review of water
relations can be helpful. Without water, there is no plant life.
It is the milieu in which chemical reactions inside a plant cell
take place. Water is the major component of the protoplasm which
makes up the content of cells. Water also serves as a raw material
for many metabolic processes, including photosynthesis.
Aside from its role in photosynthesis and other chemical reactions,
water plays a significant role in the physical processes of plant
growth. As new cells form, they expand in response to turgor pressure,
that is, the pressure exerted on the cell membranes and walls by
water entering the cells. Following this expansion, the cell adds
additional layers of cellulose to form a more permanent cell structure.
This is a fundamental process in the elongation of newly formed
cells and the growth of woody plants.
The uptake and transport of mineral nutrients occurs in water.
Minerals dissolved in soil water move into plant roots and then
to the vascular system, more specifically the xylem, for transport
throughout the plant. There they combine with proteins to form enzymes
that control the biochemical reactions essential to plant health
and growth.
In the simplest terms, the driving mechanism for the transport
of water through a plant is transpiration or the evaporation of
water from plant leaves. Tiny pores or stomates in leaves control
the amount of transpiration. Stomates open and close in response
to water availability as well as other internal and external factors.
Transpiration occurs as long as stomates are open. Transpiration
sets up a negative pressure that drives the movement of water from
the soil to the plant and through the plant via the xylem. The movement
of water from soil to roots to xylem to leaves to air can be viewed
as a continuum. Interruption at any stage of the continuum stops
or reduces the flow of water, leading to wilting of leaves.
Drought effects on plants
Drought conditions, or more specifically, soil water deficits, can
influence the normal physiology and growth of plants in many ways.
For example, when a soil water deficit exists, the result may be
an increase of solute concentration outside the roots compared to
the internal environment of the root. Such a situation leads to
reverse osmosis, i.e. a net movement of water from the cell to the
soil solution. As this happens, the cell membrane shrinks from the
cell wall and may eventually lead to death of the cell. This type
of injury can also occur when a build up of road salts or excess
fertilizer occurs in the soil environment around plant roots.
More commonly, when a water deficit occurs, soil water tends to
shrink away from the interface with water-absorbing roots, creating
a gap in the soil-plant-air continuum. As the plant continues to
lose water via transpiration, water is drawn from root cells resulting
in shrinkage of cell membranes.If these conditions persist, the
integrity of the cell membrane and the living cell itself may be
destroyed.
In the root system of a tree or shrub, the delicate root hairs
that extend from epidermal cells and feeder roots at the extremities
of the root system are responsible for the bulk of water uptake.
Confined to the upper 15 inches or so of the soil profile, they
are also the first part of the root system affected by dry soil
conditions. With the death of root hairs, the water absorbing capacity
of the plant is severely reduced.
The effect of drought is particularly acute for newly transplanted
trees and shrubs since they are already devoid of a sizeable portion
of their water-absorbing roots - the roots being lost in the digging
and transplanting process.
In addition to the potentially catastrophic effect of reverse osmosis
and loss of water absorbing ability of roots on plant health and
survival, other responses of a plant to drought may also occur.
Closing of stomates is usually the first response. There is some
evidence that abscisic acid, formed in roots in reaction to soil
water deficits, is transported to leaves and initiates the closing
of stomates. Recovery from stomatal closing is slow and may not
return to normal for hours, days or weeks despite availability of
water. Increase in abscisic acid production also leads to an inhibition
of bud and leaf development, and to promotion of leaf abscission.
The consequences of stomatal closing are many. Since plant leaves
take in carbon dioxide through these leaf pores, it is not surprising
that there would be a reduction in both carbon dioxide fixation
in photosynthesis and subsequent carbohydrate production. This in
turn leads to a decrease in leaf expansion and eventual shedding
of leaves. There may be as much as a ten fold decrease in the photosynthetic
production for a given plant specimen as a consequence of leaf surface
area reductions.
The carbohydrates produced in green tissue by photosynthesis are
used as substrate for other synthesis reactions in plant cells.
Among the products normally manufactured from these carbohydrates
are fats, proteins, growth regulators, and many secondary metabolites.
Growth regulators, such as cytokinins and gibberellic acid, are
responsible for controlling growth in plants.The production of growth
promoting cytokinins and gibberellic acid decreases in plants stressed
by drought.
Secondary metabolites are responsible for many of the defense mechanisms
a plant needs to thwart infectious diseases and certain insect attacks.
These metabolites include oleoresins, tannins and alkaloids. Initially,
there may be an increase in the production of secondary metabolites
in drought stressed plants as carbohydrates are redirected to the
synthesis of these products. However, as the severity of drought
persists, the amount of secondary metabolites decreases and plants
become vulnerable to secondary attacks by certain insects and diseases.
With prolonged drought, there is also a break down of the photosynthetic
machinery itself, further compromising the health of the plant.
Before a plant can resume normal growth rates, rebuild damaged plant
structures, and resume synthesis of growth regulators and secondary
metabolites needed for self-defense, it must re-establish the photosynthetic
machinery and normal functioning of the stomates. In the meantime,
food reserves are consumed to compensate for reductions of photosynthetic
product. Therefore, visible symptoms of drought-related decline
in plant health may not be evident for weeks, months, or years after
the drought event. Further decline will often take place while repairs
to root system, photosythetic apparatus, and morphology are made,
again taking months or years depending upon the severity and duration
of soil water deficits.
Symptoms
It is hopefully clear from the preceding discussion that the effects
of drought can be characterized as short term and long term. Immediate
visible effects of drought damage include wilting, scorch, and some
defoliation due to loss of turgor in plant cells, irreversible shrinkage
of cell membranes, and increased synthesis of abscisic acid. Long-term
symptoms of drought include dieback of branches and death of the
plant as the plants capacity to absorb water is damaged. These are
the primary or direct effects of drought.
Secondary effects
There are also secondary effects that relate to the reduction in
synthesis of secondary metabolites. Susceptibility to disease infections
and insect invasions is heightened as the plant's ability to ward
off these problems is diminished.
It is difficult to attribute plant disease problems to drought a
year or two after a drought event, especially if the current year
is a moist one. However, there are certain diseases that are more
likely to occur because of drought related stress on the plant.
These diseases in combination with weather history at the site can
be used in the diagnosis of drought related problems. Among the
types of diseases likely to occur in response to drought related
stress are root rots, cankers, wood rots, and wilt.
A good example of delayed secondary effects of drought on trees
and shrubs is Armillaria root rot, also called shoestring root rot
because the fungus forms shoestring-like structures called rhizomorphs
that invade roots. Armillaria is an opportunistic fungus, that is,
it is generally present in soil and on the surface of roots but
only invades when roots are weakened or damaged by some type of
stress, such as drought. In a healthy plant, the disease can be
isolated or compartmentalized. A drought stressed tree with reduced
metabolic activity may not be able to compartmentalize the disease.
The fungus invades cambium tissue and is capable of killing a small
tree in a season or two. Large trees with an extensive root system
may not begin to show symptoms of decline until half the root system
is killed by the fungus. This may occur several years after the
drought event and initial invasion by the Armillaria fungus.
Among the canker type diseases likely to be encountered are Nectria
canker and Cytospora canker. These diseases are almost always associated
with drought stress. Again, it is the inability of the plant to
synthesize protective chemicals and to compartmentalize wounds that
allows for infection and development of the diseases. Drought stressed
trees and shrubs are also predisposed to several other diseases
including Diplodia tip blight, Rhizosphaera needlecast and Verticillium
Insect problems
The invasion of wood boring insects such as bronze birch borer,
black stem borer (Ambrosia beetle), and other bark beetles, noticeably
increases in trees that are drought stressed.
Studies have shown a correlation between the levels of the secondary
metabolite, oleoresin, and a plant's susceptibility to invasion
by wood boring insects. A healthy tree produces oleoresin that acts
to deter feeding by borers.When a tree is severely stressed, there
is a decrease in oleoresin production and a corresponding increase
in borer attacks.
Conclusion
While drought conditions seem to have been alleviated in many previously
stricken areas of the country, plant practitioners need to be aware
that the negative effects of drought on plant growth and health
may continue to be a problem for years to come. It is important
to keep this in mind when evaluating the causes of plant problems.
Ronald F. Kujawski, UMass Extension Educator
9/00
Return
to Plant Culture and Maintenance fact sheet index
|
