Drought and Its Effects on Austin Trees

For only the third time since the year 2000, most of Travis County, including Austin, is now experiencing "Exceptional" drought conditions. This is according to the U.S. Drought Monitor that was released on December 4th, 2008.

So what happens to trees and plants during a drought or when they aren’t receiving the amount of water they need? First lets look at how trees use water.

Aside from its role in photosynthesis and other chemical reactions, water plays a significant role in the physical processes of plant growth. 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.

When drought conditions occur, a trees 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.

During a drought, roots shrink and 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.

When a tree recognizes a crisis such as with a drought, the first thing it does is close its stomates. Recovery from stomatal closing is slow and may not return to normal for hours, days or weeks despite availability of water.

A secondary problem for a tree when water is reduced is that the tree’s metabolites are reduced. These metabolites helps the tree protect itself from diseases and insects. Sometimes we notice a tree is in bad shape when we see it being attacked by beetles or other types of insects.

Some effects of drought on trees are apparent, and other effects are unseen, though just as damaging. Here are three major damages that drought can do.

1. Dead feeder roots
The fine feeder roots that collect moisture and nutrients for most trees are located within the first 15-18 inches of topsoil. If that soil dries out for a long time, the feeder roots will die. Without feeder roots, the tree cannot effectively absorb nutrients and water, even when it rains again.

2. Reduced photosynthesis
Some trees apparently drop leaves as a drought-survival mechanism. Without leaves (or with fewer leaves,) they lose less moisture through evaporation. However, photosynthesis is reduced when leaves fall prematurely, so there are risks as well as benefits to this drought response.

3. Wilting
Wilting is a visible effect of drought. As leaves dry, turgor pressure in leaf cells decrease causing leaf petiole drooping and leaf blade wilting. The amount of water lost before visible leaf wilting varies by species. Temporary wilting is the visible drooping of leaves during the day followed by rehydration and recovery during the night. Internal water deficits are reduced by morning in time for an additional water deficit to be induced the following day.

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, photosynthetic apparatus, and morphology are made, again taking months or years depending upon the severity and duration of soil water deficits.

Supplemental watering of trees can be timed to help trees recover water and minimize pest problems on surrounding plants. Watering from dusk to dawn does not increase the normal wet period on plant surfaces since dew usually forms around dusk. Watering during the normal wet period will not change pest/host dynamics. Watering that extends the wet period into the morning or begins the wet period earlier in the evening can initiate many pest problems.

If you detect that your tree has already been damaged from drought, contact your local arborist for an evaluation on what you should do.