Impact of Drought on Trees
By Noel Mues, Extension Educator,
Summer has just begun and it’s already been extremely hot, dry and windy. Drought takes its toll on landscape plants, including trees, which have a more extensive root system to help them through extensive periods of dry weather.
To understand the long-term impact of drought on plants, it is important to understand the water relationships that occur in the plant. Without water there is no plant life. It is the center of chemical reactions, which occur within the plant cell. Water is the major component of protoplasm, which makes up cell content. Water also serves as a raw material for many metabolic processes. The uptake of mineral nutrients from the soil occurs in water.
In the root system of a tree or shrub, the very fine, absorbing 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 the fine roots, the water absorbing capacity of the plant is severely reduced.
In addition to these losses, other plant responses to drought may also occur. For example, the closing of the leaf pores or stomates is usually the first response. Since plant roots take in carbon dioxide through these leaf pores, it is not surprising that there is a reduction in carbon dioxide fixation in photosynthesis and subsequent carbohydrate production. Carbohydrates produced in green tissue by photosynthesis are used as a substrate for other synthesis reactions in plant cells. Among the products normally manufactured from the carbohydrates are fats, proteins, growth regulators, and many secondary metabolites.
Secondary metabolites are responsible for many of the defense mechanisms a plant needs to thwart infectious diseases and certain insect attacks. Thus, susceptibility to disease infestations and insect invasions increases. This may occur a year or even two years after a drought, even though the current year is moist. There are certain diseases that are more likely to occur because of moisture related plant stress. These include root rots, cankers, wood rots, and some of the wilts.
In a healthy tree, a disease of one type or another can be isolated or compartmentalized.
A drought stressed tree with reduced metabolic activity may be unable to compartmentalize the disease. As an example, canker diseases such as Cytospora canker, a common disease of cottonwoods, are almost always associated with drought stress. It is the inability of the plant to synthesize protective chemicals and to compartmentalize wounds that allows for infection and development of disease. Drought stressed trees and shrubs are also predisposed to other diseases including Sphaeropsis tip blight, one of the common diseases of pine trees.
Studies have shown a correlation between the levels of a 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. Bronze birch borer and other bark beetles often result.
Proper year around watering can reduce or eliminate drought damage. However, the negative effects of drought may continue to be a problem for years to come. It is important to understand that when evaluating the cause of a tree problem.
Periods of extended drought, even into the fall, are very important times to water landscape trees. Watering may be done right up until the ground freezes and that may not occur until after Thanksgiving. During recent winters, the ground probably didn’t freeze at all in some locations. It is important that the upper soil profile be moist. For trees, this means the upper two feet of soil. This requires a good soaking over time. Getting the top three inches of soil moist during extended dry periods will not help a great deal since evaporation will be a factor. The very best thing that you can do for your landscape trees is to apply a layer of coarse, wood-chip mulch in a 4 to 5 foot diameter around the base of the tree. This will go a long way to help conserve moisture and protect the tree from mowers and weed whips.
Upcoming Activities
The Holbrook Pre-Fair Beef, Lamb and Goat Show will be held on Saturday, June 30, 2012. Beef weigh-in begins at 6:30 a.m. and the show begins at 8:00 a.m. Lamb weigh-in begins at 9:00 a.m. and the show begins immediately after the beef show. Goat weigh-in begins at 11:00 a.m. and the show begins immediately after the lamb show.
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The 4-H Presentation Contest will be held beginning at 7:00 p.m., Monday, July 2 at the
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The Furnas County Fair will take place the week of July 16-21, 2012. County fair premium books are available at the Extension Office or from the Bank in your local community.
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