Drought Influence on Trees
The following article written by Dr. Wayne Clatterbuck, Associate Professor, Forest Management and Silviculture, appeared in a newsletter published by the UT Institute of Agriculture Department of Forestry, Wildlife, and Fisheries.
A tree strives to be in equilibrium with its environment. Any time the tree is not in equilibrium, the tree is stressed and must spend extra energy to survive. Drought has killed and will continue to kill trees. Drought leads to decreased rates of diameter and height growth, poor resistance to other stresses, disruption of food production (photosynthesis) and distribution, and changes the timing and rate of physiological processes, like flower and fruit production.
Most of the variation in tree growth is associated with water supply problems. The term "drought" denotes a period without precipitation during which the water content of the soil is reduced to such an extent that trees suffer from lack of water. Water deficits in a tree are formed when transpiration (the process by which leaves emit moisture and oxygen) exceeds the water supply available to the leaf. Trees need a drink of water too!
Droughts are common in southeastern landscapes. Many trees are stressed by prolonged periods of hot, dry weather. Selecting trees that use water efficiently without the need for frequent watering or irrigation is one way to make your landscape more resistant to droughts. With impending water shortages in many urban areas leading to prohibitions of irrigation or water, planting trees that are more tolerant to drought conditions is the best long-term solution to a healthier, low-maintenance landscape.
A few factors to consider when selecting trees that use water efficiently are:
• Native trees are better adapted to local soil, moisture, climate and pest conditions than non-native trees.
•Trees with small leaves (linden, elm, ash, willow oak) are more easily cooled and have better water-use efficiency than trees with larger leaves (sycamore, cottonwood, basswood).
•Upland species are usually more drought-resistant than bottomland species.
•Early successional species, those that colonize old fields and disturbed sites (pines, black locust, elms), use water more effectively than late successional species (sugar maple and beech).
•Trees with deep, upright crowns are more effective in water use than those with flat, wide-spreading crowns.
•Trees with multi-layered crowns having many branches and leaf layers (oak, ash, gum hickory) are more water efficient than those trees with leaf canopies that concentrate leaves in single layers along the outer edge of the crown (beech, sourwood, redbud, magnolia).
•Drought-tolerant plants usually have thick leaf waxes and bark, efficient stomatal control and extensive root systems.
Examples of a few trees that are not drought-tolerant include black cherry, basswood, beech, birch, buckeye, cottonwood, dogwood, sassafras, sugar maple sycamore and yellow-poplar. These species respond to drought by shedding leaves prematurely or wilting.
Although there is not an ideal drought-resistant tree for every landscape, many trees have drought resistant features (small leaf size, degree of leaf wax, crown form) and are more tolerant of dry conditions than others. Refer to the following UT Extension publications for more information on trees with drought-tolerant attributes and information on why trees become stressed and die.
http://www.utextension.utk.edu/publications/spfiles/sp570.pdf
http://www.utextension.utk.edu/publications/spfiles/SP615.pdf
For more information contact Wayne Clatterbuck at 865.974.7990 or wclatterbuck@utk.edu.
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