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Drought and how it affects trees



By Sun Advocate

These trees in a desolate area of Carbon County, despite their tough nature, are suffering from the drought. Many trees in the area where water once flowed freely are beginning to suffer after four years of little moisture.

While most people in eastern Utah are concerned about the drought affecting agriculture and power generation, some are starting to reflect on what a bad long term drought can do to trees.
Right now the hills are full of dead trees, some of which have died of old age, some from disease and some from the lack of water. Now some places in the valleys are dry enough that trees are beginning to show the stress of drought as well.
A tree must always 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. Trees can only react to their environment in genetically preset ways. People can help minimize tree stress by understanding them.
Water is the single most important substance for tree life. All the life processes of a tree take place in water. The most apparent use of water is for transporting materials from the roots to the shoots.
As water moves from the soil through the roots and into the leaves it carries with it many essential elements that trees need to survive and grow. As water and elements move from the roots to the shoots, chemicals are added by the roots. The shoots are continuously informed by these chemicals of the water and nutrient status of the roots.
Water use in trees is primarily physical. There are a few points of biological control that override the physical process of water movement. The soil/root interactions, vascular system and leaves all provide resistance to water movement. Water movement and evaporation is a function of temperature and energy in the environment.
Trees act as conduits through which water passes. Instead of water evaporating at the soil surface, the tree provides an elevated surface for water evaporation. At the junction between tree and atmosphere is a biological control valve called a stomate. Throughout the entire water stream moving in a tree, the leaf stomate is the only portion that can be actively controlled by the tree to conserve water.
Drought has killed and will continue to kill trees. Drought leads to deceased rates of diameter and height growth, poor resistance to other stresses, disruption of food production and distribution and changes the timing and rate of physiological processes, like flower and fruit production.
Eighty to 90 percent of the variation in tree growth is because of water supply problems. Effects of drought occur throughout a tree.
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 tree leaves emit moisture and oxygen) exceeds the water supply available to the leaf.
Trees cope with drought generally by following these priorities.
•Sensing root zone stresses
•Changing stomatal conductance (opening and closing)
•Increased absorbing root production
•Using stored food
•Root suberization (development of waterproof layer around damaged areas)
•Initiating foliage, branch and/or root abscission.
The movement of water in a tree is governed by stomates (stomata). Stomates are tiny valve-like openings usually on the bottoms of leaves. Stomates can be closed or opened by the tree. As the stomates are opened, water begins to evaporate from the leaf. Anytime the atmosphere has less than 100 percent relative humidity, water will evaporate. As each water molecule evaporates, it pulls another water molecule into its place. Long chains of water molecules can be pulled by one end because this cohesive force.
Through the millions of stomates on a large tree, water is continually pulled up from the soil by the roots. All other plants in the area are also pulling water from the soil. The results can be measured by tensiometers as a landscape or tree starts to move water in the morning.
When water becomes scarce, the tree begins to have problems. With open stomates, water is still evaporating from the leaves but it is more difficult to pull water from the soil for replacement. Water is lost more quickly from the leaves than the roots can pull in it, creating a water shortage in the tree. When the water shortage becomes too great, the stomates close until water uptake in the roots catches up. When the stomates are closed, no carbon dioxide can get inside and the leaf cannot conduct photosynthesis (make food).
Water in a tree is in long columns or tubes. These long columns of water that stick together are like rubber bands. When the stomates are open, the rubber bands (water columns) are being pulled from both ends. If the stomates close, the rubber bandlike water columns are still under tension.
At night, the tension left in the water column, like a stretched rubber band, continues to pull up water even though the stomates are closed. When there is plenty of soil water available, the tension in the columns is greatly reduced by the next morning.
Water movement through a tree is controlled by the tug-of-war between the water availability and movement in the soil versus the water loss from the leaves. The normal seasonal rate of water movement in some trees can be rapid.

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