Wilderness Underfoot: Trees damaged by drought

StoryImage( ‘/Images/Story//Auto-img-11273316544373.jpg’, ”, ‘A long dry season – The U.S. Drought Monitor report, issued this month by the National Drought Mitigation Center (NDMC), noted that spring of 2005 was the seventh driest on record for our region. Summer was even worse, ranking as the third driest. Year-to-date rainfall is about 10 inches – half the normal rate. This is considered an extreme drought here. (See the Web site at: www.drought.unl.edu/dm/monitor.html) Unfortunately, watering trees in many localities is not an option. Where drought has had an impact on the water table and municipal supplies, water is too precious to spare on gardens, lawns and trees. We can only wait and hope at this point. To end the drought, a month of heavy rain (over twice normal rainfall) followed by several months of normal precipitation would help to replenish the ground water in time for the next growing season.’);

Of all ecological resources, water will have the greatest impact on a tree’s growth

This year’s drought has devastated plants throughout our region, drying up prairies, forests, and agriculture. For some plants, the lack of rainfall means a season of growth is lost. Others will die off.

Trees are especially threatened. More than 80 percent of the variation in a tree’s growth can be associated with whether it has an adequate water supply. In growing seasons when water is limited, trees can suffer from slower growth, lowered resistance to stress and pests, and interruption of normal processes such as food production, fruit and seed production. Extreme drought will eventually begin to kill off trees – especially those transplanted within the last five years. Root systems of young and recently transplanted trees are often too shallow to reach water.

Trees have natural defenses to protect them from drying out. Water is normally drawn out of a tree through leaf surfaces and leaf “stomates” – tubes that allow gas and vapor exchange. It is natural for stomates to close when heat and dryness are intense, particularly in the middle of the day. During drought, an early line of defense is for a tree to close its stomates, but this will only limit water loss, not prevent it. Water continues to evaporate from leaf surfaces, twigs, and limbs.

As the supply of water diminishes, leaf tips are first to be cut off from the tree’s water supply. In an extended drought, leaves begin to dry, curl and turn brown. At this point, if normal rainfall occurs, it may be too late for the photosynthesis to start again in the leaves. As the drought becomes more severe, a tree will sometimes shed its leaves, often before their stored nutrients have been absorbed into the tree. After this, it may not be possible to save the tree.

From the Sept. 21-27, 2005, issue

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