tt-103 BY DON MEEHAN

DROUGHT INJURY TO TREES

We have gone through two exceptionally dry summers and now our weather seems to be providing us with the moisture we need to keep our yards and crops growing. One of the problems which will continue to remain evident this year is the damage done to trees by drought. 1987 was one of the driest years in the last 100 years for western Washington. I have received many calls, as have forest managers throughout the region, about evergreen trees turning red. To help in understanding what is happening to evergreens, I am sharing comments by Department of Natural Resources Forest Pathologist Ken Russell from Olympia. Ken points out that evergreen conifer trees are more affected by droughts than deciduous trees because conifers keep their needles year round, hence transpiring year round. Here is an abbreviation of Ken's comments. Copies of the full text can be obtained by contacting my office. We will be glad to mail it to you free of charge. Drought injury should not be confused with normal fall yellowing of cedars, firs, and pines. This occurs in late summer just before deciduous tree leaves begin to turn. Evergreens lose their three to five year old needles from the insides of branches each fall, giving them the appearance of dying from the inside out. Winter storms often remove most of the dead needles. WHAT HAPPENS WHEN A TREE SHOWS DROUGHT SYMPTOMS? Imagine the tree's water transport system as a series of microscopic tubes in a bundle, the trunk. Water is pulled up the cells of the trunk in an unbroken string of molecules extending from the root cells to the stomates, the microscopic openings in leaves and needles. The energy for this water lift is provided by the sun as it evaporates water from the leaves. As each molecule of water vapor exits a stomate, a replacement molecule enters a root cell. Everything is fine as long as the soil has adequate moisture. When drought strikes, and the soil is exhausted of moisture, the sun keeps pulling water through the stomates and the water column stretches until it is tensioned like a rubber band. Eventually, the tension increases to a point where the water column breaks. It is almost impossible to rejoin the column because of the shortage of water near the roots. The sun relentlessly pulls more water vapor through the stomates and the permanent damage is done. Branches and foliage above the break being to fade and die because of the cut off water supply. Breaks may occur at several places throughout the tree, resulting in tops, individual branches or whole trees fading and turning red. The same moisture tension can build in a conifer tree in winter even under a wetter soil condition. When soil temperatures are below about 45 degrees, there is very little uptake by roots. Sunny winter days in mild western Washington can pull even more water vapor through stomates, causing additional mortality and finishing off those trees which are already at the brink of survival. Trees growing on drought-prone soils in western Washington can go into severe water stress after only three weeks without rain. Early drought stress may be hidden because symptoms don't appear until late in the summer, and by that time it may be too late to save them. WHICH TREES SHOULD BE WATERED? While drought injury could affect any tree, some are more susceptible than others, mainly due to their location, and soil or site conditions. Trees growing in deep loamy soils that stay moist may not be affected. Consider watering trees in exposed places on gravely or sandy soils, or those nearly surrounded by paving or gravel. Soils that contain as much as 50 percent rock are probably most drought prone for conifers. Trees recently exposed by adjacent timber cutting or construction, and tree groups where the undergrowth has been removed will have high moisture stress and potential damage. Watering should begin about two weeks into a hot spell. Trees in rocky soils and in exposed places can be mulched in early summer to minimize soil water loss. Lawn clippings, bark, or alder sawdust spread to the edges of the drip lines around trees conserve water. Mulches should be about six to eight inches thick and loose enough to allow air movement. Valuable small trees could be wrapped in burlap and deep watered and mulched to minimize winter injury. Christmas trees cut early may be drier than normal, and should be carefully watered. Trees are tough. Although those growing on gravely, rocky soils on south or southwest facing slopes will have the most difficult time surviving, most trees will make it. Severe droughts like the 1987 one will change forests slightly by altering species. Washington's trees are exposed to moisture stress nearly every year because of our characteristically dry summers, and they are genetically geared to handle it. A FINAL COMMENT Dr. Ralph Byther, WSU Extension Plant Pathologist, adds these comments about drought injury to Douglas Fir. Damage to this species is often characterized by death of only the top of the tree, or random branch dieback, as well as sudden reddish-brown discoloration and death of the entire tree. Affected trees are often randomly distributed among normal-looking trees. How can this be? Random problem trees are often a result of genetic variation in drought hardiness, variations in soil conditions, or micro-climates.




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