On young Douglas-fir, these dead areasare reddish-brown and contrast well with the gray/green color of healthy bark. Swellings and splits in the bark, tiny holes, and sawdust from boring insects also maybe visible. Many trees do not have cankers or visible indicators of insect activity.The pattern of branch death on a tree often is very disorderly. Branches on only one side of thetree may die. On other trees, scattered branches in the mid-crown may die, while those above andbelow remain green. Dead branches may occur in the top only, the bottom only, or randomlythroughout the crown. On some trees only the smallest branches die, while on others the entiretop may turn red.
DISTRIBUTION OF DAMAGE
Damage occurs throughout the state and is most severe in urban areas, on the fringe of forestedareas, and on shallow, rocky, or droughty soil types. Damage also appears on very wet or season-ally flooded sites. Trees growing near roads, ditches, pastures, or in areas of soil disturbance orabundant competing vegetation are most frequently affected. Damage to Douglas-fir is especiallycommon on former grass or pasture lands, and on dry south aspects. Damage occurs much lessfrequently in the undisturbed forest than in fringe areas. During 1999, damage has been particu-larly noticeable in interior southwest Oregon (Rogue/Umpqua basin) and in the WillametteValley.
TREE SPECIES AFFECTED
All conifers can be affected, but damage is most common in young Douglas-fir (usually less thanabout 30 years old). Drought-tolerant species such as Ponderosa pine are damaged less frequentlythan Douglas-fir or true fir. Douglas-fir generally does not tolerate extreme drought conditionsor waterlogged soils as well as Ponderosa pine. Trees growing beyond their natural range or fromnon-local seed sources generally are at an increased risk of drought damage compared to locallyadapted trees.
THE PRIMARY CAUSE - WATER STRESS
The primary cause of branch and tree death described in this note is water stress inside the tree,although other contributing factors may be involved. Water stress, which often is called droughtstress or moisture stress, refers to a shortage of water inside the tree which results wheneverwater loss exceeds uptake long enough to cause plant damage or disturb physiological processes.It usually results from a lack of available soil moisture due to drought (drought is a period with-out rainfall that causes depletion of soil moisture and reduction in plant growth). The length oftime without rainfall required to produce drought depends mostly on the water storage capacityof the soil and on the rate at which plants take up water through their roots and evaporate itthrough the foliage (the process is called evapotranspiration). Water stress often affects groups of trees because they share common soil and environmentalconditions that can affect the rate of evapotranspiration and the degree of water stress in trees.Because water storage capacity varies among soil types, water stress will develop in trees onsome sites sooner than others under the same weather conditions. Trees exposed to full sunlightand free air movement tend to lose water faster than trees growing among other trees in a closedcanopy. Sudden changes in the stand that expose the crown to wind greatly increase rate ofwater loss. Competing vegetation can intercept water from tree roots during periods of lowrainfall. Soil compaction or disturbance can damage roots, alter drainage patterns, reduce aera-tion, and prevent water infiltration--all of which influence water uptake and tree water stress.Rooting habit and structure of the water conducting tracheids in the tree also play a role insusceptibility to water stress.Trees respond to water stress in a number of ways. Low levels of water stress will reduce stemand root growth. As water stress increases, trees become increasingly susceptible to certaininsects and diseases. Water stressed trees even attract certain bark beetles. Many of the tree'sinternal responses to water deficits occur without visible outward indicators of stress. Undersevere drought conditions, water content may drop to a critical level where trees are irreversiblydamaged and entire trees or just portions of the tree may die. Tops of trees and branch tips oftendie first because they are farthest from the water-absorbing roots. Roots and lower boles are lastto die from moisture stress, and often remain living even though above-ground parts are dead. In Oregon, water stress injury usually occurs in late summer or fall after trees have formed buds.Ample moisture and cool temperatures of winter improve the tree's water balance and preventdrying of needles. By late winter and spring, warm, dry conditions increase evapotranspirationand dead needles dry and turn red. Even though damage occurred in fall, it usually is not visible until late winter or spring of the following year. Examination of discolored branches and tops inlate winter has shown little evidence of fungal pathogens or insects, probably because they werenot active when the extreme water stress occurred (many of the bark and twig beetles fly inspring and early summer). However, by April of 1999, insect attacks and stem cankers havebecome obvious as these agents become active in warmer temperatures.Following a drought, trees often are observed with one or more dead branches in various posi-tions within the crown. Although secondary insects or disease could account for this, it couldalso result directly from moisture stress. Why would one branch and not another die on the sametree? A plausible explanation relates to the water conducting system of trees. If you injectedcolored water into a single tree root and traced its path upward, you would find that it follows afairly narrow, often spiraling path upward into certain parts of a tree and not others. The systemcan be thought of as a network of microscopic tubes similar to a bundle of very small diameterdrinking straws that begin at root tips and end in the foliage. Because of the variation in soilconditions (rocks, impervious layers, etc.) around a tree's root system, some roots may experi-ence drought conditions sooner and more severely than others. The branch or branches connectedto the roots growing in a localized poor situation will experience the most severe moisture stressand therefore may die sooner than other branches on the same tree. This oversimplifies watertransport in a tree, but it's a useful model.Water stress inside a tree also can result from excessive soil moisture. In waterlogged or floodedsoils, Douglas-fir roots are deprived of oxygen and may be killed or damaged to the point thatthey can no longer take up water and nutrients efficiently. As the soil dries, the damaged rootsystem cannot support the water needs of the top of the tree.Low temperatures can cause water stress in several ways. Unusually low temperatures, especiallyfollowing a warm period, can damage the sapwood and impair transport of water from roots tobranches and foliage, causing them to dry out and die. Another type of water stress occurs mostcommonly in the Columbia River Gorge and is referred to as winter desiccation or winter drying.In this case, low temperatures (not necessarily below freezing) slow the soil water movement anduptake by roots. Very dry easterly winds and sunny weather can cause water loss to exceeduptake, inducing severe water stress. Damage usually is most prevalent on the east side (facingthe prevailing dry winds) of trees and on trees growing in exposed locations. The damage occursin winter, but symptoms (red-brown foliage) are most visible in early spring.Water stress also induces loss of older foliage in the fall, immediately following a summerdrought. This foliage loss (and occasionally branch death) is a drought-resistance mechanism inwhich the tree reduces the total surface area of the foliage and subsequently reduces the rate ofwater loss. It is most apparent in pines and cedar.