Backyard Forest Stewardship in Western Washington
Forest Health
Photo by: R.S. Byther
Many factors can influence the health of trees and forests in western Washington. The following are some of the most common ones that you may observe:
Environmental Factors
Drought. Extended periods of drought during the growing season can kill or damage trees. Trees on gravelly or sandy soils and species such as western hemlock and western redcedar are particularly susceptible. Whole branches or tree tops whose needles have turned bright red may be a symptom of drought damage. Sometimes, the damage may not be apparent until the growing season after the dry conditions have occurred. Western redcedar trees often exhibit scattered patches of red foliage throughout their crowns in the fall (see above); this is a normal occurrence and is not a concern.
High winds. Storms from the Pacific periodically damage or blow down trees. Check for root disease and hazard trees before the next wind storm. Stands suddenly exposed to wind, such as those at the edge of land clearing or timber harvest areas are especially susceptible to windthrow. Thinning stands beginning at an early age will help individual trees to resist windthrow.
Temperature extremes. Sudden heat waves or deep freezes often cause damage. Small trees may be killed. Large trees usually recover.
High water. Poor soil drainage and high water tables during the winter can "suffocate" tree roots. Flooding can also damage or kill trees if water stands for an extended period. Avoid planting Douglas-fir in wet sites. Shore pine (lodgepole pine), grand fir, and western redcedar are better choices. You may also wish to encourage alder to grow in these areas.
Ice and snow. Periodically, freezing rain and heavy snow cause excessive damage to trees and forests. Tops and branches are often broken, and whole trees can be bent over or uprooted. Corrective pruning and staking may be required for ornamental trees. Salvage harvesting may be needed in forest stands. Thinning forest stands starting at an early age will help trees withstand ice and snow breakage.
Overcrowded forests. Many forest stands are too dense. Competition between trees results in small crowns, poor diameter growth, and reduced tree health and vigor. One way to assess whether a stand is overcrowded is to examine the understory. A healthy forest will have a variety of shrubs growing throughout, whereas an overcrowded stand will be dark with little to no live vegetation on the ground. Thinning will help to keep trees healthy and growing vigorously and allow a healthy understory to develop.
Dry east winds on sunny winter days can severely dehydrate foliage and may kill tree tops.
Diseases
Root diseases are widespread in our area and are caused by native fungi that persist in soil and tree roots. Laminated Root Rot is a particular problem, primarily in Douglas-fir stands. The root rot fungus spreads by root-to-root contact. Conifer trees of all ages and sizes may be killed or blow over in high winds. Sparse or yellow foliage, short top growth, and excessive cone crops may be signs of root diseases. Trees blown down with little or no roots attached usually indicate root disease. Look for forest openings or groups of affected trees called "root rot pockets." Trees near the center of such pockets will be most severely affected, perhaps dead or blown down. Visible signs decrease as you progress outward from the center. Trees on the edge of the root rot pocket will be infected before visible signs appear. Once trees are infected, there is no direct control. Remove affected trees, especially if they are near structures or other areas of heavy human activity. Avoid replanting with Douglas-fir. Western white pine, western redcedar, or hardwoods such as alder and maple species are good choices for replanting.
Foliage diseases. The cool, moist climate in western Washington is ideal for many foliage diseases. Foliage diseases can weaken but rarely kill trees. Thinning and pruning to increase air circulation is helpful. Preventive applications of fungicides may be needed for ornamentals or Christmas trees. However, this is rarely an option in forest stands. Periodic fertilizer applications can sometimes help trees maintain sufficient foliage and grow out of disease-susceptible stages.
Insects
Bark Beetles. These insects are usually not the primary damaging agent in western Washington. They typically attack trees already under stress from some other factor such as drought, root damage, or overcrowding. Bark beetle-infested trees normally cannot be saved. Salvage of large blown-down Douglas-fir within one year will help control the spread of Douglas-fir bark beetles.
Foliage Insects. Several species of insects can cause foliage damage or loss. Aphids, adelgids, tent caterpillars, sawflies, leaf beetles, and loopers are just a few common examples. Direct control may be possible for ornamental trees but is rarely feasible or necessary in forest stands. Damage is often cyclic, i.e. worse in some years than others. Most trees will recover from this type of insect attack.
Human Activities
Human activities such as excavating, filling, road construction, and timber harvesting can damage trees directly or indirectly by affecting root function or changing soil drainage. Heavy equipment traffic underneath and around trees can cause soils to become compacted, which is also damaging to roots. Plan to protect trees and their root zones during development or construction activities (and be aware that the root zone extends well beyond the drip line of the tree). Damaged trees may die suddenly or deteriorate slowly. Trees stressed by human-caused damage are often highly susceptible to secondary damage-causing agents such as bark beetles.
People also sometimes unknowingly introduce exotic weeds and pests that have the potential to cause extensive damage to western Washington trees and forests. The European Gypsy Moth, which "hitchhikes" on vehicles and possessions moved here from the Eastern U.S., is one example. Your alertness and cooperation with Washington State Department of Agriculture pest monitoring programs will help minimize the potential for damage from exotic pests. Noxious weeds are common and may influence the development of your forest floor vegetation. See Noxious and Invasive Weeds, later in this publication, for more information.
Animal Damage
Both wildlife and domestic animals can damage trees and shrubs, particularly young seedlings. Use fences to keep horses and cattle away from trees and forest vegetation, particularly along streams. Paper or fabric bud caps, rigid plastic tubing, and repellents can help control feeding damage to trees by deer, elk, mountain beaver, rabbits and rodents.
Livestock left to graze in forested areas for prolonged periods can cause damage to both mature and young trees. Forest soils are easily compacted by animal traffic that may in turn lead to root damage. Horses and cattle may also rub against trees, causing bark damage. If you own horses or other livestock, keep them out of your forest, especially during wet months.
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Diagnosing the Problem:
Your local Extension office can help you diagnose tree and plant problems. They also have numerous publications which will give you more detailed information about specific tree and forest health problems. The Washington Department of Natural Resources forest pathologist and forest entomologist can also answer questions. You can reach them at forest_health@dnr.wa.gov. For questions related to non-native or ornamental trees, contact the Master Gardener program in your area.
A few things to remember:
In general, if many trees of different species exhibit common symptoms, the cause is probably environmental or human. If damage is limited to an individual species, the problem may be insects or disease.
Most insects and diseases are specific to individual tree species, or closely related groups of species, and will not spread to others in the forest.
Learn more:
WSU Extension Forest Health Notes
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