Dead and dying trees are an all-to-common sight throughout Oregon. People are quick to blame insects, but while insects are commonly present in unhealthy trees, they are generally not the underlying problem. The vast majority of tree problems occurring in Oregon can be attributed to weather conditions.
If you are noticing individual dead or dying trees, or patches of trees that all died in the same year, the problem is most likely drought and/or heat related. (If your trees are dying over a period of years in slowly expanding clusters you may have a root disease problem and should contact your local Extension Office for recommendations.) Here is an overview of what is currently happening to our trees, and some tips on how you can keep them as healthy as possible.
Trees need adequate moisture to keep their defense mechanisms fully functional. When subjected to drought, trees may lack the resources needed to resist disease-causing organisms. Oregon’s tree-growing season tends to start with plenty of available moisture, but then enter a long dry period during which trees and other plants use up the water stored in the soil. Ultimately, trees growing in overcrowded conditions; in marginal soils; in hot, dry sites (such as south and west facing slopes); or those predisposed to disease will succumb to the forces of natural selection.
Excessive heat can cause trees to display drought symptoms, even when moisture is available in the soil. The vast majority of water consumed by trees is used to cool their leaf surfaces. On a very hot day a tree simply may not be able to move and evaporate enough water to meet its cooling needs, resulting in leaf damage and stress. Many areas within Oregon have experienced hotter than normal temperatures for extended periods during the past few summers, leading to early leaf loss in hardwoods (broadleaf species) and mortality in conifers. This is particularly evident where tree species are at the “hot” edge of their natural ranges.
Many of the conifer trees that appear to be dying now actually started their decline over a year ago. Lack of moisture or too much heat led to stress, which led to reduced resistance, which facilitated insect invasion in the stem and branches. The insects laid eggs and the resulting larva fed under the bark and in the wood. This further weakened the trees and encouraged additional insect invasion. Eventually, the trees were overwhelmed and died.
Once started, this process is very difficult (or even impossible) to reverse - and it’s common to see conifers die in the spring or early summer from damage caused in prior years. It is important to recognize that the insects are seldom the direct cause of these trees’ deaths, they merely take advantage of the trees’ weakened condition. Killing the insects will not save the tree if the underlying moisture deficiency is not addressed (and even this may not be sufficient if the tree is too badly damaged before action is taken).
Several common forest pests take advantage of drought/heat stressed trees. These include bark beetles and wood borers that populate tree stems, beetles and weevils that invade branches, and fungi species that cause stem and/or branch cankers. Insect and fungi species are generally specific to their host tree species, so the things that attack Douglas-fir (for example) are unlikely to also attack ponderosa pine.
Bark beetles and borers spend portions of their lives in the stems (under the bark) of stressed, dying or dead trees. Signs of their presence include holes in the bark, boring dust, or “chewing” sounds. Beetle species often prefer a specific portion of the stem or branch where the bark thickness is most conducive to their needs. You may find one species near a tree’s top or in branches, another in the mid portion of the stem, and a third at the base of the stem. Note that by the time you notice the damage (such as reddening needles), the insects may be long gone.
Treatment of bark beetles and borers using insecticides is challenging in forest settings – this approach is probably best reserved for high value trees in home landscapes and recreation areas. Applying insecticides to the stems (either to keep the beetles out or to penetrate the bark and kill them) is difficult to effectively administer on large trees. High value ornamental trees may be treated through stem injections or soil drenches - techniques that are fairly well developed for hardwood trees, but less so for conifers. It’s generally best to have these treatments carried out an arborist or landscape professional with the appropriate training, equipment, and certifications.
Thinning is the best procedure for reducing stress in trees - it allows each remaining tree to have a larger portion of the available moisture. Trees harvested during thinning may be sold as logs if they are still in good condition, or used for firewood. Small trees and slash (tops and branches) are typically piled and burned, scattered to decompose (applicable on moist sites), chipped or ground (masticated) on site, or sold for biomass fuel. (Be sure to file a notification with the Oregon Department of Forestry before selling logs, and to comply with burning regulations.) You do not need to remove all the dead and dying trees – leaving some to form snags or downed logs can provide important wildlife habitat.
Douglas-fir and grand fir appear to have been the hardest hit by our succession of hot, dry summers. Our native pines, oaks and madrones are more resistant to drought and heat, and thus less likely to be affected by disease organisms following a difficult summer. However, trees of any species have limits to their tolerances, and in eastern Oregon even ponderosa pine is experiencing increased mortality from western pine beetle (secondary to drought/heat).
General advice on keeping trees healthy
Thin to reduce tree-to-tree competition. Trees growing close together will compete with each other, and the effect will become more extreme as they get larger. Eventually some will be out-competed and perish. You can improve the health of a forest stand as a whole through appropriate thinning. Reducing the number of trees in the stand increases the amount of water and nutrients available to remaining trees. Thinning works best when done before trees become unhealthy from over-competition, so don’t expect it to save a stand of trees that has already become weakened.
Manage vegetative competition around young trees. Grasses, herbs, and shrubs can be very competitive with young trees, consuming vital moisture or (in the case of shrubs) limiting trees’ access to light. Mowing or trimming competing vegetation may alleviate shade issues, but it does little to reduce moisture consumption. Appropriate vegetation control with herbicides or manual removal of grasses and forbs from an area three to five feet around the base of a young tree will reduce competition and increase both tree seedling survival and growth rate. This is most important for the first three to five years after the tree is planted or established.
Individual yard trees will benefit from infrequent, deep watering during the dry season. Use a soaker hose (spread around the tree about 2/3 of the length of the branches from the stem) and let it run for several hours to get the soil well saturated. As a rule of thumb, trees need the equivalent of one inch of rainfall per week from June through September, but allow soil to dry out between waterings – many trees will not tolerate persistent flooding in the root zone. Trees on steep slopes or other fast draining areas, or those competing with other vegetation, may need more frequent watering. Cease watering by the end of August – many tree species need some drought stress to induce them into winter hardiness, and watering too late in the year can leave them susceptible to freezing damage. Consider mulching over the rooting zone of yard trees to conserve moisture, reduce soil temperatures, and help reduce vegetative competition.
Plant or favor trees well suited to the site. If your tree does perish, consider replanting with a species better adapted to the soil, moisture and temperature conditions that it will experience (or let a native tree already growing on the site take over that area). Each tree species has its own range of environmental preferences, and there are genetic differences within species. If you’re planting, obtain seedlings grown from seed appropriate for your location and elevation. If you’re thinning, retain well-formed, healthy trees of species best suited to your site’s conditions.
Pitch on your conifer stem?
You may have noticed streams of sticky “pitch” running down the stem of your conifer. This is called resin and is one of the tree’s most effective means of protection against insect attack. When the bark is penetrated the tree produces resin to flood the wound, sealing out disease-causing organisms and often flushing out or drowning bark-boring insects. Without adequate moisture the tree is unable to produce enough resin to do the job, and insects can successfully invade. If your tree is producing a lot of clear or white pitch it tells us that the tree has been damaged but is still fighting back. You should resist the urge to cut down a tree producing a lot of light-colored pitch, as it still has a fair chance of recovery. If the pitch is reddish the tree had been more extensively damaged and is less likely to survive.
It’s not all about drought and bark beetles.
Trees face a wide variety of pests and disease, and while drought, heat, and bark beetles are common culprits in tree mortality they are certainly not the only ones. Trees can experience insect feeding on, or fungal infection of, leaves/needles; fungal decay or disease in stems and roots; insect or fungal damage to buds, cones or fruit; invasion by parasitic plants (such as true or dwarf mistletoe); or damage from animals such as deer and elk, bear, porcupine, squirrels, beaver, etc. (See "Want more information?" below.)
Dead branches on your Douglas-fir?
Twig weevils and cankers affect small, drought- or heat-stressed Douglas-fir trees. Twig weevils cause scarring of bark tissue of small diameter stems and branches. Cankers are caused by fungi and appear as small, sunken areas of dead tissue on the stems and around the branch collars. Both may cause individual branches to die, may occasionally kill the tops of small trees, and on rare occasions may kill the tree itself. While weevils can be treated with an insecticide, the recommended treatment is to prune off and burn the dead branches. Be sure to cut approximately two inches back on a dead branch into the green living material, as the weevil works its way toward the trunk of the tree under the bark and is typically found where the dead and living material meet. There is no common chemical treatment for cankers, but infested branches may be pruned out if desired.
One final point...
It's normal for trees to die. The conditions we are currently experiencing in Oregon rarely represent an insect or disease epidemic. It is much more likely the dead trees you’re seeing are the result of drought or heat stress. Even with perfect weather and trees perfectly suited to their site there will still be mortality as trees grow and compete. Healthy forests include dead trees; it’s part of nature’s plan.
Want more information?
Check out the wide range of fact sheets and user-friendly publications available at https://knowyourforest.org/learning-library/forest-health. If you still have questions, consult your local OSU Extension Forester or Oregon Department of Forestry Stewardship Forester.
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- Read the pesticide label—even if you’ve used the pesticide before. Follow closely the instructions on the label (and any other directions you have).
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¡Use los pesticidas con seguridad!
- Póngase ropa de protección y equipo de seguridad según las recomendaciones de la etiqueta. Báñese después de cada uso.
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