Why Are My Trees Dying?

John Punches
Extension Forestry & Wood Products

(Click here for a pdf version.) 

Trees all over Douglas County are displaying signs of poor health. Most people are quick to blame insects, but while insects are almost always present in sick trees, they are rarely the underlying cause of the problem. The vast majority of tree problems occurring in Douglas County can be traced back to weather conditions. If you are noticing individual dead or dying trees, rather than a cluster of trees, the problem is most likely drought related. (If your trees are dying in slowly expanding clusters you may have a root disease problem and should contact the 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. Under drought (or any other stress mechanism) any number of normally weak disease-causing organisms may be able to overwhelm a tree. This is why we’ve seen so many “sick” trees around the county lately. Our summers tend to start off with plenty of moisture available to the trees, but a long dry spell follows. Throughout that period, trees and other plants drink up any 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 that were simply predisposed to disease succumbed to the forces of natural selection.

Many of the dying trees evident now actually started their decline over a year ago. Lack of moisture led to stress, which led to reduced resistance, which facilitated insect invasion. 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, and often impossible, to reverse. It is important to recognize that the insects are seldom the direct cause of the tree’s death, they merely take advantage of the tree’s 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 are taking advantage of our summer droughts (which weakened the trees), mild winters, and wet springs (ideal living conditions for the pests). These include several species of bark beetles and borers, twig weevils, and a fungus called phomopsis. All of these are considered weak organisms that are only effective in killing a tree when it is weakened by other mechanisms.

Bark beetles and borers normally live and breed in the stems of dead and dying trees. Signs of their presence include holes in the bark, boring dust, and you may even be able to hear them chewing away. They live under the bark of pole-sized and larger trees and are only rarely found in small thin-barked trees. The recommended procedure for dealing with these insects is to remove and properly dispose of the dead, or nearly dead, trees that they are infesting. Disposal may include selling the logs to a processor, burning, or debarking the material. Remember that you must file notification with the Oregon Department of Forestry before selling logs, and be sure to comply with burning regulations.

Bark beetles and borers are very difficult to treat with insecticides. Treatment would have to cover the entire stem of the tree and it takes a potent insecticide to effectively kill insects living under the bark. An alternative would be to use an insecticide as a preventative treatment applying several times during the beetles’ active period to prevent them from invading the tree. Both of these treatments are difficult to effectively administer on larger trees. Without the proper knowledge and equipment this treatment would be difficult, possibly risky, and produce minimal results.

Twig weevils and phomopsis cankers primarily affect small trees. Twig weevils cause scarring of bark tissue of small diameter stems and branches. Phomopsis appears 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 live material meet.

You may have noticed streams of sticky “pitch” running down the stem of your Douglas-fir or other 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 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 pitch, as it still has a fair chance of recovery.

Douglas-fir appears to have been the hardest hit by our succession of summer droughts. Our native pines, oaks and madrones are more resistant to drought and thus less likely to be affected by disease organisms following a dry year. However, trees of any species have limits to their drought tolerance and some individuals have been effected. One should note that disease-causing organisms are often specifically adapted to a single tree species or group. Thus, the insects that invade your Douglas-fir are very unlikely to attack the pine next to it, regardless of how badly the fir is affected.

Here is some general advice on keeping trees healthy. Inadequate moisture is the underlying cause of many tree health problems. If you have too many trees planted close together some will be out-competed and perish. Similarly, if weeds are allowed to grow in and around young trees, vital moisture is taken from the trees. Mowing or clipping weeds does little to reduce their moisture consumption and may actually stimulate root growth and water consumption. Appropriate weed control with herbicides or manual removal from an area three to five feet around the base of a young tree will reduce competition and increase both tree survival and growth rate. 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.

Individual yard trees will benefit from infrequent, deep watering during the dry season. Use a soaker hose 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. Trees on steep slopes or other fast draining areas, or those competing with other vegetation, may need more frequent watering. Allow soil to dry out between watering – many trees will not tolerate persistent flooding in the root zone. If your tree does perish, consider replanting with a species better adapted to the soil, moisture and temperature conditions that it will experience.

One final point – it is normal for trees to die after a hard drought year. The conditions we are currently experiencing in Douglas County do not represent an insect or disease epidemic, this is just what should be expected under the weather conditions experienced. Healthy forests include dead trees, it’s part of nature’s plan.

If you have any further questions or would like more specific information, please call the Douglas County Office of the Oregon State University Extension Service at 541-672-4461 or 1-800-883-7568.

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