Western Oregon forest health update, July 2021

Dying or declining trees stand out in contrast to the lush green of a new growing season in the forest. Forest Health Highlights based on the statewide aerial survey, provides a good annual update on major tree problems and the state of forest health in Oregon. In western Oregon, the combination of drought stress, bark beetles, and root disease continues to account for most of the tree mortality we see. Aside from wildfire, bark beetles and other insects are the most common cause of dying trees across Oregon.

You may have heard Oregon Department of Forestry (ODF) Forest Entomologist Christine Buhl on the radio talking about threats from bark beetles. She notes that while not every infestation will spread, the likelihood is greater when trees are struggling due to drought or root disease. Christine and other forest entomologists in Oregon and Washington provide some basic guidance for reducing risks due to bark beetles.

Landowners and others in charge of tree care are advised to take special precautions to prevent bark beetle outbreaks and avoid attracting bark beetles to their trees. The first flight of bark beetles usually emerge from infested trees and tree debris beginning in April, with periodic emergence through the summer.

Though bark beetles are native to Oregon forests, recent episodes of drought, fire, and storm damage have increased bark beetle hazards. Major tree species affected are Ponderosa pine and Douglas-fir. The recommendations for dealing with bark beetles are different for each tree species.

With pine species, tree debris produced during the spring and summer is highly attractive to breeding bark beetles. When possible, schedule tree work such as thinning, pruning, and tree removal for fall and winter (October-December). Tree debris and slash may be left on the ground during fall and winter time (best if it is scattered). When work on pine trees is done during spring and summer (hazard trees or other urgent needs), tree debris three to eight inches in diameter should be disposed of within two months via chipping, burning, or hauling away. For more details see Slash management for bark beetle protection

The Douglas-fir beetle prefers larger trees or logs, at least 10 inches in diameter. Local outbreaks of Douglas-fir beetle can be caused when groups of fallen trees or logs are left in the woods. The beetles breed in the down wood and their population may grow large enough to infest green trees nearby after a year or two. Fire damaged trees may also become infested to hazardous levels. Recommendations to reduce beetle hazards are to remove fallen trees, logs, or infested trees larger than 10-inches in diameter before April of the first or second year after the tree falls. When removal is delayed (or not done), the repellent pheromone MCH can be effective in protecting green trees in the area. For more information read Douglas-fir beetle and MCH for Douglas-fir beetle management.

If a standing tree is completely dead and brown in color (dead for one year or more), bark beetle hazards are minimal. In this case, if trees are not considered hazardous, leaving dead trees standing is quite beneficial for wildlife. 

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