Drought takes heavy toll on state's forests

After the 2015–2018 drought, trees were still dying or showing symptoms of mortality in 2019 because of the stresses they endured during the previous three years.

Drought kills trees through a process called vapor pressure deficit. Vapor pressure deficit is the difference between the amount of moisture in the air and how much moisture the air can hold. The higher the vapor pressure deficit, the more the air pulls water from the plant. Temperature plays a role in this process too.

Our summers have been hotter as well as drier. Warmer air can hold more water. So, on hot days the vapor pressure deficit is even higher and will exert more pressure on the plant to lose water. This causes faster mortality during the drought. Drought symptoms include the crown dying from the top down. Some trees are also producing a “stress cone crop.” These cones are typically smaller than usual and are the trees' last effort to reproduce before they die.

Douglas fir

For Douglas fir, drought has been a huge issue, particularly at low elevations and the inland valleys. Especially in the oak zones. Trees that haven’t been killed by the drought are likely stressed.

Stressed trees can sometimes also be affected by beetles that take advantage of the trees' lowered defenses. Douglas-fir beetle is associated with drought stress on trees and they have been commonly found in dead and dying trees experiencing drought stress.

Flat-headed fir borer of Douglas fir has also been taking advantage of trees' weakened by drought and dense stands. Flat-headed fir borer is a wood-boring beetle that acts like a bark beetle. Both of these insects make distinct galleries under the bark.

Grand fir

Grand fir is a live-fast, die-hard type of tree. They are a prolific seeder but are more susceptible to health issues. In 2019 we saw lots of dead trees in the Douglas-fir and oak areas.

The main driver again is drought, but there are a few insect friends that are helping to make things worse: the fir engraver beetle and the non-native balsam wooly adelgid.

Fir engraver will cause top and branch kill, and sap streams running down the bole in all true firs. This beetle can kill the tree in one year or over a few years. The balsam wooly adelgid arrived in our area in the 1950s and used to be a major grand fir killer, but now it is a greater problem in subalpine fir. The insect sucks sap from the branches and bole of the tree while injecting toxic saliva inducing changes in the sapwood that result in decreased water and nutrient transport.

Oregon white oak

Oregon white oak has a host of insects and diseases, but only a few cause major problems.

Root disease is likely the biggest issue for older trees. The rot is caused by a fungus called Inonotus dryadeus. It has a yellow conk that looks like it is bleeding at the base of the tree in the fall. This fungus eats away at the roots and base of the trunk, which can cause the tree to fall over in a heavy windstorm.

Defoliators and gall-forming insects come and go, but this year was a good one. Leaf issues are usually caused by caterpillars of micro-moths including leaf rollers, leaf miners, blotch miners and leaf skeletonizers. Branch flagging, sections of dead branches, can be caused by the cynipid twig-gall wasp, which lays its eggs under the bark which causes small galls to form and girdle the branch causing the branch tips to die.

Squirrels exacerbate the problem by peeling the bark off around those areas, but we aren’t sure why they do it. Maybe to get at the insects?

Oak mistletoe typically doesn't cause major problems, however, can eventually intensify within a crown and cause decline.

Summary from the Oct. 25, 2019, presentation by Dr. Dave Shaw, OSU Extension Forest Health Specialist, to an enthusiastic group of landowners about some of the issues of native trees in the south valley forests. To learn more about these forest health issues, visit Know Your Forest and the Oregon Department of Forestry's forest health page.

Previously titled
South valley forest health

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