Brad Withrow-Robinson, Forestry & Natural Resources Extension Agent, Benton, Linn and Polk Counties.
Conifer trees around the Valley continue to show signs of severe drought and heat stress this year. This should not be news to many readers: young dead trees are now a common sight throughout the Valley. Also, I wrote about this problem in past Tree Topics blogs (See stories from May and September 2015 for background) but have new updates for this season.
I think you can expect to continue seeing similar damage to Douglas-fir this year and that symptoms will continue to unfold as the season progresses. Some of the trees damaged late last year did not show that damage immediately. The damage did not become evident until the trees came out of dormancy and began to grow this spring. Also, the various insect and disease organisms that take advantage of weak and damaged trees are likely to continue with their business this year, causing new signs of drought damage to show up during the season. Happily, those players like Douglas-fir cankers and twig weevils do not typically blow up and kill healthy trees. This suggests things will look much like what we saw and described last year and is likely to continue to unfold this season and maybe longer, whatever weather we get. “It is important to understand that the effects of drought damage do not go away suddenly when the rain starts again” cautions Christine Buhl, ODF Forest Entomologist “drought can impact the tree’s whole plumbing structure, and affect the growth and vigor of the tree for years.”
What we are beginning to see and anticipate may be different this year is more damage to stands rather than just individual trees, and damage to older and larger Douglas-fir trees than was typical last year. The drought is likely adding to and exacerbating other problems lurking out of view, so crowded stands, existing root disease and marginal sites (wet or shallow soils, southern aspects) can all be expected to contribute to the problem.
Unfortunately, this implies potential economic or forest health issues. Any merchantable tree lost to drought represents an economic loss if not salvaged. But larger (>8” dbh) drought-damaged Doug-fir trees can also support growing populations of bark beetles, such as the Douglas-fir beetle. Under the right conditions Doug-fir beetles’ numbers can increase to the point where they can overcome the defenses of healthier trees in the stand. Drought stressed trees are not generally considered as good a nursery material as winter storm damaged trees but can support a damaging increase of beetles if conditions are right. I may need to write more on that later in the season.
We will also likely see drought stress issues in other conifer species. In our local Valley ponderosa pine, it is already causing some limited outbreaks of the California five-spine Ips, a tiny but destructive beetle. With several generations a year, Ips can rapidly increase in numbers when trees are stressed and conditions favor the insect. Also, the Ips is able to use much smaller wood (just three inches or more in diameter) than the Douglas-fir bark beetle mentioned above, so even a young planation can provide brood material for the beetle. Sanitation of dead and dying trees as well as slash materials >3” is a very important control measure for Ips. For more information on the Ips life cycle and management, see this 2014 article about Ips or follow links to other resources provided below.
The Oregon Department of Forestry has a series of fact sheets on insects, disease, drought and slash management. Several are currently being revised, so be sure to check back in July to see the updated versions.