By David Shaw, Forest Health Specialist, OSU Forestry and Natural Resources Extension
The summer of 2015 is shaping up as a big year for drought and drought related forest health issues throughout Oregon, but especially in the Willamette Valley, SW Oregon, and in Eastern Oregon.
In late summer, it can be very difficult to discern whether insects, disease, or drought and heat are causing tree dieback and deaths, but we are becoming pretty confident that drought and heat together are influencing much of what we see. In this report I outline and describe some of the more common problems we are seeing with conifers and hardwoods as of early September.
Drought related issues
In severe drought, trees may die with no associated biotic agents such as bark beetles or canker diseases. However, it is very common to find dead trees with these agents too.
Douglas-fir in the oak zone of western Oregon (the drier area of the Valley and on heavy clay and shallow dry soils at lower elevations) is having an especially hard summer, with some sites outside the oak zone also showing drought effects. The general symptoms are top dieback, branch flagging, and whole tree mortality. These symptoms may or may not be directly related to a biotic organism. The major ones are branch cankers, bark beetles, and twig weevils. All these organisms seem to do well on Douglas-fir during drought, and this year is no exception. We also believe root diseases are exacerbating the issue, but it can be difficult to discern. Twig weevils and branch canker diseases are very common on young Douglas-fir during drought, and both are known to increase attacks on drought stressed trees.
Bark beetle attacks on conifers increase during drought. This is the case for Douglas-fir, grand fir and other true firs, as well as pine. In these conifers it often results in top dieback, but can also result in whole tree mortality. We do not have the results from this year’s the statewide survey yet, but it appears that bark beetle activity is going to be really up. However, symptoms of bark beetle attack vary with beetle type and drought effect. For example, typically when a Douglas-fir is attacked by Douglas-fir beetle in April or May, the tree crown does not go red for many months, perhaps not until late fall or even early the next spring. However, this year many trees that were attacked in the spring were turning red right away, by mid-summer. This may be because they were already dying from drought, and this may also be exacerbated by existing root diseases.
Many declining Douglas-fir trees have an associated stress cone crop, a smaller than normal abundant cone crop that is hypothesized to be related to the last gasp of the tree to reproduce before death. For a stress cone crop to hang on a tree in early 2015, means they likely formed in 2014, indicating many of the trees with top-dieback this summer have been suffering for two years or more.
True fir/Grand fir
The fir engraver bark beetle attacks all true fir, but is especially important on grand fir and white fir during drought. We anticipate a lot of grand fir mortality this summer, but it will not become evident until fall, as the trees may take a few months before showing red foliage. Throughout the range of grand fir, the species has expanded its site occupancy with fire suppression, even in the Willamette Valley. During drought, many of these sites are not suitable for fir and mortality may become very common. Root diseases may also exacerbate the mortality.
Foliage loss in conifers
Many conifers lose foliage in a drought, theoretically as an adaptation to reduce water loss through leaves. Although this is poorly understood, this summer it is quite common to see conifers like Douglas-fir or ponderosa pine losing two-year-old foliage and older. There may be interactions with foliage fungi/diseases, but it is very difficult to differentiate what is happening during mid-summer.
Foliage browning in hardwoods
Foliage browning in hardwoods is becoming more and more common throughout the region as drought intensifies this summer. Partial tree crown and whole tree crown foliage browning is already present in big leaf maple, Oregon ash, and cottonwoods. We anticipate Oregon white oak will also begin showing symptoms within the next month. We believe this is an adaption to prevent whole tree mortality, a type of early season senescence, and next spring most these trees will flush and be healthy if rains return this winter. Significant foliage browning is also being reported in California black oak in southern Oregon already this summer.
And, a few NON-drought issues…
Oregon white oak has had a big year for small branch dieback in some areas of the Willamette Valley. This is associated with a complex which involves a twig gall wasp (Bassettia ligni)(Hymenoptera: Cynipidae) in which the grubs develop under the bark of small twigs. The western gray squirrel is attracted to these areas with gall wasp grubs and the squirrel debarks the twig. If you see branch dieback in oak (red-dead foliage in clumps), check just below the dead foliage and see if you see the twig debarked. It is very characteristic and easy to see generally. This issue is common in the Valley, but year-to-year it varies in locations and intensity. This year it is particularly common in the Corvallis area and along the west side of the valley.
Bigleaf maple also begins showing branch dieback this time of year. This often is associated with western gray squirrel feeding damage, but no gall wasp is involved, the squirrels just like young maple bark. Again, to verify this is squirrel damage and not drought or other issue, check to see if the branch has been debarked below the dead leaves.
Swiss needle cast along the coast is still persisting and the aerial survey again showed over 500,000 acres of visible disease symptoms from the air. This was restricted to sites within about 20 to 30 miles from the coast. Occasionally young stands also show symptoms along the Cascade foothills. However, in the Willamette Valley and SW Oregon in general, the foliage loss we are seeing this summer is likely not caused by Swiss needle cast. See the Swiss Needle Cast Cooperative website.
Please suggest remediation measures. Or, at least information regarding disposal of dead or diseased branches or trees.
If you see evidence of canker disease, you can prune off and destroy affected branches to reduce disease spread.
Dead trees can be left if desired, or if deemed a fire hazard you could try one of these options: remove entirely from the site; or fell the tree and remove the limbs, leaving the bole on the ground where it can help retain soil moisture; or chip the tree and leave on the site.
Maintaining tree vigor is a preemptive strategy to reduce the impact of drought to individual trees. Thinning out dense stands will free up resources for remaining trees, reducing their stress level and susceptibility to drought and associated abiotic agents (insects and diseases).
Here’s a good publication from ODF with some other recommendations on page 3: http://www.oregon.gov/ODF/Documents/ForestBenefits/interactionofdrought.pdf