Brad Withrow-Robinson, Forestry & Natural Resources Extension Agent for Benton, Linn and Polk Counties

Many of our readers have been tracking this issue over the last several years through the articles on this site. But I imagine many of you are wondering what to expect next, or are being asked by friends and neighbors who are concerned about your trees or theirs. Or just wondering what is going on. What to say?

So, here is a synopsis of what I generally see going on, and the consequences. A bit risky to do, given the role of individual sites and specific conditions. But this is such a widespread phenomenon, it warrants some interpretation, even at the risk of over-generalizing. But I provide the antidote at the end: links to more detailed information.

It is hard to miss all the dead and dying trees in the area. I have been getting dozens of calls about them. So what is going on, and what is to blame? It seems time to revisit this sylvan whodunit: What is killing all these Willamette Valley trees?

Who is involved? Douglas-fir is by far the most frequent casualty, along with other conifers such as grand fir and some ponderosa pine. But trees of many sorts are being affected –

hardwoods as well as conifers, both native and non-native. Many of the usual suspects – different beetles and fungi- can be found at the scene too.

What is happening? Symptoms often include dying branches and dead tops, low growth and vigor, sparse crowns, what we have called the “Willamette Valley crud”. It is now often progressing to the death of the tree. This may be happening to individual trees or groups of trees. The younger trees are usually the first involved at a site, eventually joined by older trees.

Where is it happening? This is certainly a Valley-wide phenomenon. But within the Valley, we are seeing the most significant damage in certain situations more than others. Sites with seasonally wet, poorly drained soils, or sites with rocky or shallow soils, exposed south facing aspects tend to be most hard-hit. These are places that we think of as marginal sites for most conifer trees. Our conifers are well adapted to the area, but not every site.

When did this begin? This is an on-going event that began with a vengeance in the spring of 2013.

Why is it happening? Despite the many insect and disease suspects who can often be found at the scene, our investigation clearly indicates the real culprit is the weather. The first calls started in late summer 2012. It was a particularly warm summer with no rain until mid-October, which put trees, especially those on marginal sites, under stress. I think this was the triggering event, even though many symptoms were not expressed until the following spring.

With that stress came the usual suspects – bark beetles, twig weevils, stem cankers – that caused many of the symptoms described above (dead side branches and tops). But the true culprit in this mystery drama is the string of unusually long and warm summers we have experienced many of the past five summers. The fungi and insects are coconspirators that are able to take advantage of the situation. They may take the blame, but they generally are not the real cause of the problem.

We seem poised for another stressful summer following a dry spring and another warm dry summer forecast. I think we can expect to continue to see more sick, dying and dead trees this summer, which will be the topic of my next post.