By Brad Withrow-Robinson, Forestry & Natural Resources Extension Agent, Benton, Linn and Polk Counties
Young Douglas-fir trees with dying branches or tops turning brown, then red have become a common sight all around the Willamette Valley this spring. What is going on?
This “flare out” of branches and tops are classic drought symptoms in Douglas-fir, which we are linking to last year’s weather when we had a particularly long, dry and very hot period late in the summer. Late season drought injuries to the stem and leader do not always show up when they occur, but often express themselves the following spring as trees start to grow. We have these drought damage events from time to time here in the valley, most recently in 2013 and again before that around 2000. Older trees typically have milder symptoms, but the many older, flat-topped Douglas-fir trees you see are a reflection of past droughts and non-fatal damage.
It is important to keep in mind that the Willamette Valley can be a challenging environment for trees. Summers are significantly hotter and drier in the Valley than in the mountains, and we have many poorly drained or shallow soils that are not well-suited to many kinds of trees. So, we tend to see most drought stress damage on more marginal sites, where wet or shallow soils limit tree root growth, water availability, or both. It is also often more common in younger trees (20 years and younger) whose root system may be having trouble keeping up with rapid expansion of their crowns.
Heat and drought can kill trees outright, or often just put the trees under stress. Stress can then lead to problems with secondary pests (including insects such as the twig weevil and diseases such as stem cankers) which take advantage of a stressed tree’s weakened condition. Right now we are mostly seeing the effects of drought in Douglas-fir, but can probably expect to see problems emerge among some other conifers as the year progresses, especially if we stay as dry as we are now. Let’s hope for some more rain!
For more information:
Here are two good articles from the ODF Forest Health team. They are a few years old but very relevant, explaining Dead tops and Branches (with good pictures) and about Drought and Mortality.
More photos below.
