Extended Dry Period of 2012 Continues to Stress Western Washington Douglas-fir

drought-stressed tree
Douglas-fir trees between 5 and 15 years old appear to be the most commonly affected by last year’s drought conditions. Photo: Karen Ripley/DNR.

Throughout western Washington this spring, the needles on many young conifer trees have suddenly turned red. Douglas-fir trees between 5 and 15 years old appear to be the most commonly affected, but some larger trees are also showing symptoms, such as entirely red crowns, red tops and red branches. In a typical year, this type of damage may have many causes, but this year it is primarily the result of an extended period with little to no rain during August and September 2012 and a drier-than-normal spring in 2013.

The summer months are a critical time for a tree’s water needs, especially in western Washington. On warm days, trees lose water through their leaves or needles during the natural process of respiration. The lost water is normally replaced by pulling more water up from the soil through roots. But during this dry period, trees in rocky soils, especially young trees with shallow, less-developed root systems, were unable to draw up enough water to survive. Extreme drought stress can result in failures within the tree’s circulatory system. Sometimes the whole tree dies. Sometimes all the tissue beyond the point of failure dies, causing a dead top or dead branch.

Many affected trees that have rapidly turned from green to red in spring 2013 were likely killed from extreme drought last summer. Conifers that have died often remain green for months, especially over winter. Once the weather heats up, the needles dry rapidly and turn red. The timing of the color change is much like a cut Christmas tree discarded outside after the holidays. It appears green for a long time, and then suddenly dries to red.

Results of inspections

The Washington Department of Natural Resources (DNR) Forest Health Program has examined affected trees at several sites in an area from Shelton, DuPont and Auburn south to Vancouver and along the Columbia River Gorge. The majority of trees examined show no indication of being killed by pathogens, insects, or other animals. Some trees have been attacked by one of three different species of bark beetles.

The attack pattern we’re seeing indicates that these beetles have moved into the trees after they were already dead. Beetles identified include Douglas-fir pole beetle (Pseudohylesinus nebulosus), Douglas-fir engraver (Scolytus unispinosus), and a Pityophthorus species. The Douglas-fir twig weevil (Cylindrocopturus furnissi) has not been observed this year, but will also attack drought-stressed trees. None of these beetles are aggressive tree killers. They typically only infest stressed or dying trees, especially during drought conditions. The next generation of beetles emerging from eggs laid in the killed trees or tree parts are not likely to attack healthy trees nearby.

Drought damage
Extreme drought stress cause in failures within the tree’s circulatory system. Sometimes the entire tree dies. Photo: Karen Ripley/DNR.

Damage has been most severe in areas with rocky soils, such as in glacial outwash around the Puget Sound. Water drains quickly in these soils and trees depend on occasional rains during summer to replenish their water supply. Fortunately, even in the hardest-hit stands, most trees had adequate water and are unaffected. Landowners may see an increase in the number of red trees as the weather heats up, but if green trees have put out a flush of new expanding bright needles on branch tips this spring, they are likely to survive.

Extended dry periods in summer months are characteristic of the Pacific Northwest’s climate and influence which plants can grow and thrive here. Most years are dry in August and September, but rainstorm patterns reestablish themselves in October. In unusual years, the rains don’t return until late October or even early November. Widespread tree drought injury symptoms become noticeable in the spring months (now) following those unusual years like 2012 was.

A report on the symptoms of water stress injuries to Douglas-fir and other conifers was developed by the Forest Health Monitoring Unit at the Oregon Department of Forestry. It provides additional explanations and informative photographs.

By Glenn Kohler, Forest Entomologist, Washington Department of Natural Resources glenn.kohler@dnr.wa.gov