Drought hits Douglas-fir trees hard

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Dying trees have been spotted frequently in western Oregon this spring. Photo by Glenn Ahrens.
Last Updated: 
May 29, 2015

CORVALLIS, Ore. – Many Oregonians have noticed widespread damage in landscape and forest trees this spring – and weather may be the culprit.

“Browning or dieback is often caused by weather-related stress, sometimes in combination with pests and diseases,” said Glenn Ahrens, a forester with Oregon State University’s Extension Service. Douglas-fir trees are the most common victims, he said, but stress due to weather is affecting many tree species and a variety of problems are showing up.

On some Douglas-firs, branches and tops are turning red or brown. Sometimes the entire tree dies. Older trees typically have milder symptoms.

“This sudden mortality or ‘flaring out’ of branches and tops is a classic symptom of drought in conifers,” Ahrens explained.  

Possible stressors include last year’s long, dry summer ending with a hot period, followed by an early freeze in November and then a relatively warm winter, he said.

Drought-related injuries to the stems and leader are not always apparent when they occur, but often show up the following spring as the weather warms up and trees begin to grow.  That seems to have begun with the warm weather of February and March, with symptoms becoming obvious in April. 

Similar drought damage has occurred periodically over the last 15 years, most recently in 2013, according to Ahrens.

Heat and drought can kill trees outright or put the trees under severe moisture stress. Subsequent problems can happen when insects or diseases take advantage of a tree’s weakened condition.

Douglas-fir trees are most commonly affected, but similar problems occur with other conifers, including grand fir, noble fir, western redcedar and western hemlock. Grand firs around the Willamette Valley are notorious for health problems due to drought followed by secondary problems such as bark beetles and fungi.

Ahrens said drought-stressed Douglas-fir trees are often troubled by stem canker, normally caused by weak pathogens that become active in trees under stress.  The cankers can coalesce to girdle branches or stems, and also can become sites of attack by bark beetles.

Insect pests that take advantage of drought-stressed trees include the Douglas-fir engraver and the pole beetle. Grand fir and noble fir are vulnerable to engraver beetles that attack true firs of all sizes.

Douglas-fir trees in some foothills around the Willamette Valley are afflicted with Swiss needle cast. The disease produces a pale overall appearance and sparse crown as individual needles turn yellow and drop.

“Swiss needle cast disease has been a problem in coastal Douglas-fir since the 1990s,” says Ahrens. “But last year we had increased reports of the disease in the Willamette Valley and we are seeing it again this year.”

Weather is also a contributing factor and the disease is most severe in years with a combination of a warm winter and abundant spring moisture, Ahrens said. Indicators of Swiss needle cast are progressive yellowing and shedding (casting) of needles, beginning with the older needles. A healthy tree may carry four to five years’ worth of needles, while heavily infected trees may carry only one or two years’ worth.  Although the disease is not generally fatal to the tree, it often has a significant impact on growth.

The Extension publication Forest Health Fact Sheet: Swiss Needle Cast in Douglas-fir in Oregon gives more information.

Ahrens offers the following tips for keeping trees healthy

  • Prevent soil compaction caused by vehicle or animal traffic near trees. Compaction can damage roots, especially in clay soils.
  • Avoid direct damage to trees and roots by animals or machinery.
  • Reduce competing vegetation.
  • Irrigate landscape trees during dry spells. Apply water slowly over many hours; avoid frequent shallow watering. Apply mulch to maintain soil moisture.
  • Do not alter drainage near established trees (ditches, ponds, fill or removal of soil).
  • Plant trees that are well suited for the site. Where Douglas-fir mortality is occurring, consider planting Willamette Valley Ponderosa pine or hardwoods.
  • If insects or branch/stem cankers are evident, prune and destroy affected branches to reduce spread.
  • Do not fertilize during drought conditions; fertilization can increase a tree’s water requirements.
Author: Mary Stewart
Source: Glenn Ahrens