The summer drought of 2015 was one of the most severe droughts on record in Washington state. According to the U.S. Drought Monitor, the entire state was under severe or extreme drought conditions from late July through early October. Drought damage and mortality in Douglas-fir, western redcedar and pines was immediately noticeable; symptoms included entirely red crowns, red tops, and scattered red branches.
Some affected conifers that teetered on the verge of needle loss but remained green through the winter may be showing drought symptoms now with the onset of unusually hot, sunny days early this spring. Delayed drought symptoms appearing in spring are common to western hemlock in particular. Hemlocks may drop foliage without noticeable symptoms of drought stress. In March and April of this year, the Washington State Department of Natural Resources (DNR) received numerous reports of hemlocks suddenly dropping needles. A similar phenomenon among hemlock trees was widely observed after a drought in 2002.
Common symptoms of drought damage in conifers include rapid needle loss, desiccated (dried out) buds and wilted shoots. Conifers that narrowly survive drought stress may have compromised defenses and be more susceptible to attack by bark beetles or other insects. Drought stress can also increase symptoms of otherwise minor foliar diseases.
Internet searches for information on dying hemlocks may raise concerns about a non-native, sap-feeding insect known as hemlock woolly adelgid (HWA). HWA has caused severe damage to hemlocks in the eastern U.S., however, western hemlocks are tolerant of this pest and rarely suffer significant damage. Hemlock defoliation by HWA will look similar to drought symptoms, except that a tree with HWA will have twigs covered with tiny clusters of white, cottony “wool,” which HWA exudes to protect itself and its eggs.
Weather events can also trigger drought symptoms in mountainous areas. Earlier this year on the west slopes of Snoqualmie Pass, hot, dry easterly winds increased desiccation in Douglas-fir and other conifers. Also in the Snoqualmie Pass area, distinct patterns of brown trees can be seen in elevational bands and in low drainages where inversions of unseasonably warm air alternate with freezing night temperatures to cause desiccation. Desiccation does not typically harm unopened buds; trees may lose older foliage but will likely recover.
Consider monitoring conifer trees for a normal flush of green buds this spring—a positive sign that trees may survive and continue to grow. A drought damaged tree that retains most of its foliage is likely to recover with improving weather conditions. Western hemlock is less tolerant of foliage loss than other conifers. If more than half of the foliage is lost, a hemlock may not survive. Surviving conifer trees should also be monitored for evidence of bark beetle attack.
By Glenn Kohler, DNR Forest Entomologist