In 2015, Washington state experienced a record low snowpack, below-normal spring and summer precipitation, and record high temperatures for most of the year. As a result, by August eastern Washington was experiencing extreme drought conditions that lasted through the end of October. The visible effects of this drought on tree health are already apparent and may be evident for several years, especially related bark beetle-caused mortality.
Effects of Drought Stress on Trees
During a drought, water loss through the foliage (transpiration) can exceed water uptake via the root system, resulting in increased tension within the columns that are transporting water from the roots to the crown. As drought conditions escalate, water columns can break, having deleterious effects on trees.
Symptoms of drought usually progress from the top of the tree down and from the outside in. The effects may not appear right away, but over time you may see tree foliage wilt, become chlorotic (turn yellow due to lack of chlorophyll), or redden. Newly emerging shoots may appear shrunken. Shoots and branches may die, resulting in top kill, or producing an irregular pattern of flagging in the crown. Growth loss may result from loss of foliage and damage to cambium, potentially leading to mortality.
Bark beetle outbreaks and more root disease are often associated with drought-weakened trees. The current increase in bark beetle activity in Washington is likely related to the drought conditions experienced last year.
Managing Drought Stress
Although drought stress is common in eastern Washington, it has been exacerbated by decades of fire suppression practices that caused tree stands to become more dense. Trees growing in dense, overstocked stands tend to get less water and lack the defenses to fend off invaders such as bark beetles.
Thinning stands can increase the vigor and resilience of trees because there will be fewer trees competing for scarce resources–water, in particular. When thinning, it is important to leave species that are appropriate for the site, such as pine and larch which are more drought tolerant than many types of fir. Controlling competition from other plant species in the understory can help as well.
By Melissa Fischer, Forest Health Specialist, Northeast Region, Washington State Department of Natural Resources, melissa.fischer@dnr.wa.gov
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