In 2015, Washington state experienced a record low snowpack, below-normal spring and summer precipitation, and record high temperatures for most of the year. By August, eastern Washington was experiencing an extreme drought that lasted through the end of October. Not only are the visible effects of this drought on tree health still with us, they may be evident for several more years as weakened trees fall victim to bark beetles.
During a drought, water that is lost through the tree’s foliage (transpiration) can exceed the amount of water the tree can intake through its root system. The result of this imbalance is increased tension in the interior columns of the tree that transport water from the roots up to the crown. As drought conditions increase, these water columns can break, causing damage to the tree.
Symptoms of drought usually progress from the top of the tree down and from the outside inwards. The effects may not appear right away, but over time you may see the tree’s leaves wilt, become chlorotic (turn yellow due to lack of chlorophyll), or redden. Newly emerging shoots may appear shrunken. Shoots and branches may die, which can cause the treetop to die or produce an irregular pattern in the tree’s crown. The loss of foliage and the damage to tree’s cambium can slow new growth and, potentially, lead to the tree’s death.
Bark beetle outbreaks and root disease are often associated with drought-weakened trees. Bark beetle activity is increasing in Washington state and it is likely related to last year’s drought conditions.
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 these stands can increase the vigor and resilience of the trees that remain because there will be fewer trees competing for scarce resources, such as water. DNR advises forest landowners to design their thinning projects to leave the tree species that are appropriate for the site. In many locations these species include pine and larch, which are more drought tolerant than many types of fir. Controlling competition from other plant species underneath the trees helps, too.
Caring for trees suffering from drought
If you live on an urban plot or have just a few trees, try these tips if your trees are turning yellow:
- Water to a depth of 12 inches below the soil surface.
- Saturate the soil around the tree within the “dripline” (the outer edges of the tree’s branches) to disperse water down toward the roots.
- For evergreens, water 3 to 5 feet beyond the dripline on all sides of the tree.
- Water slowly so that the water gets deep down to the tree’s roots. Watering for short periods of time encourages shallow rooting which can lead to more drought damage.
- Don’t spray water on tree leaves during a drought because it wastes much of the water; instead, water the ground around the tree.
If you own forestland and are caring for dozens, if not hundreds or even thousands of trees, it’s more practical to monitor the health of your tree stand for signs of disease, insect damage and tree mortality.
Find free tree care and garden advice printouts from Washington State University Extension.
(This post is based on an article that appeared recently in Forest Stewardship Notes, a free e-newsletter about forestland management published quarterly by DNR and Washington State University Extension)
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