Trees in Washington state are showing the damage caused by dry conditions. Photo: DNR.
When the rainforest in Olympic National Park catches fire, you know that Washington is dry. However, increased fire risk is not the only summertime threat to trees and forests. Drought conditions can cause cell and tissue dieback in trees and can also give pests and diseases a leg up in the battle for forest health.
According to DNR’s recently published Forest Health Highlights in Washington–2014:
“Trees experiencing drought stress can become more susceptible to insect and disease attacks and are less likely to recover from damage. In eastern Washington, trees growing in dense or overstocked stands have a higher likelihood of experiencing drought stress.”
Trees in urban landscapes that may be disproportionately affected by drought are those that are newly planted, victims of root damage, or growing in tough planting sites that are heavily compacted, poorly irrigated, or space limited.
In some cases, such as with water-dependent diseases like Sudden Oak Death, drought can hinder the growth and spread of disease organisms. However, many pests and diseases are more resilient in drought conditions than their host tree species.
For example, bark beetles thrive on drought stressed trees. In recent years, pine bark beetle populations have been exploding throughout the western U.S. as a result of drought and other complicating factors. Many types of tree diseases may also worsen in drought conditions including root rots, cankers, and wilts such as Dutch elm disease.
Check out this recent story from King5 News about the effects of drought on Seattle’s elm tree population.
For more information on this topic, consider reviewing the following resources:
- Dead Branches, Dead Tops & Dead Trees: The Interaction of Water Stress, Insects & Disease, From the Oregon Department of Forestry
- Long-term Drought Effects on Trees and Shrubs, from the University of Massachusetts Extension
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