Western Redcedar: Facts, Bugs and Cruds

In their book Northwest Trees – Anniversary Edition, authors Stephen R. Arno and Ramona P. Hammerly begin their description of this native species, “Western redcedar is an icon of the Northwestern forest, beloved for its majesty, beauty, and excellent wood.”

Western redcedar (Thuja plicata), however, is not a true cedar but instead finds a taxonomic home in the cypress family – Cupressaceae.

Now with that dendrology lesson, let’s take a closer look at our native western redcedar.  We oftentimes visualize majestic groves of enormous, long-lived trees.  As a climax species on moist sites, it will survive long after competitor trees drop out of the stand.

The wood of western redcedar is naturally resistant to decay as the two most common stem decay fungi that infect it – cedar laminated butt rot and cedar brown pocket rot – seldom kill the tree.  But the decay pockets attract cavity-nesting birds and other animals.  Carpenter ant nests are also common at the base of the tree.  While these conditions may cull a tree for merchantability, western redcedar is a survivor and can live for centuries as a hollowed-out shell.

Occasionally cedar bark beetles (Phloesinus spp.) will attack western redcedar trees.  These attacks usually result in top kill or dead branches, only occasionally killing the tree.

For centuries this majestic species has done quite well occupying moist sites, surviving all but the most intense fires, fighting off occasional bark beetle attacks, and still standing even though its bole rings like a bass drum when pounded by the timber cruiser’s axe.  (Using a skill involving more art than science, old-time cruisers estimated the defect in cedar trees by the hollowness of its sound.)

But now, there is something amiss in the deep, dark woods where cedar thrives.  For the past few years, western redcedar growing west of the Cascades Mountains have been exhibiting uncharacteristic symptoms including top kill, dead branches, yellowing needles and, in some cases, mortality.  Don’t confuse these symptoms with the natural needle shed that occurs every year in early fall as evergreen trees drop foliage in preparation for winter.

There are indications that this condition is now appearing in North Idaho and eastern Washington.

This malady baffles landowners and foresters.  Without knowing for certain, forest health specialists refer to this phenomenon as western redcedar dieback.

At first glance, there does not appear to be a significant increase in the insects and pathogens that cedar trees have evolved with over past centuries.  Furthermore, no new pests affecting cedar have been identified.

Absent of any biotic explanation, abiotic causes might provide an answer to this unusual condition.  Climate change in the form of warmer, drier summers may be affecting western redcedar trees.

A concerted research effort involving federal, state and university forest health specialists is underway to better understand western redcedar dieback.  Are there common factors such as topography, soils, climate relating to this condition?  What size and age classes of trees are most affected?  How long does it take a tree to die?   Answers to these and many other questions will eventually solve this dilemma.

In the meantime, I offer two suggestions.  First, call a forester if you notice any visual changes to cedar trees in your forest.  Secondly, take a walk through the Ross Creek Cedars above Bull Lake in western Montana to appreciate this, “icon of the Northwestern forest, beloved for its majesty, beauty, and excellent wood.”

~Bill Love