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Wood Decay Associated with Pileated Woodpecker Roosts in Western Redcedar

The pileated woodpecker (Dryocopus pileatus) is a species of concern to forest managers because it uses cavities in large snags and decadent trees for nesting and roosting. A radio-telemetry study of pileated woodpeckers on the Olympic Peninsula of western Washington (1) found that large western redcedar (Thuja plicata Donn ex D. Don) were frequently used for roosting. These roost trees were partially hollow or softened by internal decay. Western redcedar is thought to be less susceptible to most fungal pathogens than are other conifers. This misconception probably arose because of the high resistance to decay of cedar heartwood under service conditions. Many heartwood fungi do, however, attack living redcedar during their lifespan (often over 1,000 years), and large western redcedar have a high incidence of heartwood decay (2). Two to four wood cores were removed below openings used by radio-tagged woodpeckers at heights of 7.5 to 10.5 m on each of 10 large (200 to 300 cm diameter at 1.4 m above ground), living western redcedars. Wood samples were cultured on malt extract agar. No pure cultures of wood-decay fungi were obtained from two trees, although the samples were visually decayed. Oligoporus sericeomollis (Romell) Bondartzeva (= Poria asiatica (Pilát) Overh.) was the only wood-decay fungus isolated from the remaining eight trees. O. sericeomollis causes a brown, cubical heartrot in living western redcedar. In late stages of decay, the rot column typically forms a piped or tubular shape, can extend 25 or more meters up the bole of the tree, and can spread out into large branches (2). Pileated woodpeckers often roost in large, longitudinal tree cavities having multiple openings that provide a means of escape from potential predators. The decay columns produced by O. sericeomollis may create optimal roosting conditions in western redcedar for pileateds. During the telemetry study, woodpeckers were observed entering redcedars at one opening, but exiting from another opening some distance away. Large western redcedars that contain heartrot before they die may be particularly important to pileated woodpeckers because these trees persist in the landscape and provide potential roost sites far longer than other tree species in western Washington, perhaps even for centuries. Information from this and similar studies may be useful for developing inoculation techniques that use fungi with specific decay characteristics to create habitat for woodpeckers.

References: (1) K. B. Aubry and C. M. Raley. Northwest Env. J. 6:432, 1990. (2) D. C. Buckland. Can. J. Res. C. 24:158, 1946.