Nurse Logs, Voles, and Hemlock Regeneration

Copyright © 1997, Daniel B. Wheeler

Hemlock as a forest tree has developed some nearly unique strategies for survival. Because soil and elements are often well colonized by Douglas fir, true fir, or hardwoods by the time hemlocks normally are introduced into the forest mix, the climax forest trees such as Hemlock, Sitka spruce and Western red cedar have learned to use downed logs as seed beds. As the logs degrade, the seedlings roots fully colonize the rich humus reservoir. Elements which already allowed one tree to grow, are recycled by the fungi into the next generation of trees.

As large woody debris (larger than 3 inches in diameter) contact the forest floor, a host of animals move into them for shade. But even before they fall to the floor, many have already been colonized by fungi. In general, within a year of tree death, the cambium layer between the bark and wood has been completely consumed by fungi. These fungi then slowly grow into the remaining wood. As the fungi do this, the degraded wood takes on a consistency of a sponge. It holds onto water, and traps mycophagous (fungus eating) insects, which feed on the fungal mycelium. In essence, a downed log becomes a perfect seed bed for hemlock, cedar and spruce seeds to sprout and begin growing. Additionally, the bed logs are often of large diameter, clearing small spaces in the forest canopy which then allow anything growing on the logs access to light.

In shallow soils or on very steep slopes, large woody debris also becomes important habitat for California Red-backed voles, which use the down-hill side of the logs as runways. Usually these voles are underground animals. But when shallow soils and insufficient humus layers require, they utilize logs instead. Indeed, they often burrow into the soft wood for shelter and to find truffles, which are their main food source. In turn, the truffle spores are dispersed through the vole fecal pellets, making downed logs literally into nurse logs for the next generation of tree succession. This assures that cedar, spruce and hemlock will become climax forest trees, simply because they can survive in the low light and dense canopy of their parents.

Some truffles are so involved with woody debris that David Aurora in Mushrooms Demystified calls them Wood Truffles (Hydnotryas). Other important mycorrhizal fungi (MF) with this type of forests include Elaphomyces truffles, which Aurora calls Deer truffles. Deer, elk, squirrels, bear, mice, voles, rabbits and other animals all find Elaphomyces to be an important food source. Unfortunately for us, they are mildly poisonous to humans.

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