Older black locust trees in our region are often rotten on the inside. Fungi worm through the tree trunks, digesting their cores. When these fungi are mature enough to reproduce, they sends filaments (hyphae) to break through the tree bark. The exposed hyphae grow into “shelves” on the trees’ trunks. These shelves are comprised of the remains of thick-walled dead hyphae, intermingled with thin-walled living strands. The “wooden” feel of the shelves comes from the walls of the dead cells; the spores are made by living hyphae. “Wooden” is not quite accurate: fungi cell walls are strengthened not with the stuff of wood, cellulose, but with chitin, a molecule that also finds use in the exoskeletons of insects. The shelves are tough enough to persist for many years.
Rotten trees provide habitat for many animals. Much of this utility is mediated through the work of woodpeckers. In excavating a fresh nesting hole each year, woodpeckers leave a trail of convenient roosting and nesting sites for other species, most obviously many bird species (titmice, chickadees, great-crested flycatchers, wood ducks, owls, etc), some mammals (flying squirrels), and bees (feral honey bees love big hollow trees). Most woodpeckers will only attack trees that are partly rotten, so it is the combination of bird and fungus that produces this real estate boon in the forest.
Woodpeckers and fungi may also have more short-lived associations. My friend Joseph Bordley pointed out to me that the tree bark under shelf fungi is often scratched up. This seems to be particularly true for shelves of Phellinus robiniae on locust trees. I found one such example this weekend on the trail to Bridal Veils falls near Sewanee. As the photos below demonstrate, the distressed bark sits directly below the shelf. Are woodpeckers using shelves as shelters in the rain? Despite keeping my eye on locust trees for many years now, I’ve yet to see any birds under shelves in rain or shine. I’d be interested to hear whether anyone has any other relevant observations. Might animals be gnawing the wood to slurp mushroom spores? Is there some other reason for the mysterious scratchings?