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?
David George Haskell 
What’s amazing to me is the first picture of the Phellinus robiniae. It looks like an arial photograph taken from high up of some distant land features.
The lower tier on the left has a tall mesa that towers over a lush and verdant forest, which is in stark contrast to the desert plains beyond the tree line.
The upper tier on the right has dwellings carved out of the mountainside by local natives that live high above the surrounding forest and grasslands; similarly to the lower tier, excursion towards the outskirts brings the natives to a barren, arid land.
Interesting perspective! I’m one of those natives, but my dwelling is not nearly so beautiful.
Black locust is named Robinia pseudoacacia after Jean Robin, herbalist to king Henry IV of France in the 1600s, who sent or brought seeds to Paris from Louisiana. There is a black locust you can see across the river from Notre Dame supposedly planted from those seeds in the 17th century. I think it’s the Cumberland Plateau French who love mushrooms who are scratching around under the fungi.
Larry, I am very interested in early plant explorers, particularly in the US, and wonder where you came the reference to Jean Robin. Would you mind sharing?
Hi Larry, I did not know this. Fascinating. I grew up in France and never paid attention to the locusts near Notre Dame. One more thing that I missed (!) but I’m glad to know it now. And, yes, the French love their mushrooms!
To me, this is a fascinating example of how so many of us go through our lives looking at our surroundings every day, but missing so much of what is in front of us. I wonder how many of us have seen this and similar things and simply never noticed them? Scratches on wood and shelf fungi are common sights, but sometimes a light comes on and the observation is made that they are often found together. Then we go and look some more and “discover” something that we have seen, but not “seen”, for years. I’m always trying to train my eyes and brain to look past the obvious, but it’s a hard slog, sometimes. Nice article and a great lesson in observation.
Thank you. Seeing what is in front of our eyes is a challenge sometimes.
Kay, the reference to Jean Robin is in Donald Culross Peattie’s A Natural History of Trees of Eastern and Central North America, p. 417, one of my favorite tree books. I happened to be walking on the east bank about six years ago, not far from Notre Dame, when I came across a little park with an ancient, stooping black locust with a detailed plaque claiming it was planted by Jean Robin in the 16oos. I went back later in the day and spent a long time looking for seeds in the cracks in the brick pavement and found 8 or 10. But where they are now I do not know. In my native state of New Jersey, ancient large black locusts often surround the oldest houses. They were popular plantings in the 18th century. Here in north Alabama, they are more fugitive and don’t attain great size. The larger ones in our woods are mostly in death throes and have the fungus David describes.
There’s a shattered old locust much set about with bracket fungi in our field. I’d been past it many times but it took your post to make me REALLY look at it. Sure enough, several of the larger fungi had lots of scrapes under them. But one was particularly telling: not just scraped but drilled in several places in woodpecker fashion. Several holes an inch or so across and deep. We wondered if the woodpeckers, being perhaps drawn by the smell of the fungus to an easy-to-drill tree, worked particularly enthusiastically under the shelves where spores might have impregnated the wood and made it smell particularly enticing? Do woodpeckers relish the taste of mushrooms? Or are there lesser creatures nibbling on an abundance of hyphae under the brackets that the woodpeckers are winkling out? Your ponderings only seem to produce more ponderings – but I blame the woodpeckers in some way or another – perhaps more as feeders than as taking shelter from the elements.
Anne, Thank you for this observation. I’m particularly interested that you found drilling. Woodpeckers are insectivorous so I suspect that they may be after the grubs that are feeding on the fungus (this is speculative, of course). Ponderings lead to more ponderings! Please let me know if you see any birds or other animals on the old locust.
A further observation – a smallish dead locust in the woods here has a couple of bracket fungi on its trunk, one above the other, the lower being a couple of feet above the ground. The area directly below that lower fungus is much scraped for a distance of about 18″ directly below the fungus. The bark looks shredded as if raked from the top down by… I initially thought claws? But when I looked more closely, I saw that the “claw marks” are actually minute double scratches – mousey tooth marks to my eye.
Excellent observation! Perhaps the mice are chewing on spores or other goodies from the fungus? We need to set up some remote cameras to figure this one out…