Deadwood — every bureaucracy has some! Yet each “deadwood” individual supports a vibrant community of personal bacteria, and perhaps a fungus or a tapeworm. And each such individual is as good as everyone else at harboring viruses and passing them on to others. Yet those hundreds, maybe thousands, of kinds of mostly-unseen organisms in one piece of “deadwood” are a miniscule fraction of the number of organisms hosted by a dead tree.
A dead tree standing often has lichens festooning its remaining branches and plastered onto its loosening bark; it is often easy to find five or six easily distinguishable kinds. Mosses and sometimes ferns adorn its sides. Conks and mushrooms of fungi emerge from the wood; these are the reproductive structures of the fungi, whose main portion consists of filaments (called hyphae) that grow through the wood. Squirrels perch on branch stubs to peel their spruce cones, chickadees store sunflower seeds in crevices, downy woodpeckers search for small insects and spiders in the bark and lichens. Sapsuckers and other woodpeckers drill nesting cavities in the trunk or large branches. And that’s just the readily visible set of critters that find a standing dead tree useful — there are myriad but unseen bacteria and other microscopic organisms on the surfaces, inside the tunnels of wood-boring insects, or in the lichens and mosses.
A dead tree fallen hosts a wealth of mosses, which seem to dominate the lichens. A quick and casual survey of over twenty logs discovered that many of them bore stands of six or seven kinds of readily distinguished mosses (and who knows how many less obvious ones). Small hemlocks and occasional spruces get established on the upper surface of the log, the seeds having found homes in the mosses. In some locations, red huckleberry shrubs get a good start on old logs, along with other shrubs such as blueberry, rusty menziesia, and dwarf dogwood. There are occasional herbaceous flowering plants, including goldthread and wintergreen, and ferns, perhaps a few leafy lichens. That’s all readily visible.
The less visible world on a downed tree is astonishingly diverse. The mosses are teeming with tiny inhabitants. Mites and small insects live among the moss fronds. There are likely to be some tardigrades, those amazingly tough little animals that can survive without water or food for decades, if necessary. Springtails find food and refuge among the mosses, exercising their prodigious jumping ability to escape predators or in hopes of landing in a better spot. Nematodes (round worms) look for detritus to eat. Little pools of water held by the moss leaves might harbor rotifers, microscopic critters with a whorl of hair-like cilia (used for feeding and locomotion) that gives them the informal name of “wheeled animals.”
Innumerable bacteria and one-celled organisms are ubiquitous.
Cavities in a decaying log offer housing for mice, shrews, or ermine. Centipedes and millipedes creep under the loosened bark. Terrestrial snails and slugs may nestle into protective niches in a nice, soggy log. Those oft-maligned slugs are effective dispersers of mosses, when moss leaf fragments pass through the slug’s gut, and they can also disperse the seeds of salmonberry and other fruits that they eat.
The inside of a log can be a rather lively place, being home to some very interesting wood-boring insects. Ambrosia beetles are small, only a few millimeters long. They bore straight into the wood a short distance, making galleries; both male and female beetles help keep the galleries clean and open. Along these galleries the females deposit their eggs. When the larvae hatch, they make small side galleries of their own, just off the main gallery. They are chewing their way through the wood but not eating it; the chewed-up wood accumulates as sawdust. The larvae feed on mutualistic fungi that the female beetles brought in, carried in special chambers under the dorsal part of the thorax; having arrived in a new location, thanks to the beetles, the fungi grow on the wood and feed the growing larvae. When the larvae mature in late summer, the adults emerge from the wood and go into hibernation in the leaf litter. Then in spring, they burrow into recently dead wood, mate on the bark outside the new gallery, and start the cycle again. They emit a pheromone that says “Calling all ambrosias!”, so a number of these beetles often congregate on a tree. There are many species of ambrosia beetle in the world, each with its own particular kind of mutualistic fungus, but just one is prominent in our region.
There are round-headed wood borers, too, whose larvae feed on the wood itself, often in decaying logs. Some of them can be important agents of decomposition in the forest ecosystem. Flat-headed borers are more likely to attack wounded, sick, or recently fallen trees, eating the wood. The tunnels made by these and other burrowing insects allow entry of a great variety of bacteria and fungi, which play a role in decomposition of the wood.
Carpenter ants feed on a variety of organic material, but they tunnel into wood for protection and brood-rearing. (Around here, we don’t have the usual carpenter ants (defined taxonomically) but rather a different species that does much the same thing.) They set up colonies in rotting logs (and sometimes houses …), making extensive galleries. Winged adults emerge in spring and mate; the males then die and fertilized females set up new colonies. These queens spend the rest of their lives laying eggs, most of which become workers that guard and tend the larvae; some of the larvae become the adults of the next generation.
There are even wasps that bore into wood. Using a long ovipositor that looks like a stinger (but isn’t), female wood wasps or “horntails” (so-called for the spike at the end of the abdomen) deposit eggs under bark of newly fallen logs, dying or recently dead trees. At least some species of wood wasps have special places on the abdomen where females carry mutualistic fungi that grow on the wood and are eaten by the larvae. When the larvae hatch, they create galleries that penetrate the wood. Each larva may take two years to complete its development; then it pupates near the surface of the wood, often on the upper side of logs, emerging in spring or summer.
All those exciting beetles and wasps, as well as some tunneling moth caterpillars, are victims of tiny parasitoid wasps or flies that lay their eggs on the host larvae. Then the larvae of the parasitoids eat the host’s body.
A fallen log is really a small but very busy ecosystem, one that interacts with the larger ecosystem of which it is a part.
Thanks to Dr. Elizabeth Graham, USFS Forestry Science Lab, for helpful discussions.
• Mary F. Willson is a retired professor of ecology. Her essays can be found online at www.onthetrailsjuneau.wordpress.com