Perched on the bare limb of a standing snag, an owl calls to its mate. Nearby, a mouse scampers along a fallen log and a spider spins its web on a rotting stump. From standing snags to lying logs, dead wood is essential in a forest, though its importance is often overlooked. At each stage of decay, snags and logs are hubs of activity, providing food, shelter, perches, travel corridors, and many other functions in the forest ecosystem.
Some trees die suddenly, caught in fires, hurricanes, or struck by lightning, but most trees die in stages, succumbing gradually to disease, drought, old age, or a combination of factors. As a tree decays, it provides a home for a parade of plants, animals, fungi, and bacteria that further the process of decomposition. Each community of organisms inhabits the tree at a particular stage, from standing snag to rotting log, and then is replaced by a different complement of inhabitants as the conditions change. The occupants alter the wood in ways that make it no longer suitable for their own kind, but ideal for the next wave of inhabitants – a process called ecological succession.
Snags – any standing dead or dying trees – are a magnet for birds of all kinds. Many cavity-nesting birds, like chickadees and bluebirds, wood ducks and nuthatches, raise their young in standing snags. Other birds, like osprey and herons, build their nests on snags. Hawks and owls perch on snags to hunt or to sit and eat their prey. Crows use snags for surveillance, and woodpeckers use them for drumming. Because of the many insects that infest them, snags provide a rich food supply for insect-feeding birds. Some, like nuthatches, glean insects from the surface, while woodpeckers probe for beetle grubs and ant pupae in the wood.
Many kinds of wood-boring insects tunnel into snags, some near the surface and others deep inside the trunk. Engraver beetles make channels just under the bark, leaving spider-like patterns in the wood. One kind is the ash bark beetle, which attacks newly dead wood. The male excavates a small chamber just beneath the bark where he mates with a female. The female then excavates a horizontal gallery, with notches along the sides in which to deposit her eggs. When the larvae hatch, they tunnel away from the main gallery, some up and some down, following the grain of the wood. Each tiny tunnel gets larger as the larva grows and chews out a bigger space. Larvae get what nutrients they can out of the inner bark of the dead or dying tree. At the end of each tunnel, the larvae make a small oval chamber where they turn into pupae. When the new adult beetles emerge, they chew their way out through the bark and fly off to another tree. Each species of bark beetles leaves a characteristic pattern of tunnels in the wood of its host tree.
A tree’s bark is its armor, and once it is breached by insects, spores of fungi can enter those openings. The fungal threads that grow from the spores penetrate the wood, breaking it down and softening it as they digest it. Shelf-like bracket fungi growing on a tree are the fruiting bodies of a fungus and a sign of decay inside the trunk. The yellow-bellied sapsucker, a woodpecker that relies more on tree sap than insects for its main course, excavates its nest chamber in dying birches, aspens, and other hardwood trees infected by fungi. The decayed wood inside the trunk promises easy digging while the still-hard outer wood provides good protection for the young.
A pile of sawdust near the base of a snag or stump is a telltale sign of carpenter ants in residence. These social insects live in large colonies, excavating a maze of galleries in decaying wood. Unlike termites, carpenter ants use the tree not as a food source but as a home. Pileated woodpeckers gouge out large holes in snags as they search for carpenter ants inside. Carpenter ants tunneling inside a snag and woodpeckers digging from the outside are powerful forces that weaken even the toughest trunk.
Animals are drawn to snags and stumps for many different purposes. Squirrels and raccoons use hollow snags for nesting or shelter in winter. Bats roost under the loose bark of decaying snags, flying out at dusk to hunt for insects. Mice and chipmunks store food in cracks and crevices. Fishers, skunks, and bears visit snags and stumps in hopes of a tasty meal. Under the snow a tangle of fallen limbs and branches provide cover and travel corridors for small mammals, and the punky wood in a snag or log provides insulation from the cold. Uprooted trees offer nesting habitat for birds and protected hollows, perhaps even a bear-sized den, underneath.
When trees end up as logs on the ground, they change dramatically. Once dry and lofty, they become refuges for damp-loving, earthbound critters. Logs soak up water from rain and snow and the damp earth, becoming reservoirs of moisture. Mosses soon carpet the surface, while fine thread-like strands of fungi weave their way into the log, and mushrooms spring up upon it. Some are colorful, like bright yellow witch’s butter, a jelly-like fungus, or the eye-catching scarlet eyelash cup. Green stain, as its name implies, dyes the wood a deep blue green color. More muted are turkey tails in frilly rows and papery brown puffballs that release their spores like tiny smoke signals. Fungi secrete enzymes that break down the tough lignin holding the wood together, an important step in the recycling of trees.
A rotting log attracts numerous small creepy-crawly critters that scuttle about inside its dark interior, lurk on its surface, or hide underneath. Spiders set their snares in cracks and crevices and beetles scurry through tunnels. Fly and beetle larvae burrow in the soft wood. Snails and slugs leave trails of slime and tiny translucent eggs. This soggy domain is home to isopods, or sowbugs, tiny crustaceans with seven pairs of legs and flattened bodies that live on land and breathe with gills. Also called roly-polies or pill bugs, some isopods roll up into a ball when disturbed, like tiny armadillos. These creatures produce frass, a sawdust-like material which if dark-colored is made of their droppings or if light-colored, from wood that was chewed and pushed aside.
These small creatures provide food for other woodland animals, like the red-backed salamander, a common inhabitant of rotting logs and one of the most numerous animals in the forest. These slender amphibians are lungless, breathing only through their skin, and so require a moist environment. The female hangs her eggs like a cluster of grapes inside the log, then guards them with her body. The log provides her with food, water, shelter, and a place to rear her young.
The rate at which a log decays depends on the type of tree and many other factors, from rainfall to the workings of its residents. Eventually the wood fragments into shreds and bits of organic matter, called humus, an important moisture-holding component of the forest floor. As portions become more soil than wood, seeds take root and the log becomes a nursery bed for plants. The tunneling of worms, shrews, ants, and salamanders mixes the fragments into the soil, providing nutrients that sustain the life of the forest.
Standing snags and rotting logs are a key attraction in any forest, magnets for a procession of inhabitants and visitors, from mammals to microbes, seeds to salamanders. A certain amount of dead wood is an important element of a healthy forest ecosystem, and it always draws a crowd.
Holland, Mary. Naturally Curious. North Pomfret, VT: Trafalgar Square Books, 2010.
Kricher, John C., and Gordon Morrison. Ecology of Eastern Forests. Peterson Field Guides. Boston: Houghton Mifflin, 1988.
Wooster, Chuck, ed. The Outside Story. Corinth, VT: Northern Woodlands, 2007.