Why dead trees are not empty
To many people a dead tree looks like waste, something to tidy away. In a forest it is closer to a crowded apartment building. A large share of forest insects, fungi and some birds and mammals depend on dead or dying wood for food, shelter or nesting sites. Where dead wood is removed consistently, the species that need it often decline, and this has been recognized by many forest managers.
Dead wood comes in two main forms. Standing dead trees are usually called snags. Fallen trunks and large branches on the ground are logs, sometimes called coarse woody debris. Each supports a different set of species, and a forest with both tends to support more variety than one with neither.
Wood is food, but hard to eat
Wood is made largely of cellulose and lignin. Lignin in particular is tough, and few organisms can break it down. This is why fungi are so important: certain groups are especially good at decomposing wood, and many insects either feed on the fungi or carry them into the wood. Beetle larvae tunneling through logs are often working alongside fungi rather than digesting wood on their own.
As wood decays it also softens, holds water like a sponge and gains nutrients, making it more useful to many animals over time. The same material that is nearly inedible in a living tree becomes a rich resource a few years after death.
Stages of decay
A dead tree is not one habitat but a series of them, changing over years or decades. Speed varies widely with species, climate and the size of the piece, so the sequence below is a general pattern rather than a timetable.
- Recently dead. The bark is still tight. Bark beetles and wood borers often arrive first, feeding under the bark and in the outer wood. Some of these insects attract woodpeckers and other predators.
- Early decay. Bark begins to loosen and fall. Fungi are establishing, and beetle galleries open pathways for more fungi and for moisture.
- Advanced decay. The wood is soft and crumbly. Many fungi fruit on it, and a wide range of insects, snails and other small creatures live inside. Mosses and young seedlings may begin growing on the surface.
- Final stages. The log breaks down into soil-like material that blends into the forest floor, returning nutrients to the soil much as fallen leaves do in leaf litter.
Standing dead trees and the animals that use them
Snags are valuable because they combine height, hollow space and a supply of insects. Woodpeckers excavate nesting cavities in dead or decaying wood, because it is easier to dig than solid wood. Those cavities are later reused by other species that cannot excavate their own, which often include small owls, some ducks, chickadees and similar birds, and bats and small mammals. This reuse is one reason woodpeckers are sometimes called ecosystem engineers.
Loose bark on a snag gives bats or other creatures a place to roost in some regions. The open crown of a dead tree can serve as a perch for raptors with a wide view. Many forest birds rely on these features for nesting and feeding.
Snags can also be hazardous. A dead tree near a trail, road or building may fall, so the decision to keep one standing depends on where it is. Where there is a safety concern, some managers leave the trunk at a reduced height or let it fall on the ground rather than remove it entirely.
Fallen logs on the ground
Logs on the forest floor create cool, damp, sheltered microhabitats. They hold moisture through dry spells, which can matter for salamanders, amphibians, beetles, millipedes and fungi. Small mammals use hollow logs for shelter and runways. Some birds drum or forage on them.
Logs also act as seedbeds. In some forests, tree seedlings establish on rotting logs more readily than on the surrounding ground, because the decayed wood stays moist and may be free of heavy competition from other plants. Fallen trunks can also slow the movement of water and soil on slopes, which relates to slope stability and soil erosion, though the effect depends on log size and position.
Large logs generally last longer and hold more moisture than small ones, so large-diameter dead wood is typically valued highly. Logs in streams are a separate case: they create pools and shelter for fish in forest streams.
Why dead wood exists: how trees die
Dead wood is produced by many ordinary processes. Old age, shading, drought stress, disease, fire, lightning, windthrow and attacks by forest insect pests can all kill trees. A young forest that has recently been cleared or managed tends to have little of it, which is partly why old-growth forests are often rich in snags and logs of many sizes and decay stages.
Dead wood in managed forests and woodlands
Many modern forestry guidelines encourage leaving some dead wood during harvesting, and some landowners keep a few snags or logs deliberately as part of wildlife habitat management. Practices differ by region and by goal, and there is no single correct amount. Several considerations commonly come up:
- Safety. Keep trees that could fall on paths, roads or buildings in mind.
- Fire. In some dry regions, large amounts of dead wood add to fuel loads. Managers there may weigh habitat benefits against fire risk, and the right balance depends on local conditions.
- Pests. Freshly dead or stressed trees can sometimes be breeding sites for insects that also attack living trees, though most beetles that use dead wood do not harm healthy trees.
- Variety. A mix of species, sizes and decay stages usually supports more life than a single type.
The lesson of a rotting log
Dead wood is a bridge between the life of a tree and the life of the soil. It stores carbon for years or decades as it slowly breaks down, feeds fungi and insects, shelters vertebrates and eventually becomes part of the ground in which the next trees will grow. Leaving some of it in place is often one of the simplest steps a forest owner can take to support a more complete community of forest life.