Dead tree ecology

Dead trees, or snags, may not look like they belong in a healthy forest, but they are thriving habitats for all sorts of wildlife. Photo: Ryan Campbell

If there is one fundamental principle underlying all of ecology, it may be that no niche goes unused. Essentially everywhere we look in nature, we find plants and animals exploiting the available resources and energy to make a living. One common feature of all forest environments is some number of standing dead trees, often called snags. While they may not be part of our concept of a healthy forest, they are a bonanza for wildlife and provide hundreds of organisms with comfortable habitat.

A tree can die in any of a number of ways, including pest infestation, disease, lightning strikes or windstorms. When they do, almost instantly, swarms of plants, fungi and invertebrates begin to colonize the new habitat. Fungi, mosses, insects, spiders, millipedes, centipedes and worms begin to invade the dead tissue, processing it for their own use, facilitating the decomposition and recycling of the tree. As this initial process proceeds, dozens of species of birds and mammals find the first wave of exploiters to be attractive food sources. The most familiar examples are woodpeckers and other birds foraging on snags to find the underlying insects that often burrow just under the bark. But raccoons, squirrels and bears, among others, also use the richness of the “dead tree ecosystem” to find food. As the tree tissue begins to break down, it becomes much easier for birds, mammals and insects to excavate it to make cozy dens and nests.

Recently in the midwest the emerald ash borer, a beetle that devastates ash trees, caused the near-extermination of the ash tree population and an increase in the number of snags in forests. This is not a new phenomenon. We have lived for decades with periodic outbreaks of tree pests. While their effects can be undesirable, the potential for creating more usable wildlife habitats is obvious. But this is true only if we manage forests in an ecologically conscientious way, preserving as many of the dead standing trees as possible.

At Fermilab, we have begun removing doomed ash trees, especially in the Village, but only those that would pose a safety risk if left standing. In fact, we have a general policy regarding snags that requires us to leave them as wildlife trees unless they present a hazard. This practice encourages species that might not be interested in a forest with only living, healthy trees.

Rod Walton