Notes from the Wildlife Naturalist:
The Science of Snags
by Emily Hutto

“Snag.” What does this word bring to your mind?

Perhaps your clothing getting caught on some briars, or grabbing a drink from a convenience store. Did you know that “snag” has a different meaning to biologists? A snag is a word that scientists use to refer to standing dead trees.

We don’t see a lot of snags in our everyday lives when we drive around town or walk through our neighborhoods. Generally, humans see dead trees as a nuisance or an eyesore, and we cut them down. We may even think that because a tree is dead, that it is no longer useful, and it has no more life in it.  And this couldn’t be farther from the truth.

Snags are not dead—the tree may no longer have living tissue, but these woody skeletons are absolutely teeming with life, and are the lifeblood of the forest. Trees have many more years of life-giving purpose after their death. When a tree dies, its wood becomes soft as it rots away.  Fungi colonize the tree and further decompose it. The rotting wood attracts insects, especially wood-loving insects such as carpenter ants and termites, who eat or make their home in the dead tree, making the timber even softer. These insects attract insectivorous predators, especially woodpeckers. Not only are the snags a smorgasbord of food, but the soft, rotting wood is much easier for the woodpecker to drill into than the wood of a living tree. In addition to a buffet, the snags also provide a home for the woodpecker. Woodpeckers are cavity nesters.  Instead of building a traditional bird nest, woodpeckers excavate holes in dead trees and lay their eggs in the cavity they have created. Some species of woodpeckers will use the same cavity for years, and some will excavate multiple cavities for a mate to choose from.  Once the woodpeckers leave, the rejected and used cavities sit vacant—a tempting home for many other animals.

Once vacant, the cavity becomes a home to “secondary nesters”—Eastern Screech-Owls (Megascops asio), Southern Flying Squirrels (Glaucomys volans), Purple Martins (Progne subis), Tree Swallows (Tachycineta bicolor), and many more. Secondary nesters are animals who need to nest in cavities, but who cannot excavate the cavity themselves.  This means that they rely on cavities abandoned by woodpeckers in order to reproduce.  When the woodpeckers leave, you may find these species raising their babies in the abandoned holes.

After many years of providing a warm home to nesting species, the snag eventually falls. But its job is far from over. For the next few years, it is a haven for hundreds of species of ground-dwelling and burrowing millipedes, spiders, insects, gastropods, salamanders, snakes, and more, who use the snag to protect them from the elements and predators.  The fungi and animals in these rotting logs are a critical winter food source for woodland animals, including rodents, snakes, deer, birds, turtles, foxes, and frogs. Finally, after many years of providing warmth, food, and shelter to hundreds of creatures, the snag disintegrates back into the soil, becoming a nutrient-rich natural fertilizer for future trees, and perhaps its own offspring, completing the circle of life.

Without snags, there are no woodpecker cavities and less food for the insectivores. Without woodpecker cavities, there are no Screech-owls, flying squirrels, or dozens of species of cavity-nesting birds.  Without fallen trees, there is significantly less food available for woodland animals, especially in winter.  When we remove snags, we remove valuable resources and life from the forest. While you may not see many snags in your neighborhood, if you visit Ruffner or other greenspaces, you will see many snags, even on a short journey.  We want to be great stewards of our forest, and that means leaving as many snags as we can, only removing them if they pose a fall risk to buildings or property.

The next time that you visit Ruffner, pay attention to the “dead” trees around you, and notice all of the life teeming within them.