Snags: One species' eyesore is another's habitat

by Pam Owen, Fall 2004

What do you think when you see snags—dead or dying standing trees—on your property? According to RLEP director Bruce Jones, many visitors to his property cheerfully offer to help him them cut down. When Bruce tells his helpful visitors that he "planted" the dead tree, they’re usually dumbfounded—planted a dead tree?

What Use Are Snags?

Pileated woodpecker. Photo courtesy Bruce Jones.

As Bruce explains, he and his wife Susan love to see birds on his largely naturalized property and so have a lot of bird feeders scattered around. It occurred to him that, while he had set up several dining rooms, the birds that visited them have no living rooms—no place to rest and hang out. Birds, like us, also like to scope out their eating establishments before actually partaking. They’ll land in a nearby tree and check out the bill of fare and who the other diners are. They’ll also look to see if any predator is lurking nearby.

While living trees make great living rooms, too, they also camouflage birds when the trees are in full foliage.  Dead trees, without the leaves, provide both a great staging area for birds and a great view for birdwatchers. So, Bruce "planted" some dead trees in strategic locations around the property.

Many predators (such as hawks) also use snags to scope out their meals—smaller birds and mammals. At the same time, they can keep an eye on members of their own species who invade their territory. The cycle of life is played out every day in these trees, with one species devouring another, making a home, breeding, and raising young, and then in turn often succumbing to another species higher up the food chain.

Large birds—such as eagles, ospreys, red-tail hawks, herons, and egrets—often prefer the bare branches of snags to perch or roost on. Perhaps this is because the bare trees are the best place to keep an eye on potential invaders or just because it’s hard for large birds to maneuver wings through foliage.

All classes of terrestrial animals nest in the cavities of dead or dying trees. In some forests, 30 to 45 percent of the bird species are cavity nesters. Overall, 85 species of North American birds (35 in the Northeast) use tree cavities for cover and feeding. Many of these species will not nest elsewhere and so, without snags, would become extinct.

Why are tree cavities preferred as dwellings? They provide shelter from wind, rain, and snow, and the temperature inside cavities in large trees stays more constant than the surrounding air, giving added protection. For many species, that’s also where their food is, so it’s convenient for dining.

How Are Snags Created?

Tree cavities usually begin with an injured tree. A large limb may break off, or lighting or fire may damage the tree, leaving an open wound that allows the tree to be infected by heart-rot fungus. Water and feeding by invertebrates (including insects) further weaken the tree. Invertebrates find dead trees a bit easier to devour, and fungi and some plants find they can root better there. Birds, reptiles, amphibians, and mammals find the buffet these first feeders provide just as enticing.

Which Trees Make the Best Snags?

The tree you leave for a snag doesn’t have to be high-grade lumber, but a snag’s size, species, level of decay, and location affect its usefulness to wildlife. Deciduous trees, for example, are more likely to develop larger cavities than conifers. Large beech and sycamore trees often provide the best cavities. Other deciduous trees that tend to develop cavities include black gum, black locust, maples, and sassafras.

Optimum habitat for the widest number of species includes a variety of sizes and species of dead or dying trees, with at least one large snag per acre. The more snags that are available, the less competition each cavity nester will face.

Live trees can also be "girdled" to produce snags. The easiest way to girdle a tree is to use a chainsaw to make two cuts 2 inches deep all the way around the circumference of the tree trunk. A little chipping with an ax ensures that the cuts don’t close up.

Aren't Snags Dangerous?

Like the idea of providing habitat but are concerned about the safety of having dead trees around? What if they fall over? Won't they infect the surrounding trees with whatever caused the snags' deaths?

For safety, make sure snags are unlikely, considering prevailing winds, to fall on your house or utility lines. Cutting some of the top branches can make the tree less top heavy and less likely to fall over, but be sure to leave at least a couple of large, horizontal branches to accommodate larger perchers. Snags that are sheltered by other trees or a slope are more likely to last than those alone on hilltops.

If you’re going to take that extra step in fully embracing the idea of snags and actually plant them, Bruce suggests two things to prevent the part going into the ground from rotting (and the tree subsequently falling over): strip all the bark from the portion going into the ground and seal it with asphalt cement.

In some cases, removing a dead tree can be more dangerous than letting it stand, especially when it’s in a forest. If you have plenty of snags already, let them fall and create great habitat for other species on the ground.

Leaving—or creating—snags also does not threaten the health of a forest but is rather a part of the total ecosystem. To be safe and provide the best habitat, consult a wildlife-friendly forester who can give you good advice on which snags to retain and how to manage them.

Aren't Snags Ugly?

Maybe you find snags "messy," defying your sense of order and neatness. Remember that you're seeing them through human eyes, and what might look like an eyesore to you can look like a million-dollar condo to other species. Personally, I find many snags a graceful contrast in structure, color, and texture to the sea of green we Virginians enjoy so much of the year.

To get more information on snags, search on the following words on the Web: tree snag wildlife. Add Virginia if you want information specific to Virginia tree and wildlife species.