Picture yourself immersed in a forest for a few seconds. Towering, powerful trees surround you. Glints of sunlight might be squeaking through the dense, green canopy. The birds are chirping and singing, flying past you. Something that might be missing from your mental image, but is a significant piece of a forest ecosystem, is a snag.

What Is A Snag?

The large dead tree in the fence row at the farm, down the road where all the vultures roost? Snag. The dead tree near your house that doesn’t have bark anymore and you constantly see woodpeckers pounding away on? Snag. The dead tree that appeared in the forest this year that you saw covered in spongy moth caterpillars last year? Snag. When you’re standing along the bank of a stream casting your line hoping for a fish, but it goes into the branches instead? Snag. Oh, that’s that wrong kind of snag (sorry anglers). The kind of snag I’m talking about is generally found in the forest and is defined as a dead or dying tree that is still standing.

Three tree snags in an open field with a forest in the background.

Three snags in a pasture at a southeastern Pennsylvania farm (Photo credit: Tim DeWalt).

During the summer months, snags are quite noticeable because of their gray and brown coloration against the backdrop of the deep green sea that is the rest of the forest. If you have ever driven along Route 22 northwest of Harrisburg, PA you may (or may not if you keep your eyes on the road) have noticed a multitude of snags on the ridges. My family drives along Route 22 several times a year during the different seasons while traveling to our cabin. One year during the mid-2010’s, the ridges were a blanket of lush green leaves while we drove up. The next summer the same stretch that used to be fully green was peppered with the grays and browns of snags, signaling that insects made their way through the area a couple of years prior. Each year since, I’ve noticed different patches of gray and brown where fresh snags have appeared from the previous year’s insect damage. This tree mortality may look unsightly, but dead and dying trees create natural habitat and food sources for many of our native wildlife species.

What Causes Snags

There are many reasons that a tree may die; insects, diseases, human activities, weather events, and old age are the most common reasons. Currently, insects have been a primary culprit for killing large numbers of trees throughout the watershed. Within the last two decades, the emerald ash borer has swept through the Chesapeake watershed and the eastern United States and has killed most of the ash trees in its path. The spongy moth (formerly called the gypsy moth) is an insect that defoliates trees by eating the leaves, rendering the tree unable to photosynthesize and susceptible to other harmful insects or diseases. The spongy moth can be found on a number of species, but can be regularly found on oaks and in the past few years it has done a great deal of damage. Both of these insects are invasive to the region and unfortunately have killed millions of trees, but the silver lining is that many of those dead trees are left standing as snags providing a valuable resource to the ecosystem.

Ecological Value

Snags hold a valuable place in the ecosystem. Many wildlife species rely on snags for food or homes, including a countless number of insects, fungi, and lichens. Woodpeckers, like the pileated woodpecker, use the dead trees as food sources and eat the insects that are decomposing the tree. Along with being a food source for woodpeckers, the soft wood of the dead and decomposing tree allows the woodpecker to excavate cavities for nesting purposes. The cavities in snags provide nesting opportunities to a wide variety of species from bees to bears. Owls, squirrels, raccoons, martens, and swifts are a handful of the many other species that nest in cavities provided by the snag. Birds of prey, like owls and hawks, use snags to perch on while they are hunting. The exposed branches allow the birds to have a better vantage point of potential prey. Bats and other small animals will roost under the loose bark of snags.

Snags in Residential Areas

In a forest setting, we know that snags are important for many wildlife species, but in some situations they can become hazardous. If a snag is located near a house, building, utilities or other residential areas there is a possibility of it falling and causing significant damage. In these cases, consider consulting with a professional to have it removed. The image below shows an example of a snag that could be removed. If it falls, It has the potential to cause damage to the structure or the propane tank. If you remove a snag and don’t have any uses for the wood, consider creating a brush pile with part or all of the felled snag so that it can continue providing for wildlife.

An oak tree snag near a structure and utilities.

A recent oak snag near a structure and utilities (Photo: Tim DeWalt).

Snags in Forest Management

Across the forestry field, it is common for snags to be retained as a component of a forest management plan. A standard recommendation is 3-4 per acre in order to help meet wildlife and biodiversity objectives. Like with much of forestry, killing some trees is often done in order to accomplish gains for other tree or wildlife species. Creating snags by girdling trees is often done in order to give “crop trees” (valuable timber trees that can become even more profitable if they grow bigger) more space to grow. Canopy or sub-canopy trees are also often killed to reduce the amount of shade that reaches the forest floor, allowing the next generation of tree seedlings to become established. As always, be sure to consult with a forester before undertaking forest management actions.

What happens if a snag falls onto the forest floor? It becomes considered coarse woody debris (dead and fallen trees and large branches on the forest floor). In addition to most of the same benefits that snags provide, coarse woody debris provides nutrients for the soil and plants through the decomposition process. Trees fall where they want to and this includes streams and rivers; the trees and logs you see in their waters are called large woody debris. The logs and trees in streams may be in your prized swimming hole, make the stream seem “messy”, or make for a frustrating day of fishing; nonetheless, wood in streams is essential! What benefit does large woody debris provide? You guessed it; habitat and food. Large woody debris is the snag and coarse woody debris of the water!

A multistem oak snag in a central Pennsylvania forest

A multistem oak snag in the central Pennsylvania forest (Photo credit: Tim DeWalt).

Final Thoughts

I hope you view the dead trees you encounter in the forest a little bit differently now and see the value snags add in the forest ecosystem. If they’re in a forest, don’t cut them down if you don’t need to!

If you live in a more residential area or where not many trees are present or you can’t safely leave standing snags, consider adding a few nesting boxes to your property. It will provide potential nesting sites for the animals which would otherwise use snags in a more wild context. It feels good to share space with other animals, and easy wildlife viewing is an added bonus!