Why dead trees may be full of life

Many species of mammals, birds, insects use dead trees for shelter, food

When visitors ask me if I live here at Stillman Nature Center, my normal response is, “Yes, in the third hollow tree from the left.”

Humor aside, what’s really living in dead trees? At least 43 species of birds and 26 species of mammals use dead trees for shelter or food.

Add to this species of snakes, amphibians, pill bugs, snails and slugs that call dead wood home. Speaking of which, here’s a spelling bee word for you: thigmotactic. This term is used to describe creatures that are attracted to places in which they are touched on all sides (e.g. silverfish). In addition, there are hundreds of fungi, moss and lichens that add color and texture to a decomposing log.

We’ve hit a snag

Tree holes come in all sizes. Small ones can be made by insects such as wood borers or small woodpeckers such as a sapsucker. This article will focus on larger holes, or cavities. These cavities might start with the damage done to a tree trunk after a branch breaks off. Or, a nest hole used by a woodpecker could later be enlarged by a squirrel.

Please keep in mind that trees with cavities, often called snags, could well be alive.

A snag is a dead or partially dead standing tree. Just because parts of a tree are dead does not mean it will be entirely moribund in a matter of months. The rate at which a tree dies varies with the species and what is triggering its decline.

For 40 years, I have been aware of two sizable silver maples growing at either end of the building that houses Stillman’s office and equipment. Both trees had large branches that eventually fell off. In addition, the main trunks were somewhat hollow as well. Over the decades, in one tree or the other, I have seen bees, red-bellied woodpeckers, squirrels, raccoons, and woodducks nest in these large veteran maples. And yes, both trees are still putting out green leaves.

Some folks would call these maples “hard” snags since they are only partially dead and have some limbs with fairly sound wood. A “soft” snag rarely has branches and is in a more advanced stage of decomposition. Admittedly, these modifiers are somewhat subjective, so we’ll just stick with snags.

Each of these maple snags have had a large dead branch fall down. In one case, the branch fell into our pond. It soon became a popular perch for herons, ducks, and turtles. Interestingly, a stub was left on the trunk where the branch had broken. It was roughly 20 feet off the ground.

Guess what nested on this arboreal shelf? A Canada goose. I kid you not. However, the eggs did not hatch. Most likely, a raccoon enjoyed dining on these enticingly elevated eggs.

Some exceptions

All right, I realize that if a dead tree is leaning over your porch or garage, you need to take it out. Also, if it is a dead elm that still has the bark attached or a red oak that died of oak wilt, you should have it removed.

This will help reduce the odds of spreading oak wilt and Dutch elm diseases. With the latter, bark beetles that lay eggs underneath elm bark are one of the means that the disease is spread from dying to living elms. If the bark is off, these thigmotactic beetles will move on.

I should note that while removing a dead elm (near a parking spot) with loose bark, one of our volunteers made a fascinating discovery. It was a silver-haired bat. This solitary migratory species, like our other local bat species, are great insect eaters. They are quite fond of roosting under loose bark during the day. Dead trees and loose bark both have pros and cons.

Back to the ‘holetels’

Now, if an animal can’t migrate like many bats and birds, or hibernate like a woodchuck, where do you go to escape frigid temperatures? That’s right, book a room in the nearest holetel.

The first animal that jumps to mind is this country’s only marsupial, the opossum. Think of this critter with its bare pink ears and long tail, not much insulation there.

As you might have guessed, an opossum will seek shelter in a snag, hollow log, or brush pile.

I say “an opossum” since they live a solitary lifestyle in winter.

Other winter snag users are the more sociable tree squirrels such as the gray, fox, or flying squirrel species. At just over two ounces, flying squirrels can avail themselves of pretty small cavities.

At the other end of the size scale would be a raccoon. Raccoons will use snags during bitter cold spells, but since they can weigh up to 40 pounds, they need a much large suite at the snag “holetel.”

Ideally, a snag should be at least six inches in diameter and 15 feet tall. Of course, even a smaller dead tree could provide food for birds such as nuthatches and woodpeckers.

In urban areas, snags are often quickly removed due to the damage that might be caused by fallen branches. Folks living in these neighborhoods will sometimes have some of these mammals, mentioned earlier, in their attics and garages. Perhaps there might be a correlation between attic animals and available snags.

If you have or are considering putting up a nest box for any bird from a wren to a woodduck, you are simply providing an artificial cavity. But what uses an artificial cavity may not be the species it was designed for. Here at Stillman, for example, we had southern flying squirrels move into a woodduck box. The flying squirrels were delightful tenants to greet in our rectangular cavity, which was, not surprisingly, mounted on a tree.

Long term planting

This brings me back to the two large silver maples at either end of this building. Many decades ago, long before air conditioning was widely available, these maples were probably planted to provide cooling shade. I know they did just that.

Since then, generations of humans have come and gone. During that same time, one can only imagine the countless generations of birds, mammals, insects, spiders, reptiles, and amphibians that have passed through those maple branches and inhabited the aging trees’ developing cavities.

When either of these maple trees finally collapse, it will not take the decomposers long to finish with the rotten trunk. A rich and fertile compost will return to the soil and, with any luck, a seedling will sprout not far from where the maple snag once stood. Tree to tree, soil to soil, ashes to ashes, dust to dust.

Which reminds me, you know what Mozart is doing in his grave right now? Decomposing.

Mark Spreyer is the executive director of the Stillman Nature Center in Barrington. Email him at stillmangho@gmail.com.