The importance of snags and downed logs to wildlife

by Christine Hanrahan

Walking through our local forests and along trails at the city’s edge, your eye may be caught by the stark form of a standing dead tree or by a fallen log stretched across the forest floor. Perhaps you have seen a woodpecker fly from a hole in the tree’s trunk, or noticed a squirrel running along the log, using it as a sort of elevated highway through the forest, and recognized the value of this dead wood to birds and other forest creatures. To many people, however, standing dead trees represent a threat to their safety, or an eyesore to be felled. Yet these standing dead trees and downed logs are an important feature of forest ecology.

A forest is a living entity, constantly changing and evolving. Old trees die, new ones sprout up and, over many years, the very composition of a forest changes as climax species eventually come to dominate the early and middle succession periods of the forest community. An important component of all forests are dead and dying trees, whether standing as snags or lying on the forest floor as downed logs. So vital is their role in the forest ecosystem that it is not an exaggeration to say that dead trees give life to the forest. Norse (1990), writing of a Pacific Northwest rainforest, states:

“Rotting snags and logs provide the tunnels, dens, and nesting cavities needed by animals from black bears and spotted owls to land snails and springtails. They are the birthplaces for western hemlocks, Sitka spruce, and smaller plants…. They are sites of biological nitrogen fixation, adding to the nutrient wealth of the forest.”

Although writing of the Pacific Northwest, his words ring true for our forests as well, albeit with some species difference.

Snags or wildlife trees

Snags are standing dead trees. They are also known as den or cavity trees and, increasingly, as wildlife trees. The latter term is especially appropriate for their value to wildlife is immeasurable, as they provide food, safe nesting sites in the form of cavities and platforms, roosting and denning sites, hunting perches, display stations, and foraging sites for a wide variety of species (Guy 1994).

From the time a standing tree dies until it falls to the forest floor, its many stages of decomposition attract different birds, mammals and invertebrates. Charles Elton (in Kennedy, 1991) observes that “dying and dead wood provides one of the two or three greatest resources for animal species in a natural forest… if fallen timber and slightly decayed trees are removed the whole system is greatly impoverished of perhaps more than a fifth of its fauna.”

Not all snags occur within a forest. Sometimes isolated trees, left standing by design or chance, hold a lonely vigil over fields or cottage lots, or some other cleared area. These, too, represent an important wildlife resource, offering nesting platforms for Ospreys (if near water), or hunting perches for flycatchers, Eastern Bluebirds, swallows and other birds, as well as food in the form of invertebrates inhabiting the tree.

Death of a tree

The primary “colonizers” of snags are insects and fungi, which soften the wood allowing it to be easily shredded by birds and mammals. If you usually think of insects as pests, you might be surprised to find out that they’re essential to all the other wildlife species that depend on or make use of cavities. The variety of invertebrates inhabiting dead and dying trees is staggering: millipedes, mites, earwigs, beetles, spiders, ants, and even earthworms These insects then attract woodpeckers and other forest-dwelling animals who in the course of excavating for food, create holes or cavities that become, in turn, nesting sites for birds and small mammals. Biologists call those species that greatly influence other species, “keystone species.” Woodpeckers are one such example, for the holes they create as they search for food provide homes for countless other creatures. Fungi also provide food for other creatures, as well as being used by many insects.

A standing dead tree can remain in place for many years. Smaller trees come down sooner, but even they can last for many years, and this should be remembered when considering the “safety” aspects of snags in public places.

Downed logs

An old-growth forest is full of fallen trees, or downed logs, whereas in second-growth eastern hardwood or pine forests, logs are much less in evidence. Yet even here they form an important part of the overall ecosystem just as they do in any forest or wooded area. In fact, biologists are now calling logs the “hot-spots” of the forest ecosystem.

When a tree falls to the ground, it is quickly taken over by insects, especially beetles. Earlier, woodpeckers were referred to as a keystone species; beetles serve that same function in downed logs (Norse 1990). As they bore into the log they open up the way for fungi, which in turn help to decompose the inner bark. As the beetles tunnel further into the log they provide access for spiders, ants, millipedes, and salamanders and the process of decomposition initiated by the beetles continues.

Like snags, downed logs provide shelter and denning sites for mammals, birds, and for amphibians and reptiles such as salamanders and snakes. Small animals such as squirrels use logs as easy routes through the forest.

Logs also act as “nurseries” for plants, allowing them a nutrient-rich base in which to take root. Many plants take root on downed logs and it is a fascinating exercise to count the number of plant species growing on a single “nurse log.”

Ecologists have classified five stages of decay in a downed log, from the first stage when a log is intact and not yet decayed, to the fifth, where the log has crumbled into a mass of organic material. Because logs are more moisture-retentive than snags they decay more slowly since oxygen is excluded from wet wood (Norse 1990). Large old-growth logs can take 200 or more years to decompose completely. Smaller logs, such as those found in this region, will decay much faster.

The term “coarse woody debris” or CWD, refers to all the woody debris on the forest floor, not just logs, but stumps and branches as well, rotting or otherwise. As Fred Schueler points out, our eastern forests are more full of CWD in recent decades thanks to the influx of invasive species such as the Emerald Ash Borer. He says:

“Now, largely due to Dutch Elm Disease, and successional squeezing out of Aspens, there seems to be a plethora of CWD, and when the Emerald Ash Borer is done with us we’re going to have huge quantities of both standing and fallen wood which will presumably make the woods much more old-growthy than their age would indicate.”

According to Owen Clarkin, it has also been pointed out that various woody plants require a good layer of coarse woody debris in order to regenerate.

Thus, not only standing and fallen dead trees, but stumps and other woody debris contribute to the overall ecosystem of the forest and the wildlife therein.

Wildlife using snags and downed logs

Birds
Many people perhaps do not realize that when they put up nest boxes each year they are offering homes to cavity-nesting birds whose natural nest sites are holes (or cavities) in snags; hence, the use of the terms den or cavity trees. Some of our most familiar birds are cavity-nesters along with numerous other species, perhaps less familiar. However, nest boxes can never be a complete substitute for natural cavities, for while certain species readily adapt to man-made nest boxes, many others will not, or cannot adapt.

Not all birds make use of the cavities in snags for nest sites. For some birds, such as the tiny Brown Creeper, it is the loose bark on dead trees that gives shelter for nests, while for others such as Ospreys, standing dead trees near water provide platforms on which to build their large, bulky nests.

Not all uses of wildlife trees are for nesting purposes. Ruffed Grouse use downed logs for “drumming” in their spring courtship ritual. And as noted earlier, many birds use snags as hunting perches or display stations.

Mammals
Mammals also make use of snags for both shelter and for rearing young. Martens, weasels, squirrels, other small rodents, bats, even bobcats will den up in cavities. Black bears may sometimes find winter refuge at the base of large snags, as well as in hollowed out downed logs. Squirrels and chipmunks and other small rodents use logs as forest highways.

Invertebrates
As noted earlier, a multitude of insect species thrive on dead and dying trees whether standing or down on the forest floor. In turn, these insects provide much needed food for a variety of wildlife. When these trees are removed from the forest ecosystem, the insects associated with them are also removed, and in turn, the wildlife that feed upon the insects.

Standing dead trees in your garden

Unless you live on wooded rural property, your backyard is certainly not part of a forest ecosystem. But if you are a gardener wanting to create a healthy, viable wildlife habitat in your own backyard, you will by now recognize the role that dead trees play in attracting birds and other species. As well as harbouring food for insectivores in the slowly rotting wood, snags also offer safe nesting cavities. In the winter these cavities are often used as roost sites, providing the necessary insulation that nest boxes cannot. (However, nest boxes are a suitable supplement to natural cavities in your garden.)

If your standing dead tree is quite large, you may be worried about heavy falling branches. Cut away some or all of them and leave the trunk. If you still think the snag is too tall and overpowering, topping the trunk to a reasonable height might be a solution. A “reasonable height” depends on what you feel comfortable with and what is in the immediate vicinity of the snag (i.e., your house, neighbouring houses).

But if you cut the snag back too much, you might as well fell it completely and leave it as a log; it will have little value as a nest site if it is only a couple of metres tall. Naturally the best thing to do is nothing, leaving the tree to take its own course, but in a small suburban lot, safety concerns must be evaluated.

If you’ve left the snag at 4.5 metres or better, but want to disguise it somewhat, plant lightweight climbers such as wild cucumber vine (Echinocystis lobata) or native clematis (Clematis virginiana) to twine up the trunk. You’ll need to provide some support for these vines to get started. Virginia creeper (Parthenocissus quinquefolia) and wild grape (Vitis riparia) grow fast and can quickly cover a snag with a dense green cover, but these vines are very heavy and can hasten its collapse.

If you want to “dress up” your snag, you can hang seed feeders from its branches or from simple hanging brackets. Suet feeders can be affixed right to the trunk. If you really want to turn your snag into a work of art, hang flower baskets as you would the feeders. Plant them with nectar-rich flowers for bees, butterflies and hummingbirds (see the FWG information sheet on butterfly gardening).

When the snag eventually collapses you can either leave it where it falls, or move it to a more remote part of your garden where it will continue its work of feeding insects, birds, and your soil.

If your neighbours complain about your snag, tell them what you are doing and why; you might change their way of looking at standing dead trees. It is only by changing how we view the land around us that we can begin to help restore and nourish both it and its wildlife.

References

• Guy, Stewart. 1994. More than dead wood. Protecting the wildlife tree resource in British Columbia. BC Naturalist 32(1): 4-6.
• Kennedy, Des. 1991. Death of a giant. Nature Canada 20(2): 18-26.
• Norse, Elliott A. 1990. Ancient forests of the Pacific Northwest. Island Press.

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This is the third in a series of suggestions from the OFNC’s Conservation Committee for things you can do around your home in aid of wildlife and conservation. They are all based on personal experience – ours and colleagues’. We would love to hear your thoughts about these practices and your experience with them – good or bad. And your suggestions for further good practices are very welcome.

Other “Conservation how to” articles