Double-crested Cormorant on a snag; photo Jakob Mueller
Standing dead trees are often considered dangerous eyesores in need of prompt removal. Actually, they play such an important role in wildlife ecology that it is often said they “give life to the forest.”
Ecology of snags
Usually called snags, den trees or cavity trees, biologists are increasingly calling standing dead or dying trees “wildlife trees” in recognition of their enormous value to birds and other creatures. In Ontario alone, at least 50 species of birds and mammals rely on snags. Biologists know that, in the wild, 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.
Interestingly, not all snags are equally attractive to all species of wildlife. Some birds require a minimum diameter before they will use a snag. Perhaps not surprisingly, snags with the largest diameter support the greatest number of species.
What happens when a tree dies?
Lightning, wind storms, fire, and fungal infections can all kill trees, but injuries, which open the way to attacks by insects, are the commonest cause of tree mortality in our urban backyards.
The primary “colonizers” of snags are insects and fungi. If you usually think of these species 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. But don’t worry that these creatures will invade your house. They have no desire to be inside what, for them, is an alien and hostile environment.
All of these species help carry on the long process of decomposition. By softening the wood, they make it easier for birds and mammals to gain access. Insects also attract woodpeckers and other forest-dwelling animals who, in their search for food, excavate holes or cavities, which become nesting sites for other birds and small mammals.
Ash tree stump with shelf fungus; photo D. Gordon E. Robertson
Fresh Pileated Woodpecker hole and fresh bark scaling by Hairy Woodpeckers; photo Christine Hanrahan
Woodpeckers are an example of what biologists call “keystone species” because they greatly influence other species. The holes they create as they search for food provide homes for countless other creatures. You may already be familiar with the large rectangular holes of the Pileated Woodpecker.
How long does a dead tree stay standing?
A standing dead tree can remain in place for many years. Some of the giants of the Pacific northwest rainforests have been snags for well over 150 years by the time they fall. Smaller trees come down sooner, but even they can last for several decades. This should be borne in mind by anyone considering the “safety” aspects of snags in public places and in your own backyard.
Downed logs
When a snag eventually tumbles, it continues to contribute to the overall health of the forest. Biologists are now calling logs the “hot-spots” of the forest ecosystem, integral to biological diversity.
Like snags, downed logs provide shelter and denning sites for mammals, birds, amphibians and reptiles. Small animals such as squirrels and chipmunks use logs as easy routes through the forest.
Logs also act as “nurseries” for plants, giving them a nutrient-rich base in which to take root. It is a fascinating exercise to count the number of plant species growing on a “nurse log.”
Even a single downed log in your garden can return nutrients to the soil and, in the process, provide a temporary home for chipmunks and other small critters.
Nurse log with Painted Turtle and various flora; photo D. Gordon E. Robertson
Dead tree supporting several types of vines; photo D. Gordon E. Robertson
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 5 or 6 feet high. 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.
Red-breasted Nuthatch excavating nest in a dead standing tree; photo Christine Hanrahan
Making your wildlife tree part of the plan
If you’ve left the snag at 15 feet 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 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.
In British Columbia, snags are considered so important that a snag-tagging program is underway. Bright yellow signs identify wildlife trees and inform the public about their value. Information about this program is available at the FWG Interpretation Centre.
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
- Cavity trees are refuges for wildlife. Ontario Ministry of Natural Resources, Extension Notes, 1999.
- 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.
- MacDonald, Christy. 1992. Ontario’s cavity-nesting birds. Ontario Birds 10(3): 93-100.
- Norse, Elliott A. 1990. Ancient Forests of the Pacific Northwest. Island Press.
This page was revised 1 July 2012
Text: Christine Hanrahan
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