Habitat Feature: Snags

Photo © Bureau of Land Management, Oregon

Old Vanilla–that was the name given to the huge Western Yellow Pine Tree outside my childhood window that smelled of vanilla cookies on dry summer days–died when I was eight years old. I loved that tree. To my surprise and delight, instead of cutting it down, my dad manicured it into a standing dead tree, which I’d later learn were called snags . For the next fifteen years I watched the tree slowly decompose. It sprouted shelf mushrooms, attracted insect-eating Pileated Woodpeckers, housed Mountain Chickadee families, sheltered dreys of squirrels, and provided perches for innumerable flying creatures. This tree was as alive as a snag as it was when it was living.

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Photo © Kelle Kelle Kelle

Dead trees nurture new life in ecologically important ways. Without snags some 85 species of North American birds, numerous small mammals, insects, fungi, and lichens would be without valuable habitat. Snags are nature’s apartment complexes and cafeterias. To have thriving, healthy habitat, in many places, means snags are a part of the equation.

mushroom snag

Photo © Scott Robinson

Decomposition of a tree starts quickly when disease, damage, fire, etc., affect the outer cambium of the bark. Fungi move in and begin the critical work of breaking the tree down into vital nutrients to replenish soil. The rate of decomposition is influenced by moisture, sun/shade exposure, forest age, species composition (hardwood vs. softwood), and animal activities.open_in_new The rotting process creates ideal soft outer layers in the hard trunk for primary cavity nesters to excavate.

Pilliated

Photo © Flipkeat

Woodpeckers and sapsuckers are species of birds known as primary cavity nesters. They are the ones that actually create the holes, or cavities, in dead wood in which to build nests and rear their young. These birds are also experts at hammering holes into the bark of trees to gain access to bark-dwelling insects and sap and even drum on trees to defend nesting territories.

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Photo © Justin Kern

Woodpeckers in different regions of the United States will create cavities in soft or hardwoods, depending on the species of trees present and the birds’ habitat preferences. Changes in forest make-up due to disease, human intervention, and climate change can have a dramatic impact on the birds that live there. For instance, we know Ponderosa Pine forests in the Pacific Northwest saw a decline in primary cavity nesting birds when snags were actively removed from forests.open_in_new Privately held forests tended to have even fewer-than-average snags leading to a hypothesis that private management practices have huge potential to impact populations, even when it seems that what you choose to do on your small property can’t possibly scale to levels that matter, evidence suggests otherwise.

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Photo © Billy Lindblom

As the field of forestry emerged, the desire to prevent fires and raise trees for harvest actively lead to the practice of removing snags. Snags were viewed as ecological menaces. Dead trees were and are intentionally removed from forests to minimize the risk of fire and reduce the population of bark beetles infecting healthy trees. This destabilized interdependencies between trees, insects, birds, and other wildlife that rely on snag habitat.open_in_new

bark bettles

Photo © Cynthia Cheney

By consuming bark beetles, and other tree-eating insects, cavity nesting birds help create a balance in insect populations. When snags are eliminated the potential for insect invasions increases as the cavity nesters lose habitatopen_in_new and can’t reproduce at high enough rates to keep up with insect populations. Sometimes this isn’t a major ecological issue because bark beetles only attack weakened or dying trees, leaving healthy trees alone;open_in_new but, this is not true for all nonnative beetles, nor is it true for native species, like the mountain pine beetle. These species, when given the opportunity will advance on healthy trees when their populations rise in the absence of sufficient predation and extended foraging seasons induced by climate change.open_in_new

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Photo © vischerferry

We may think of an owl snug in their hole in a tree, but they don’t make those holes. Those are thanks to the woodpeckers.

Secondary cavity nesters, such as certain species of chickadees, owls, nuthatches, creepers, ducks, bluebirds, flycatchers, swallows, titmice, wrens, and warblers, all benefit from the hole-creating activities of the primary cavity nesters. Abandoned woodpecker nests and drilling sites will quickly be adopted by secondary nesters. Without the efforts of the woodpeckers and sapsuckers, many of these secondary nesters would lack the required habitat to raise their young. This is why primary cavity nesters are considered keystone speciesopen_in_new as they provide essential nesting cavities for other species and their absence from an ecosystem has devastating effects. An example of this is in eastern pine forests where there is a positive correlation between areas with snags and all nesting birds.open_in_new Similar findings also occur in western Ponderosa Pine Forests.open_in_new

Features of Quality Snags


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Photo © Roy Saplin

  • Large diameter, tall trees
  • Existing woodpecker holes or cavities
  • Fungal Conks (mushrooms) present
  • Wounds or scars from fire or lightning present
  • Dead areas on living trees
  • Both sound and decayed wood
  • For larger land area management, maintain snags in areas of both low and high tree density and across a range of topography (ridges, slopes, and valleys)
  • Snags arranged solitary or in small clumps of up to ten
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    Photo © Bobbi

    Snags support more than just bird diversity. In fact, in Estonian peatland forests 25% of lichens live only on snags.open_in_new Similarly, a majority of flying squirrels studied in Ontario, Canada used declining trees for nesting, while another quarter of the the population relied exclusively snags. open_in_new

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    Photo © USFWS Midwest

    Tree-roosting bats use snags as well. Bats largely prefer large snags with pieces of exfoliating bark.open_in_new They will crawl behind pieces of bark to roost and raise young. Bat houses , like nest boxes, are an option to supplement when snags aren’t present, but, preserving snags in forest ecosystems is vital for protecting wildlife diversity at a larger scale.

    snags

    Photo © Laura Molnar

    How many snags are needed to support wildlife? The answer to this question depends on the ecology of a specific region. Researchers, when pressed, will say one to three snags per acre are sufficient; but, optimum snag density probably varies by region. Knowing the local flora and fauna and the interactions between trees, fire, tree-eating insects, primary nesters, sapwood decay, and other requirements of local wildlife are important when making recommendations about snag numbers and density.open_in_new If you own several acres of forest, this is the perfect question to work with a forester on when creating a wildlife management plan for your property.

    snag in yard

    Photo © Tim Davies Landscaping

    When we envision snags in the landscape we usually think of larger forested areas. Can snags work in an urban setting to support birds as well? Evidence suggests that cavity nesting birds do well in any setting as long as snags are present.open_in_new This same research found that the bigger the snag, the more it was successfully used to nest. Tall dead trees can pose a problem for urban areas where infrastructure and people can be threatened by a falling tree. In instances where a falling tree directly threatens a building, or cars, we recommend manicuring a dead or dying tree into an artificial snag approximately six feet in height. Or, if it doesn’t put a building at risk, you can leave it as is, or select a height appropriate to the location of the tree.

    Skeptical a snag can look good?


    View this slideshow of homes incorporating snags into their landscape
    Chainsaw

    Photo © Jacob Avanzato

    Techniques for making a snag from a dead or dying tree in an urban area include girdling and tree topping. Any work involving tree pruning or modification should be done by an expert, especially in an urban area where mistakes could result in property damage.

    Making an Artificial Snag from a Diseased or Dying Tree


    bird house snag

    Photo © B.C. ministry of Transportation and Infrastructure

  • Wait until nesting season is over before creating a snag, September-December is a safe time in most of N. America to avoid disrupting nesting birds and wildlife.
  • A snag in urban areas should be approximately 6 feet tall and not endanger infrastructure or people if it were to fall.
  • A snag in a forest or more rural area can be left taller, as long as it is not a hazard.
  • Have a professional girdle or top the tree.
  • Prune the tree in a way that mimics how it would look in the wild.
  • Keep some living branches to slow the decay.
  • Avoid creating snags near structures or roads.
  • Manage and prune off new growth around the tree base.
  • Create cavities for birds by drilling various size holes.
  • Cut long slits in the top of the tree and peel back areas of the bark to create roosting areas for bats.
  • artificial snag

    Photo © John Weiss

    At the Presido National Park artists and ecologists collaborated on a project to create a fully functional snag replacement. It included nesting cavities for the Pygmy Nuthatch and Bumblebee, a cantilevered perch for the Black Phoebe, louvered crevices for the Yuma Myotis bat, cover logs for the California Slender Salamander, and a hibernaculum or winter residence for the Coast Garter Snake.

    snag

    Photo © Ed Suominen

    Snags are vital features for wildlife in urban, suburban, and rural areas. Leaving them standing to naturally decay in the landscape requires us to rethink what a beautiful, well-tended yard or home entails. This practice of leaving dead wood benefits your native wildlife and may just attract some new visitors to your yard.

    Have a Snag?


    Add it to your Map
    After completing your site outline and overlaying habitats, add a snag as an object by zooming into the location on the map you’d like to add it.

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    Photo ©

    In the tool shed, choose “Third” and scroll through the object options until you come to snag.

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    Photo ©

    Add the object to your map and tell us more about it by clicking the green Info button. Upload a picture and tell us what lives in your snag.

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    Photo ©

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