University of Vermont
Department of Plant and Soil Science

Anytime News Article
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SNAGS: DEAD TREES SUPPORT LIFE

Dr. Leonard Perry, Horticulture Professor
University of Vermont

When you hear the word “snag”, you may think of a run in a stocking or pulled thread in a fabric.  In the natural and landscape contexts a snag (sometimes called “wildlife tree”) is a dead or dying tree that, hard to believe, can support more life than a living tree.  Think of them as wildlife condos.  How is this possible? The Washington Department of Fish and Wildlife provides some answers and many details (wdfw.wa.gov/living/snags/).
   
Birds and small mammals use snags for nests, food storage, foraging for insects (a staple of the diet for many birds), roosting, and merely perching.  Hollow trunks, cavities and dead branches, even on living trees, provide similar “ecosystem resources”.  Often wonder where birds go during storms and on cold winter nights?  Many retreat to snags.  If such trees are near waterways, when they drop limbs or fall into the water, these enhance aquatic habitats.  Experts recommend at least three dead trees per acre to support wildlife.  Nationwide, snags support over 1000 species of wildlife (www.nwf.org).
   
A tree may already have large cavities from rots or other damage, or woodpeckers searching for insects to eat may enlarge the size of cavities.  The large pileated woodpecker is one of these “excavators” that, when at work, send the bark chips flying.  It is searching for carpenter ants, termites, and other insects in the heartwood of a tree.
   
Closer to the surface, the inner bark is where other woodpecker species, flickers, and sapsuckers find larval and pupal stages of insects.  This is important, especially in winter.   These birds use the cavities, also, for nesting and winter protection.  Raccoons and black bears may peel the inner bark layer, also looking for insects.  The outer bark at the surface is where nuthatches and woodpeckers look for bark beetles, spiders, and ants.  If the bark is partially detached, this space provides shelter for bats and some butterflies. Small cavities are used by house wrens, bluebirds, and black-capped chickadees for nesting.
  
Snags are used for more than food and cover.  If you hear a loud tapping in such dead trees, this resonating sound is from woodpeckers announcing their presence during mating. Large snags—those 12-inches wide or more and over 15 feet tall—make desirable perches for hunting bird species such as hawks, owls, and even eagles.  Many birds, such as swallows and mourning doves, use their branches for perches.  Tanagers and flycatchers use them for song perches.  Mice, squirrels, woodpeckers, and blue jays may use them to store food gathered elsewhere.
   
When a tree is dying, or recently dead, it forms a “hard snag”.  This is one that has the inner bark and heartwood still firm, so is most attractive to the woodpeckers and other “cavity excavators” both for food and nesting.  Eventually this inner wood rots from the action of fungi, branches fall off, and tops fall off making the tree shorter.  The dead tree is now a “soft snag”, providing a different selection of insects for other birds and wildlife. Even when the soft snag falls to the ground, this provides habitat and a food source for small mammals.  

The best stage for those nesting in cavities is in between—the outer sapwood is still firm, providing protection, while the inner heartwood has softened, providing easy excavation for a nesting cavity.  Some of the strong excavators, like the pileated woodpecker or flicker, use living trees if the inner heartwood has begun to decay.  Their large cavities in such trees are a good indication of this condition. 
   
While most tree species can be used by birds as snags, softwood species such as firs may be better for food while hardwoods such as maples may be better for nesting.  Large conifers such as firs, cedar, larch, pine, and spruce tend to rot more slowly than large deciduous (those that lose their leaves in winter) trees such as oaks, maples, and cottonwoods.  Dead trees as small as six feet tall, with trunks only 4-inches across such as birches, are used by black-capped chickadees and other small birds.

If you have room in your landscape, plan for at least one snag, even if a small one.  Keep an old or damaged tree, as well as tall shrubs near it to provide wildlife habitat.  If in a developed neighborhood or setting, snags are best located away from high traffic or high visibility areas (or passersby may wonder why you’re leaving dead trees).  A potential snag is a tree that has splits in the trunk, dead main limbs, fungi on the outer bark, or evidence already of woodpecker activity.  Snags can be relocated from elsewhere but, since these are quite heavy and so usually require a professional and crane, they’re best left or used on the ground as a log.

Living trees that are good candidates to make into a snag are those located away from high traffic and visibility, as with dead trees.  “Hazard trees”, or those that pose safety concerns from weak forked trunks or disease, make good snags.  If you have a tree providing shade where you want sun, consider making it into a snag.  Other snag candidates are trees that need thinning out of a clump, or those with invasive roots threatening paved surfaces or underground water and sewer systems.

To make a living tree into a snag, the easiest way is to “girdle” the trunk.  This is similar to what some gardeners often do unintentionally with weed trimmers, cutting off the flow of water and nutrients between tops and roots.  Do this as high as possible, removing a four-inch wide strip of inner and outer bark.  You can do this to just a branch if you want to have it die and be used as a perch.  Eventually the branch will fall off, which may leave a cavity for birds.

If you have a snag, you can make roosting slits for bats and some songbirds.  Make a slit at least eight inches deep, an inch or more wide, and angled upward.  The higher slits are (15 feet or so for bats is good), they more they’ll be used.  Make these on the south side of a snag if for bats, which like the warmth. 

If you have a snag, or even living tree you’re turning into one, you can create cavities for desirable birds.  To begin the cavity and decay process, drill a one-inch hole (use one of those drill bits you find in hardware stores for holes in doors for locksets) slightly downward into the heartwood of the tree.  Do this where water might collect, such as below where a branch joins the trunk.  Also you can remove a branch that is four inches or wider across, leaving a bit of jagged stub (unlike if you were pruning the tree properly for good health) to rot. 

If you cut, or have, a large cavity in a trunk that you want to prevent aggressive, non-native birds such as European starlings and house sparrows from taking over, make it smaller.  Use a piece of wood, leather, or metal to cover the cavity, with a hole one and one-eights (1-1/8) inches wide. 

So if you have a dead or dying tree, consider if it can be left to support wildlife.  Dying or dead trees often can remain intact for many years.  If dying branches or trunks pose a safety concern if they fall, or the tree is in a prominent location and really an eyesore visually, then a tree may need taken down by a certified arborist.  If you’re unsure about the safety of a tree, consult with an arborist or a town tree warden.  Perhaps the tree is still sound, but merely weak branches need removing.  When you take a tree or branches down, leave them nearby on the ground to rot and provide habitat, if possible.  Then replace the lost nesting cavities with nesting boxes, appropriate for the species in your landscape.

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