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The Afterlife of Logs
Snags - The Wildlife Tree
The Importance of Snags in Your Neighborhood
Dead Wood Brings New Life
Hard to believe, but trees can actually provide more habitats for wildlife dead than when they are alive. Standing dead and dying trees, called “snags” or “wildlife trees,” are important for wildlife in both natural and landscaped settings, occurring as a result of disease, lightning, fire, animal damage, too much shade, drought, root competition, as well as old age.
Birds, small mammals, and other wildlife use snags for nests, nurseries, storage areas, foraging, roosting, and perching. Live trees with snag-like features, such as hollow trunks, excavated cavities, and dead branches can provide similar wildlife value. Snags occurring along streams and shorelines eventually may fall into the water, adding important woody debris to aquatic habitat. Dead branches are often used as perches; snags that lack limbs are often more decayed and, may have more and larger cavities for shelter and nesting. Snags enhance local natural areas by attracting wildlife species that may not otherwise be found there.
All trees of all sizes are potential snags. Unfortunately, many wildlife trees are cut down without much thought to their wildlife value or of the potential management options that can safely prolong the existence of the tree. Wildlife trees offer a one-stop, natural habitat feature. In short, snags “live on” as excellent wildlife trees for all to enjoy!
 Wood duck ducklings plunging from their nest cavity in a tree. This is normal behavior for wood ducks when leaving the nest cavity which can be anywhere between 6 to 15 feet above ground and almost always above water into which they fall. Photo Credit: Mike Lentz Images | 
Pileated woodpeckers foraging on an old dead snag pulling off the bark to get to the insects underneath. Note the thick bare branch at the top of the tree perfect for bald eagles or other large birds.
Photo Credit: Patricia Thompson | |
 Red squirrel poking out of a den tree savoring a nut dropped by a passerby. Central Park, New York City. Photo Credit: Bruce Yolton |
Wildlife That Use Snags
West of the Cascade Mts 39 species of birds and 14 species of mammals depend on tree cavities for their survival. East of the Cascades 39 bird species and 23 mammal species depend on these snags (Pederson, USDA Forest Service). In total, more than 100 species of birds, mammals, reptiles, and amphibians need snags for nesting, roosting, shelter, denning, and feeding (Bottorff, WSU, Snohomish Co. Ext); nearly 45 species alone forage for food in them. Hollow snags and large knot-holes are used by many species of mammals such as squirrels, marten, porcupine, and raccoons.
In winter when snow covers the ground, northern flickers and other common backyard wildlife depend heavily on insects and other foods found in snags. Brown creepers, bats, and other small animals will roost behind loose bark and bark slits for winter warmth and shelter. Hollow snags are very valuable in winter as they are used by many species such as squirrels, raccoons, owls, and bear for denning and roosting.
Large snags more than 12 inches in diameter and 15 feet tall offer ideal hunting perches for hawks, eagles, and owls. They function as resting perches for swallows, band-tailed pigeons, mourning doves and other birds; food storage areas for mice, squirrels, woodpeckers, and jays; and song perches for tanagers and flycatchers. Woodpeckers use large dead tree trunks as a way to announce their presence during courtship, hammering their bills against the tree’s resonating surface. Small snags may be used as song posts by bluebirds, hummingbirds, and other songbirds to attract mates and proclaim nesting territories. This high use of snags by a myriad of species underscores the importance of preserving snags and including them in your landscape.
Dead Tree/Wildlife Condo
You can see where wildlife finds food and shelter if you look carefully at a snag:
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The Afterlife of Logs
My three children have participated in a Four Winds Nature Institute program that recruits adult family members to lead grade-school nature learning. I have worked with several moms and dads over the years to pull together materials for hands-on lessons about communities, habitats, and the natural world. The activities usually ended with crowd-pleasing puppet shows.
During my first year in the program, in a rare moment of advance planning, I read the entire year’s program, and was glad I did: “Snags and Rotting Logs” was scheduled for November, when I anticipated most logs would be frozen or buried in snow. Regardless of frost or snow, I expected that some interesting invertebrates would have tunneled deep into the soil to wait out Vermont’s winter, leaving little more than wood for the students to dissect.
So I pushed a wheelbarrow into Winooski’s Gilbrook Nature Area in September to load up with logs for November. I poked a screwdriver through the bark of each log to confirm that wood was sufficiently rotted to host a diverse community before bagging them for cold storage. I had stacked the odds in favor of success by selecting only the ‘best’ logs: my poking and probing quickly focused my attention on birch.
The point of the Four Winds lesson is to show how snags (standing dead trees) and downed logs, while no longer growing wood, are very much alive with other organisms. Fungi that weakened trees and hastened their return to earth continue releasing enzymes and are joined by soil fungi, further breaking down cell walls. Insects and other invertebrates, incapable of digesting wood without the help of fungi and other microorganisms, draw sustenance from the decomposing debris. Carpenter ants dine elsewhere, on sweet or high-protein foods, but they nest in decaying wood.
This early invasion of dead wood sets the table for larger creatures. Shrews, moles, and insectivorous birds chow down on the abundant six-legged protein-packed morsels. Woodpecker activity opens up the logs to the elements, accelerating the breakdown and release of nutrients from the wood to forest soil. Pileated woodpeckers are a good indicator of mature forest conditions including snags for nest sites, and logs in various states of decomposition.
For woodpeckers’ purposes and mine, not all logs are created equal. Some, such as black locusts planted in years past to grow fence posts, take a long time to decompose and are therefore poor hosts for invertebrates. Researcher Grégoire Freschet and colleagues in Holland modeled log decomposition and showed that alder, willow, and poplar lose most of their density to decomposition in eight years; logs from pine roots were far tougher lasting four times as long. However, all of these trees share one characteristic that made them less interesting for my purposes: the bark tends to rot first.
For woodpeckers’ purposes and mine, not all logs are created equal. Some, such as black locusts planted in years past to grow fence posts, take a long time to decompose and are therefore poor hosts for invertebrates. Researcher Grégoire Freschet and colleagues in Holland modeled log decomposition and showed that alder, willow, and poplar lose most of their density to decomposition in eight years; logs from pine roots were far tougher lasting four times as long. However, all of these trees share one characteristic that made them less interesting for my purposes: the bark tends to rot first.
Birch flips the decomposition process on its head. The wood rapidly rots to crumbly pulp while the bark, protected by antifungal compounds, persists as an intact cylinder. These “birch pipes” can serve as important habitats for invertebrates. With a long bladed shovel, it’s easy to lift an intact yet very rotten birch log into a bag along with all of its inhabitants.
Birch pipes have been a great way for children in our Four Winds program to make hands-on investigations, discovering how rotting logs are essential habitats for centipedes, millipedes, sow bugs, beetle larvae, and ants as well as the birds that depend on them for food. I must come clean at this point and admit that the first time I provided logs for the classroom, I sweetened the pot by adding night crawlers. The children found the night crawlers along with other earthworms and more than a dozen invertebrate species that had not required my real-estate services.
The thrill of ‘the hunt’ through decomposing logs is a good example of how kids’ science instruction doesn’t always require fancy equipment; often all that is needed is outdoor materials and time to explore. Through fun activities, children and adults learn together to appreciate that nature is everywhere, and that our fellow travelers are fascinating.
This year, with the delay in cold weather, it may still be warm enough to slide a shovel under a log for study. If you do, I recommend a well-rotted birch log.
Declan McCabe teaches biology at Saint Michael’s College. His work with student researchers on insect communities in the Champlain Basin is funded by Vermont EPSCoR’s Grant NSF EPS Award #1556770 from the National Science Foundation.
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