Subnivean Chronicles

Adventures in the woods lately have revealed fewer species than earlier in the winter. Have you noticed a change? Tracking stories these days are mostly written by little feet, and many of the stories lay hidden below the surface. Now that we actually have a thickness of snow on the ground, a new world develops beneath our feet. This ephemeral habitat is called the Subnivean layer. Since I first learned about this hidden zone as a bright-eyed college freshman, it has taken on a mythical quality in my imagination. Subnivean sounds like it could be a region of Narnia ruled by the White Witch, or perhaps an outpost neighboring Rivendell in Middle-earth.

The Subnivean layer, like so much of life on Earth, owes its existence to the unique chemistry of water. When frozen, water becomes light an airy, a wonderful insulator. Just as down feathers in your jacket trap a layer of air next to your body, retaining the heat you radiate, a six-inch layer of snow traps air that retains heat from the Earth. We don’t have hot-rock geothermal here, no geysers or hot springs. Our ground warmth, used in many local geothermal heating and cooling systems, comes not from radioactive decay in the Earth’s core, but simply from sunlight absorbed into the upper layers of soil and stone.

Because of this insulation and radiating heat, a thin zone opens up under the snow, right at the surface of the ground. As water freezes, it releases a tiny bit of heat, and as it melts, it absorbs a tiny bit of heat. In this way, the temperature in the Subnivean zone is regulated at a pretty stable 32 degrees Fahrenheit. Compared to -20 with a 15mph wind at the surface, that feels pretty balmy!

Many tracks are now being pressed invisibly into the leaf litter that carpets the Subnivean layer. Shrews, voles and mice, small mammals depend on this habitat for food, warmth, and protection from predators. Their presence is betrayed where the tiny jumping tracks of a deer mouse connect small trees with fallen logs. The tunnel of a shrew will suddenly exit the snow at the edge of a ski trail or where snowshoes have packed down a trough. The impossibly small holes can be not much wider than ½ inch. Sometimes shrews and voles will tunnel through the surface of the snow, especially the fluffy stuff, leaving a winding trough that ends in another hole when they again descend into the fabled Subnivean layer.

Although soft and concealing snow may seem like a perfect security blanket, the fat and juicy prey are not safe from their wily and well-adapted predators. Owls can hear mice through the snowpack, triangulate the sound with their asymmetrical ears, and bust through the crust with fisted talons. Foxes and coyotes can also hear and pounce through the crust, thus securing a meal without even seeing it. Our three smallest weasel species – long-tailed, short-tailed and least, are long and skinny with short legs for a reason. They can snake their way through mouse and chipmunk tunnels to catch the critters right in their own dens. Then the weasel will feast and nap at the warm hearth of its meal before going out to eagerly search every tree, log, rock and stump for another tasty treat.

You may also have noticed the prolific squirrel tracks, connecting trees to small holes littered with leaf shards and pine cone scales. Red squirrels will tunnel to find food caches stored last fall, and often put a big hole in the ski track in the process.

Grouse also make use of the warm blanket that snow provides. When the snow is deep enough, they may “roost” by doing a swan dive, leaving no tracks that would lead a predator to their warm bed. In bad weather, a grouse may stay in its burrow for a few days. Back-country skiers and snowshoers tell stories of grouse flushing from these secret burrows just inches in front of their next step.

While many tracking stories are hidden these days, it’s fun to imagine the complex chronicles unfolding in the Subnivean world. What lies beneath the smooth white surface? More than we will ever know.

Subnivean Party

Kerflumpf. Mumpf. Fump. It’s difficult to describe exactly the sound that snowshoes make in 18 inches of fresh, fluffy snow. I can tell you that the soundtrack includes some huffing and puffing, and the swish, swish, swish of nylon sleeves as arms swing their assistance. At the start of our recent dive into the polar vortex, I traded skis for snowshoes and headed out into the untracked forest.

Only snow-plops blown out of treetops marred these smooth drifts of snow.
Photo by Emily Stone.

The woods felt peaceful and serene. Only snow-plops blown out of treetops marred the smooth drifts. Winter, I thought to myself, can feel pretty lonely sometimes. If only I was smaller…

Because, for the little critters at least, there is a party going on under there. Under where? In a curious space called the subnivean zone.

Do you remember the magic of hoar frost that I wrote about a few weeks ago? Water vapor in the air crystalizes directly into ice without going through the liquid phase. Under the snow, at ground level, the opposite happens. Latent heat from the earth—stored months ago during days of summer sunshine—gives snow crystals at ground level just enough energy to vaporize back into their gaseous form. Sublimation is the scientists’ term.

The vapor rises just a bit, filtering into the snow, where it changes the structures of crystals as it solidifies again. The result is a zone of airy pockets and loose, granular snow topped by an icy ceiling; and above that a thick, insulating blanket made—counterintuitively—of ice and air. Best of all, this magical space remains at or near freezing. That might not usually seem warm, but after our weeks of subzero temps, we can probably agree that 32 degrees Fahrenheit sounds fairly pleasant.

I’ve written and taught about this subnivean zone many times before. Usually my focus is on the small mammals—mice, voles, and shrews—who make their winter homes in tunnels sewn between soil and snow. And, of course, there are the charismatic carnivores who chase those warm nuggets of protein. Short-tailed weasels (who become ermine in their white winter fur) follow mice directly into their dens. Foxes and owls take another route—pinpointing supper with their ears and then plunging in from above.

But I mentioned a party going on under the snow, and these are only the most visible revelers. Jon Pauli, a winter ecologist from UW-Madison, explains that, “In this refuge, there's a complex ecosystem of interacting microbes, insects, plants and animals that we can't see but are active throughout the winter.” In fact, says one of his research papers, the subnivium is where the “majority of biodiversity in northern temperate areas spends the winter.”

For example, Pauli and his colleagues have been studying the impacts of warming winters on the energy budget of wood frogs. These cold-blooded amphibians take refuge in leaf litter and very carefully allow themselves to freeze solid while the subnivean zone forms around them. Once frozen, their metabolisms are shut down, which preserves their limited energy stores. The moderating effects of the subnivium buffer them from energetically costly freeze-thaw cycles, and also reduce the potential for lethally low temperatures.

Frozen wood frogs have a fair amount of company. One group of ants, in the genus Temnothorax, likes to set up house inside empty acorn shells. Like a colonial version of hermit crabs, they compete for a limited number of good homes. Loathe to abandon their nut hut for the winter, they simply snuggle up with about 100 workers and a few larvae and produce glycerol as an antifreeze to prevent cell damage. Unsurprisingly, colonies in acorns that are buried more deeply in the leaves and soil have higher winter survival rates. Also unsurprisingly, the little girl in me loves that nature has its own version of acorn fairies.

Temnothoraz ants are also known as “acorn ants” because their colonies live inside acorn shells. Photo by Matt Hamer, Wikimedia Commons.

A thick layer of leaf litter isn’t just important for ants. Autumn leaves provide insulation before snow accumulates, and then continue to augment our fluffy white blankets. This protects plants’ roots from freeze damage. Dead leaves also provide essential habitat for woolly bear caterpillars, mourning cloak butterflies, luna moth cocoons, the eggs of red-banded hairstreaks, bumblebee queens, spiders, snails, millipedes, mites, and more. Some of them are frozen, some of them are active. And they aren’t the smallest subnivean inhabitants.

Woolly bear caterpillars need thick leaf litter covered by insulating snow in order to survive the winter. Photo by Emily Stone.

The microbes that Pauli mentioned are important here, too. Fungi and bacteria take advantage of the microclimate of the subnivean zone to slowly break down dead stuff throughout the winter. They release their nutrients in spring—just in time to fuel the growing season. In contrast, invasive earthworms decimate the leaves too quickly. Both insulation and nutrients diminish.

As I was saying, the soundtrack above the snowpack is pretty quiet. That allows me to cock my ears like a fox and focus my listening toward a hidden world beneath the snow. What do I hear? A wild party in that magical place we call the subnivean zone.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books. Or order it from our friends at redberybooks.com to receive free shipping!

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. The Museum is closed, but our Mysteries of the Night exhibit is available online. Connect with us on Facebook, Instagram, YouTube, and cablemuseum.org to keep track of our latest adventures in learning.

Subnivean Experiments

One evening in the 1970s, little Gary Siegwarth learned a lesson. While outside his family’s home in Iowa there had been a big snowstorm, inside there was ice cream—ice cream that had to be shared among five kids. Wanting to make sure that he got his fair share, young Gary took the cardboard carton out of the freezer, then out of the house, and hid it deep within one of the drifts. When he came back a day later, the sweet treat was just a melty, unappetizing blob.

An artist's impression of the event. ;-)

“I thought it was the perfect hiding place from my siblings,” Gary told me, “Turns out the ice cream cache quickly metamorphosized into a subnivean shake…I was surprised. I just never really thought about it.” Gary had accidentally discovered the magical realm of the Subnivean Zone.

I’ve been wanting to explore that world a little more, too, but instead of burying precious ice cream, I buried thermometers. The night before our first big snowstorm, I wrapped a few indoor/outdoor temperature sensors in plastic wrap and placed them where I thought snowdrifts would soon form.

One sensor is under the boughs of a hemlock tree. Deer and snowshoe hares are known to seek the cover of evergreens in winter, because those dense needles prevent some warmth from escaping to the stars.

Another sensor is out in the open—just to the side of the planned sledding run—and I made sure to snuggle it in under dry autumn leaves and fern stems. The third sensor is leaning against the base of an ash tree. This one I expect to become more interesting in the spring, when tree wells expand in the strengthening sun.

The final sensor is dangling about four feet above the ground, in the branches of a dying spruce, in an attempt to shade it from the sun and get the most accurate air temperature possible.

Thermometers in place, I snuggled in, too.

By dawn, the sky had shaken down about five inches of dry, fluffy snow. By the end of the day, the total was seven. I watched the digital weather station eagerly, as temperatures from each of the thermometers shifted and changed. I knew that the data would get more interesting as the air temperature dropped.

Sure enough, the evening after the storm, when air temps hit zero in the dying spruce, every other thermometer was significantly warmer, and none more drastically than the one placed in the open, under leaves, with the thickest blanket of snow. Its temperature read 30 degrees. Even as the air temp sank down to 8 below, the buried thermometer only reached 27.

Yes, I keep my house cool. The thermostat is set at 64 and this weather station sits near a window, so it's often even cooler. I wear a cozy sweatshirt, sometimes down vest, curl up under blankets, and often overheat if I'm doing chores! Among my friends I do not have the coldest house. Not even close. I've always been a fan of saving energy and saving money.

According to the International Dairy Food Association, the ideal temperature for ice cream storage is zero degrees Fahrenheit and should not rise above 10. It’s no wonder that young Gary’s ice cream melted! He had placed his stolen treat in the same microclimate as my temperature sensor in the Subnivean Zone.

As winter’s first snowflakes drift through the dark, some land on top of dead plants, fallen leaves, twigs, and other detritus of the forest floor. In many places, snow never fully reaches the ground. In addition, the residual heat of summer still radiates from the earth. This warmth causes the bottommost snowflakes to sublimate—or transition directly to water vapor without becoming liquid first. The damp air then rises through the snowpack and refreezes into a crystal ceiling above spacious (for a mouse) rooms and runs.

As the first flakes fall, they often don’t reach the ground. In the air spaces below leaves, sticks, and other objects on the forest floor, the Subnivean Zone begins to form. Photo by Emily Stone.

In this space, with a blanket of snow to trap the earth’s warmth, and a solid break against the windchill, temperatures hover around freezing. Deeper snow provides even more insulation, and all manner of critters—from mice to martens, bacteria, fungi, spiders, hibernating insects, frozen wood frogs, and more—rely on the moderated microclimate.

To Gary’s credit, it didn’t take much thinking for him to figure out what had gone wrong. As a kid in farm country, he’d heard many times that a blanket of snow was better for the alfalfa and protected it from winter kill. He also had plenty of direct experience with the relative warmth of snow forts.

And still, it can take a while for information to sink in. Years later, in junior high, teenage Gary was playing football at a neighbor’s house. They grabbed a snack of ice cream sandwiches out of the freezer and Gary buried his in the snow for safekeeping. As before, the deliciousness melted and oozed away. That was the last time he experimented with ice cream in the Subnivean Zone.

While Gary never admitted guilt to his family in the case of the disappearing ice cream (he thinks he probably got in trouble even while taking the 5th), he still learned something important from these two accidental experiments. And he went on to become a scientist.

I’m not going to put any ice cream at risk this winter, but I am going to watch eagerly as my experiment in the snow plays out. How will deeper drifts and colder air temps impact the Subnivean Zone? I’ll be sure to let you know when I find out!


Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. The Museum is now open with our exciting Mysteries of the Night exhibit. Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.

Least Weasels in the Subnivean Zone

Sometimes, when temperatures struggle to rise above zero, it takes all of my willpower to get out for a simple walk. Even when cheeks sting and the ice in my eyelashes sews up the corners, I know that getting outside will do wonders for my mood. When possible, I do try to plan any outdoor excursions for “the heat of the day.”

So it was noon on one of those bitterly cold days by the time I wrapped up some cookie baking and clomped down my driveway. Coyote tracks quilted the snow between the tire tracks, but they were softened by time and flurries. Nothing new.

And then, there was something new! Several pairs of tiny tracks stitched a line across flat ground and up the bank before disappearing into a hole no bigger than one gloved finger would poke under a mess of dead branches. Each set of prints could have fit on a nickel. In a few places, parallel lines connected the tracks to each other as the critter’s jump didn’t quite rise fully above the snow. Any signs of tiny toes were obscured within the texture of snowflakes.

Tracks of a least weasel disappear into the snow—into the subnivean zone.
Photo by Emily Stone.

“Least weasel!” I exclaimed before taking a few quick photos. Then my phone died from the cold.

With a maximum size of 8 inches long—including tail—least weasels are the smallest weasels in Wisconsin. Female weasels are smaller than males, tending toward 6.5 inches long, and weighing only 37-40 grams. Small animals tend to lose heat quickly, because of their large surface-area-to-volume ratio, so it’s odd that the smallest of the dozen or so least weasel subspecies lives in northern Russia, and the largest lives in North Africa.

There is one major benefit to being small in winter, though: least weasels can fit easily into the narrow passages and tight spaces of the subnivean zone. Inside this magical space where ground meets snow, a crystalline blanket traps the earth’s warmth, and temperatures hover near freezing. The subnivium is so important to Mustela nivalis, that their scientific name means “weasel of the snow.”

Earlier that morning, when the air temp read -13, my thermometer buried under just 6 inches of fluff registered a balmy 28 degrees. Being small means that the least weasels almost never have to venture up into the frigid, windswept winter world.

In fact, their entire body is designed to exploit the underground. A narrow head, lithe body, short legs, and flexible spine allow weasels to maneuver easily in tight spaces. Sensitive whiskers and a great sense of smell guide weasels in the darkness. They don’t dig their own tunnels, but then, why would they? Mice, voles, and chipmunks can be found in the dens they’ve dug themselves—in either snow or soil.

With a bite to the neck, least weasels are even able to take on adult rabbits—prey that is at minimum 16x bigger than this feisty little predator. In fact, least weasels are the world’s smallest carnivore. When relaying this fact, I’m often asked, “What about shrews?” Shrews are insectivores, and members of the now-obsolete taxonomic order Insectivora instead of the weasel’s order of Carnivora.

Unlike wolves—another member of Carnivora—who often gorge on a kill and then go without eating for several days, weasels cache their prey and come back for smaller snacks 8-10 times each day. To feed their fiery metabolisms, weasels eat at least half of their own weight daily, and more in the winter.

Least weasels are prey as well as predators, and they attempt invisibility by transitioning from summer brown to winter white fur. Unlike their cousins, the short-tailed and long-tailed weasels, least weasels do not keep a black tip on their tail. In the larger weasels, that bit of misdirection tricks raptors into aiming for the wrong end. In least weasels, their head and their tail are just too close for comfort.

Least weasels change from brown fur in the summer, to all white in the winter.
Photo by Cecil Sanders, flickr.com.

Speaking of comfort, I found myself a bit jealous of the “weasel of the snow” today as I ventured into a sparkling, -21-degree morning. While my car and lungs protested equally at being asked to work in such conditions, my subnivean thermometer had risen to 31 degrees under the added insulation of a fresh foot of snow.

I probably won’t have another chance to see tracks or sign from my tiny neighbor this winter. At least I know that they are staying warm under the snow as I imagine their adventures in the subnivean zone.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.

Pretty Warm Small Mammals

Just before the recent heavy snows deepened our drifts, I led a long line of third graders from the Hayward Elementary School into the woods at the North End Trailhead near Cable, WI. A thin layer of fluff covered the hard-packed trail. It recorded the recent passing of a fox, as well as the traffic of squirrels. Off in the woods, I could see little chains of mouse tracks from where they hopped through snow—trail dragging—before disappearing down a hole.

“Subnivean Zone.” I made the students repeat, as I described to them the magical, Narnia-like world beneath the snow where little critters carry out unseen dramas. The Earth gives off warmth. The snow becomes a blanket. Between the two, a realm expands. Mice sniff for food. Weasels hunt for mice. Voles convert vegetables into meat, and owls transform voles into feathers. Shrews hurry scurry in a constant search for snacks. We look out on a featureless façade and assume the winter world is asleep.

With Ally and Sarah—two scientists who have been doing research on small mammals in the nearby national forest—staying at my house, I’m reminded every day that much still moves this time of year. Each evening I burst through the front door asking “What did you catch today?” Their eyes light up and an account of how many mice, voles, shrews, flying squirrels, and red squirrels they caught spills out.

Ally holds a cute deer mouse. Photo by Emily Stone.

Scientists though they are, Ally and Sarah always have at least one story ready to illustrate the cuteness of their catches. There was the mouse that ran up to Sarah’s shoulder before jumping to a tree and posing—Vogue style—on a low branch. Another mouse leapt from Ally’s glove and disappeared instantly into the fluff, leaving only a mouse-shaped hole in the surface of the snow. Chubby voles received cooing and grandma-like comments about their size. Flying squirrels dashed up the nearest tree, then soared gracefully into the woods. And the shrews began foraging for food immediately—by nosing into the top layer of fluff and leaving a wiggly trail behind them.

Many little critters, including mice, prefer to spend winter in the Subnivean Zone where heat from the Earth and a blanket of snow help regular temperatures. This mouse wasted no time in getting back under the snow when it was released from the clutches of science. Photo by Sarah Nagel.

Cute does not even begin to describe these citizens of the subnivean, though. Persistent, tenacious, calculating, skilled, and constantly on the verge of disaster might be more accurate.

A few weeks ago, Dr. Paula Anich came down to the Museum’s annual “Wild about Winter Ecology” weekend workshop to talk about the adaptations of small mammals. She recently became famous when her research group at Northland College in Ashland, WI, discovered that flying squirrels glow hot pink in UV light.

With a few calculations, she illustrated the problem of being small in the winter. First, it’s helpful to realize that there is an 80,000-fold difference in body size between a shrew and an elk. In her estimation, those are the smallest and the biggest mammals in Northern Wisconsin. Size matters, because it impacts the surface area to volume ratio, and we lose heat through our surfaces. As a result, little critters lose more heat, and have to generate more heat in order to maintain a healthy body temperature of about 98 degrees.

Shrews burn twelve times more energy per unit of body mass than an elk. They are constantly racing toward the edge of starvation. With that knowledge, it’s easy to understand why a shrew would start foraging for food immediately when Ally and Sarah release it from a trap. Sadly, not all shrews were able to survive an entire night with only the food inside the trap, and they presented the highest death toll of the research. Death is inevitable for these critters, though, and a high birth rate is part of their survival plan.

Short-tailed shrews were plentiful in Ally and Sarah’s small mammal traps. They are 80,000 fold smaller than elk, and so much burn a lot of calories to stay warm. Photo by Emily Stone.

While chubby-looking voles are slightly bigger than shrews, they also must eat to stay warm. Some of the herbivorous vole’s food gets stored as brown fat—a handy type of tissue that can generate heat without shivering.

Of course, eating isn’t the only thing standing between a mouse and its maker. The advantage of being small is that these creatures can take advantage of microhabitats and microclimates. The Subnivean Zone is replete with cozy, windless, nooks and crannies. The mouse that leapt from Ally’s hands dove straight through the wardrobe into a protected place where no elk could follow. The mouse that scurried up a tree may also have had a microclimate in mind—deer mice are excellent tree climbers and often snuggle up together in an old woodpecker hole. There, the heat one loses soaks right into its neighbor, and windchill becomes a non-issue.

Deer mice are excellent tree climbers and will often use cozy cavities to snuggle in with their friends and stay warm. Photo by Ally Moser Scott.

Flying squirrels use the huddle technique, too, which I have the third graders experiment with on-trail. Small groups smush together with a thermometer in their midst, and I put one minute on the clock to see how warm they can get.

From hearing Professor Anich summarize the body of research in her field, to giggling over field observations with the student scientists, and to sharing just the tip of what is known with the next generation, I’m fully immersed in the Subnivean Zone. Layers of learning pile up just as quickly as the snow.

Emily’s book, Natural Connections: Exploring Northwoods Nature through Science and Your Senses is here! Order your copy at http://cablemuseum.org/natural-connections-book/.  Listen to the podcast at www.cablemusum.org!

For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: "Bee Amazed" is now open!

The Subnivean Zone Returns

Lake Superior was astonishingly calm as we walked out to a rocky point in the last rays of the setting Sun. With no wind and temperatures well above normal for late November, my fiancé and I only needed light sweaters and jackets to stay warm. It was a truly lovely day to be outside.

Lake Superior on a calm November, day. Photo by Emily Stone.

Lake Superior on a calm November, day. Photo by Emily Stone.

“Grandfather Alden would have called this a ‘weather breeder,’” I told Kevin. He didn’t read The Boxcar Children books by Gertrude Chandler Warner, but they were a staple of my childhood. In this early reader series, four brave siblings solve quaint mysteries with the help of their wise grandpa. In Snowbound Mystery they spend a glorious, bluebird day hiking around a mountain cabin before a blizzard socks them in. Their Grandfather called the bluebird day a “weather breeder.”

On The Weather Channel’s website, I found this explanation to confirm Grandfather Alden’s usage: “According to a late 19th century definition, a weather breeder is a beautiful day of ‘unusual fineness’… However, such a day is usually followed by bad weather.” A 1996 article about weather adages in The New York Times explains that the only science behind that saying is the law of averages. Good weather doesn’t cause bad weather, but since the weather is always changing, your good weather will soon turn to bad. Checking my weather app, I was thrilled to see the amount of snow in the forecast going up yet again. This day was certainly the calm before the storm!

Cold rain splattered my windshield the next day as I headed back to the shores of a smaller lake in Northern Wisconsin. Before cozying up indoors, I wrapped my digital thermometer in plastic wrap and tucked it under the bright green frond of an evergreen wood fern in the yard. This has become an annual ritual. On the weather station screen indoors, I could see that the newly placed sensor matched the air temperature in the mid-30s.

Can you see the white corner of the temperature sensor hidden under the fern?
Photo by Emily Stone.

Overnight, the wind howled and rain turned to snow.

As winter’s first snowflakes drifted through the dark, some landed on top of dead plants, fallen leaves, twigs, and other detritus of the forest floor. In many places, snow never fully reached the ground. That was surely true for the protected hideaway of my thermometer. By dawn, it was buried under six inches and counting.

Despite falling temperatures, the relative warmth of the cold rain and the residual heat of summer were still radiating from the soil. At sunrise, when I checked the weather station, the air temp had dropped to 24 degrees Fahrenheit, but the sensor cozied up to the earth under a fresh blanket of snow read 33 degrees. After two winters of thin snow, the Subnivean Zone has returned!

All winter long I will monitor the air temperature and the temperature of a thermometer I recently buried in the Subnivean Zone under the snow. In it’s first day of existence this winter, the Subnivean Zone did not drop below 33 degrees! Photo by Emily Stone.

In this magical space, with a blanket of snow to trap the earth’s warmth and provide a solid break against the windchill, temperatures hover around freezing even as the world above drops below zero. Deeper snow provides even more insulation, and all manner of Beings—from mice to martens, bacteria, fungi, spiders, hibernating insects, frozen wood frogs, and more—rely on the moderated microclimate.

In 2022-23—the winter of record-breaking snow—deep drifts accumulated on still-thawed ground, and the temperature in my front yard’s subnivium didn’t drop below 32 degrees for the entire season. The last two winters haven’t been so lucky. With thin, icy snowpacks, plant roots and mosses felt the sting of dry, bitter cold, ruffed grouse couldn’t dive into a snow cave to spend the night, and small mammals had to face the choice of staying safe and warm in a burrow, or risking being eaten or dangerously chilled while they foraged for food. Wood frogs in the leaf litter suffered without snow to buffer them from energetically costly freeze-thaw cycles.

Happily, at least for this week, those Beings are safe from the challenges of a snowless winter. Those Beings include me. Cold weather without groomed ski trails makes me sad. This week, my social media feed is full of good news about trails opening.

A weather breeder might be a day of “unusual fineness”, but I personally wouldn’t call what came after it “bad weather.” For many Northwoods Beings (the ones who don’t have to drive on bad roads or clear downed trees) snow and The Subnivean Zone are truly something to be thankful for!

Fluffy snow is a wonderful insulator to help retain warmth from the earth. Fluffy snow is also great for skiing up a gravel road! Photo by Emily Stone.

Emily’s brand-new third book, Natural Connections3: A Web Endlessly Woven, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

https://www.cablemuseum.org/books/ 

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our Winter Calendar is open for registration! Visit our new exhibit, “Becoming the Northwoods: Akiing (A Special Place). Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.

Night Magic

Headlamp stretched over wool hat, I strapped on snowshoes (fresh from basement storage) and ventured into the snowy dark.

A few flakes glittered through the air, but mostly the snow clung to trees and heaped on the ground. My light reflected brightly off white in all directions as I walked along in its bubble. The only sounds came from my snowshoes--creaking, squeaking, and shuffling.

Trees leaned out over the driveway, reaching their ice-encased, snow-frosted twigs toward my path. The lower edges of the iced twigs were scalloped with frozen droplets. Falling temperatures had slowed the drips to stillness. Unable to resist, I licked some fluffy snow-frosting off a birch twig.

Amazed at the beauty caught in every movement of my headlamp, I swept the light around in a wider arc, taking in the intricate patterns of twigs, needles, and snow. A ways off in an open area, the light caught something brighter. Two green eyes shone back at me.

Eyeshine is caused by a layer of tissue called the tapetum lucidum (which means “bright tapestry” in Latin). This layer sits behind the retina, and increases the light available to the animal’s photoreceptors by reflecting visible light back through the retina. Deep sea creatures and nocturnal animals use the tapetum lucidum to increase their night vision.

I was hoping that the two green orbs belonged to the neighborhood bobcat, but as the eyes moved, I could just barely make out the profile of a deer against the snow. Even so, I couldn’t stop my brain from imagining a monster or goblin behind those glowing spheres.

Undaunted by my overactive imagination, I followed a wandering herd of snowed-in deer tracks out the driveway and onto the trail. A snow-laden balsam arched across the trail at waist height, its tip buried under the crust. I gently swung it forward like a gate, and entered a tunnel fit for dwarves

This section of trail sneaks through a thicket of balsam on an old road grade. It is always dark and narrow. Tonight, snowy balsam branches hung especially low and close as I bent down to shuffle through. The passageway heightened my sense of expectation and suspense, as if I really might pop out into the Narnian Empire at any time.

Instead, the trail entered a spacious hemlock grove. As the trees opened up, the dark closed in. Through the open understory, I caught the shining green eyes of four more deer. As they bounded away, a soft whisper of wind tinkled through the treetops. Snowflakes drifted down. The whisper crescendoed to a rush of air, and bigger clumps of snow fell, plopping all around me. As I put up my hood and leaned toward the trunk of a large hemlock, I imagined Ents in a snowball fight. When the dull thumps of falling snow had subsided, I continued on through the aftermath of drifting clouds of crystals.

A cute string of mouse tracks made me grateful for another sign of life. Most small mammals are hiding out under the thick, fresh snow, where a new world has just developed beneath our feet. This ephemeral habitat is called the subnivean layer.

The subnivean layer, like so much of life on Earth, owes its existence to the unique chemistry of water. When frozen, water becomes light and airy, a wonderful insulator. Just as down feathers in your jacket trap a layer of air next to your body, retaining the heat you radiate, a six-inch layer of snow traps air that retains heat from the Earth.

Because of this insulation and radiating heat, a thin zone opens up under the snow, right at the surface of the ground, which stays at a pretty stable 32 degrees Fahrenheit. This becomes even more important as the temperature plunges into the single digits, and then below zero. Without snow to insulate the ground, frost burrows more deeply.

Tree roots, invertebrates, and the myriad little critters in the upper reaches of the soil suffer in cold, dry winters. Snow provides not only provides warmth, it also facilitates easy access to food, and gives cover from predators. “To the mouse, snow means freedom from want and fear,” wrote Aldo Leopold in A Sand County Almanac.

Red squirrels also use the subnivean layer to escape want and fear. In the fall, they cache seeds near the ground, and then use their great sense of smell to find them again, even below four meters of snow. During cold spells, squirrels will dig themselves a little snow den and hang out near their pantry, safe from the searching eyes of predators. I stepped over a small hole excavated by a hungry squirrel, and peeked down into the dry, leafy carpet of the subnivean zone. His tracks still ran across the top of the snow, but with temperature forecasted to plunge, my guess is that he will take a dive, too.

I emerged from the woods onto the gravel road and turned off my light. The world went gray. A grove of balsams—their drooping branches and conical shape perfectly adapted to the heavy snow—stood like statues in the White Witch’s courtyard. A rosy-pink glow in the northern sky gave an otherworldly aura to the night.

Beams of warm yellow light beckoned me back inside, but I hesitated, reluctant to leave this magical world (my snowy backyard) behind.

“And if you have not been enchanted by this adventure-Your life-What would do for you?” –Mary Oliver

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.

Warm Winter Worries

What a weird winter! In a region where people love to talk about the weather, the chatter has been constant. Skiers and snowmobilers (and the business they support) are particularly grumpy, but many people are trying to make the best of pleasant temperatures, despite the lack of snow. Humans are lucky. We have temperature-controlled homes and clothing that can be changed at the drop of a hat.

But what about non-human beings? How are they faring as temperatures fluctuate from 50 degrees to the teens with barely a skim of snow on the ground? This week, I combed back through my many articles about animals in winter to try and understand what some of the impacts may be.

In a typical winter, when we have at least six inches of snow, The Subnivean Zone forms. Because of the insulating qualities of snow and leftover summer heat from the Earth, a thin zone opens up under the snow, right at the surface of the ground, which stays at a pretty stable 32 degrees Fahrenheit.

Ruffed grouse make use of this warm blanket. When the snow is deep enough, they “roost” by doing a swan dive, leaving no tracks that would lead a predator to their warm bed. Without snow, or with icy crust, grouse may struggle to find a warm roost.

Small mammals like mice, voles, and shrews lose more heat for their body size than big ones, and therefore must generate more heat to maintain a healthy body temperature of about 98 degrees. Dr. Paula Anich from Northland College has estimated that shrews burn twelve times more energy per unit of body mass than an elk. They are constantly racing toward the edge of starvation, and all of the little beings must eat pretty much constantly to survive. Without snow, they face the choice of staying safe and warm in a burrow, or risking being eaten or getting too cold while they forage for food. “To the mouse, snow means freedom from want and fear,” wrote Aldo Leopold in A Sand County Almanac.

Even with snow, mice still need to fear American martens. These small weasels hunt along the log-lined runways where red-backed voles, mice, shrews, and squirrels travel. The snow is an excellent blanket for this lean mammal, who stores little fat and burns lots of fuel to stay warm. Snow also provides cover from bigger predators. The diets of foxes, fishers, and bobcats overlap with martens’ diets, and those larger carnivores will kill martens to eliminate competition.

Martens’ smaller cousins, least weasels, short-tailed weasels, and long-tailed weasels, all hunt under the snow as well. The subnivium is so important to Mustela nivalis, the least weasel, that their scientific name means “weasel of the snow.” These three fierce hunters need snow for an additional reason: their fur turns white in the winter, and a shift in the timing of snow cover can leave them vulnerable to their own predators.

Short-tailed weasels need snowy winters to match the camouflage of their white fur, and to provide safe, warm hunting grounds. Warm winters like this one will make it harder for them to survive. Photo by Emily Stone.

Autumn leaves can provide insulation before snow accumulates, and then continue to augment our fluffy white blankets. This protects plants’ roots from freeze damage. Dead leaves in the subnivium also provide essential habitat for woolly bear caterpillars, mourning cloak butterflies, luna moth cocoons, the eggs of red-banded hairstreaks, bumblebee queens, spiders, snails, millipedes, mites, and more. Some of these beings are frozen, some of them are active.

Wood frogs take refuge in leaf litter and very carefully allow themselves to freeze solid while the subnivean zone forms around them. Once frozen, their metabolisms are shut down, which preserves their limited energy stores. The moderating effects of the subnivium buffer them from energetically costly freeze-thaw cycles, and also reduce the potential for lethally low temperatures. After warmer winters, female wood frogs lay fewer eggs.

Like wood frogs, goldenrod gall fly larvae can freeze solid. They spend the winter in little round homes built inside goldenrod stems. While they can endure multiple freeze-thaw cycles over the winter, warmer temperatures increase their metabolism, and reduce their body size. When they hatch in the spring, the resulting adults – who do not feed – will not be able to lay as many eggs.

Beyond getting a break from shoveling, it’s not easy to find a silver lining for this weird weather. Here’s one possibility, though: deer ticks might suffer if a hard freeze comes while they are exposed without snow. If mice populations drop as well, then maybe there will be a decline in the spread of Lyme disease next summer. One can hope.

Jon Pauli, a winter ecologist from UW-Madison, once explained to me that “there's a complex ecosystem of interacting microbes, insects, plants and animals that we can't see but are active throughout the winter.” In fact, says one of his research papers, the subnivium is where the “majority of biodiversity in northern temperate areas spends the winter.” That’s why it’s so devastating when The Subnivean Zone doesn’t exist.

If you’d like to learn more about The Subnivean Zone, just wait until May 1st. Winter will be over, but the Museum’s new exhibit, Anaamaagon: Under the Snow, will give you an inside look at this special place.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our exhibit: “The Northwoods ROCKS!” is open through March 9. Our Winter/Spring Calendar of Events is ready for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.

Martens and Wind

Fierce gusts of wind rocked the treetops above me, sending shivers down their trunks. Among the boles of swaying aspens, maples, and firs, I skied with one ear to the sky. Twigs clattered, snow plopped, and cold wood fibers moaned up and down the musical scale. One tree almost buzzed with a high-pitched popping, while another groaned from deep within. I listened carefully to determine the location of the loudest trees, and to gauge their ability to strike me if they came crashing to the ground.

Although the bluster threatened above me, it didn’t quite reach all the way down to the swooping hills and rippled snowpack. I felt protected in the understory of this forest. My eyes were scanning for animal tracks in the days-old snow, but I only saw signs of other skiers. Deep drifts and bitter temperatures have driven many creatures down into the next level of protection: the subnivean layer.

Fluffy snow captures summer warmth still radiating up from the ground. A thin zone opens up under the snow, right at the surface of the ground. Here, the temperature stays at a fairly stable 32 degrees Fahrenheit. Compared to a negative 45 degree wind chill at the surface, that feels pretty balmy!

American martens are one of the many creatures that exploit the subnivean microclimate. These small weasels tunnel through the snow to find food, stay warm, and escape predators. Phil Manlick, a Master’s student at UW Madison, has been studying the “Habitat-mediated foraging and predation risk in reintroduced American Martens.” Recently he gave a dinner lecture on the topic at the Rookery Pub and Fine Dining near Cable, WI. One of his hypotheses is that martens need lots of structural diversity – lots of fallen logs – in their habitat to help them gain access to the subnivean layer.

Once under the snow, martens can forage along the log-lined runways where red-backed voles, mice, shrews, and squirrels also travel. When satiated with a tasty meal, martens have been known to curl up in the den of their prey for a nice warm nap. The snow is an excellent blanket for this lean mammal, who stores little fat and burns lots of fuel to stay warm. Snow also provides cover from other predators. The diets of foxes, fishers, and bobcats overlap with martens’ diets, and those larger carnivores will kill martens to eliminate competition.

American martens were extirpated from Wisconsin in the 1920s, due to unregulated fur harvest and widespread habitat loss. In 1986, wildlife managers developed a marten recovery plan with the goal of reestablishing two self-sustaining populations of martens in the Chequamegon-Nicolet National Forest. But even with cooperation between the Wisconsin DNR, the US Forest Service, and the Great Lakes Indian Fish and Wildlife Commission (GLIFWC), and multiple reintroductions, marten populations in the Chequamegon National Forest are not rebounding as well as wildlife managers and researchers hoped. Phil is trying to figure out why.

Although he’s still mostly in the data collection phase, preliminary results indicate that one issue limiting the American martens’ recovery in the Chequamegon National Forest might be poor juvenile survival. Adults are doing fine, he says, but the young martens, kicked out of their parents’ home range after just a few months, may not be finding enough high quality food to make it to adulthood.

Phil’s research techniques involve gathering hair samples from martens as they explore PVC tubes baited with beaver meat and equipped with a brush. By comparing the ratios of stable isotopes of nitrogen and carbon in the martens’ bodies to the ratios of those isotopes in their various prey species, Phil can get an idea of what martens are eating. Other researchers also identify animal remains found in marten scat to identify specific prey.

The surprising results are that in northern Wisconsin, martens’ diets consist of 40% shrews and 30% deer. In contrast, healthier populations of martens out west and in Minnesota eat many more red-backed voles and squirrels. Why might eating shrews and deer be a problem? Shrews, like the martens themselves, don’t store fat for the winter. With an extraordinarily fast metabolism, shrews must feed voraciously night and day. This makes them the skim milk of the small mammals.

Deer might seem like an improbable food source for a two-pound marten, but evidently, martens are feeding opportunistically on winter-killed and wolf-killed carcasses. The trouble is that deer carcasses are a risky food source. Fishers, wolves, coyotes, and other scavengers are attracted to those same banquets, and make things dangerous for the martens. That competition with other species may be another important factor limiting the martens’ recovery.

Phil suspects that habitat quality has an impact on both food availability and competition. During his summer habitat surveys, Phil found that martens need “nasty forests that you don’t want to trudge through.” By which he means forests with lots of blow downs, snags, large trees (greater than 39 cm diameter), and coarse woody debris. If further data analysis supports that hunch, it might influence how foresters manage the landscape in the future.

Thinking back to those trees groaning and swaying in the tempest above me, it might not be a bad thing if a few of them fell down...after I’ve skied past, of course.

“But these are the woods you love, where the secret name of every death is life again…”  -- Mary Oliver, from Skunk Cabbage

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.