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.