Montana Public Radio

Beneath The Snow, The Subnivean Zone Bustles

Jan 12, 2020

Many animals are able to survive the freezing cold temperatures of a Montana winter by making use of that place between the snow and the ground called the subnivean layer. This layer is created because snow is such a good insulator, holding in warm air heated by the earth, and keeping out cold air.

Intense morning sunlight warms my face as I stare up at a deep blue sky. It makes me think how out of place the freezing wind seems on a day like today. But around me, the trees are covered with new snow and the river rushes by, carrying with it chunks of ice. It's winter and the world looks cold.

My feet crunch on icy snow as I enter a wooded area near the river. Suddenly, the bright sun is gone, replaced instead by dappled light creeping in through openings between the trees. The sound of the river becomes muffled, and my ears focus on the quiet stillness.

I sit down on an old, decaying log and take in the calm and deserted feel of winter. Many animals have long since migrated to warmer climates, or are well into hibernation. All that's left, it seems, is an eerie silence.

On the ground next to the log, I notice a small mouse-sized hole in the snow, and soon I'm down on my hands and knees to get a closer look. Peering in, I can see just far enough to notice a patch of grass and a narrow tunnel leading away into darkness. I stand up to brush off my pants, and I realize that under the snow there exists an entirely different world from the one we know up here.

Many animals are able to survive the freezing cold temperatures of a Montana winter by making use of that place between the snow and the ground called the subnivean layer. This layer is created because snow is such a good insulator, holding in warm air heated by the earth, and keeping out cold air.

When the ground underneath this insulating layer of snow becomes warmer than the air above it, it causes the bottom layer of snow to melt, which creates a small space between the snow and the ground. The subnivean layer is covered by a ceiling formed when water vapor condenses and freezes on the undersurface of the snow. Temperatures here will remain fairly stable all winter long, ranging from about 20 to 30 degrees Fahrenheit, even when outside temperatures fall far below zero.

The warm layer that's formed under this literal blanket of snow is important winter habitat for many small animals, including mice, voles, shrews and some insects. In this space, animals will stay active throughout the winter, avoiding blizzards and creating runways that allow them to move freely and forage on seeds, grasses, new sprouts and tree bark.

In addition to warm shelter, the subnivean layer provides much-needed protection from predators. It is, however, not a total assurance of safety. These subnivean animals must be wary of weasels and martins, which skillfully hunt beneath the snow. They are also vulnerable to larger predators such as coyotes and foxes and owls, who use their incredible hearing to track down even the slightest movement under the snow. Once located, the predator will quickly dig through the surface and capture its prey.

Sometimes predators are able to access the subnivean layer by finding ventilation holes. Voles and other animals construct these holes, which are easily seen on the surface, so that excess carbon dioxide can escape. Perhaps the hole here by my log is one of these ventilation holes, without which the subnivean dwellers would die.

How easy it is for us to forget about this amazingly active world just under our feet, a world often overlooked by those of us whose gaze is usually focused upward on birds and trees and sky. Sitting on this log, I begin to notice more ventilation holes scattered around the snow. And soon, the cold, silent winter doesn't seem so lonely after all.
 

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Field Notes,"  12/29/15, 1/14/20 and 1/17/20. Listen weekly on the radio Tuesdays and Fridays at 4:54 p.m., or via podcast.)