This fox has tracked down his next meal by listening for sounds in the subnivium. Image Credit: Yellowstone National Park, Public Domain Mark 1.0, Image Cropped.

Sometimes the hardest places to access are the most interesting places to study. Take the subnivium, a temporary ecosystem that forms each winter in the small space between the snowpack and the ground.

The subnivium isn’t far from familiar places—it can be as close as a few centimeters beneath your skis. But in other ways it’s mysterious and easy to overlook. Though we can’t see it from the snow’s surface, the subnivium is an active space: plants and animals live, eat, and move around under the snow all winter long.

The subnivium starts forming as soon as snow starts to build up on the ground. Branches, grasses and other vegetation can hold up the snow in uneven patterns as it settles, creating pockets where space forms beneath the accumulating snow. As the snow gets to be around fifteen centimeters deep, warmer air coming off the ground heats the bottom layer of snow into water vapor, which slowly infiltrates up through the snow and freezes the bottom of the snowpack into an icy “roof” several centimeters off the ground.

At Home Under the Snow

This is great news for many small animals. Snow is an excellent insulator, so the subnivium remains relatively comfortable at close to 0°C no matter how cold and windy it is on the snow’s surface. Mice, voles, and other animals—sometimes even chipmunks and squirrels!—remain active under the snow all winter, developing a network of tunnels and living spaces where they forage for food. Tunnels for occasional access to the surface, as well as air pockets that form where tall vegetation pokes up out of the snow, ensure that there is enough oxygen under the snow.

Many insects and other invertebrates remain active under the snow too, and even some plants continue to grow through the winter by relying on faint sunlight that filters through the snowpack. These insects and plants provide a food source for the larger inhabitants of the subnivium, along with bark, leaf litter, and other vegetation on the ground. Predators have adapted to the patterns of life in the subnivium as well: foxes, owls, and other predators will listen for the sound of movement under the snow and pounce into the snowpack to capture their prey, and weasels and other long-bodied animals hunt by squeezing through small tunnels into the subnivium.

The glacier lily (Erythronium grandiflorum), native to western North America, is well known for growing and even flowering below the snowpack. Image Credit: US Forest Service Northern Region, CC BY 2.0.

Studying the Subnivian

Not surprisingly, the subnivium hasn’t historically been a major focus of ecological research. Challenges abound: the subnivium only exists seasonally, and it is small enough to make direct observation tricky. In many parts of the world, the subnivium lies beneath well over a meter of snow, so it’s hard to say from the surface exactly where a subnivium tunnel network lies underfoot and where it doesn’t—and even if you can detect it, the price of access is a good workout with a snow shovel.

Still, ecologists have recently become more interested in studying the world under the snow, and they’ve come up with a variety of creative approaches. In 2015, a team of researchers in northern Norway set up enclosed tunnels equipped with camera traps along known subnivium animal routes to keep track of animals’ daily patterns of activity. Other researchers have measured the importance of subnivean microorganisms, which release nutrients throughout the winter that are essential for spring vegetation growth. And scientists at the University of Wisconsin are currently using specialized greenhouses to simulate the effect of increased winter temperatures on the subnivium.

Of course it should go without saying that many people have been well aware of the subnivium long before modern science took an interest in it. For instance, the Saami term for the space is skoavdi, and the Inuit term pukak refers to the type of snow close to the ground where the subnivium forms. I encourage anyone interested in this blog post to also read more about the depth of traditional ecological knowledge relating to northern ecosystems (e.g., here, here, here).

Small mammals photographed with remote cameras while using subnivium tunnels. Image Credit: Soininen et al. 2015, CC BY-NC 4.0.

What Does the Future Hold?

Many scientists focusing on the subnivium cite an urgency to understand this important habitat in order to better predict how it will be altered by climate change. The number of days the subnivium exists each year is expected to decline with climate change, as is the quality of habitat under the subnivium, and researchers are eager to find ways to manage this risk before it is too late. A 2013 study by a team of prominent subnivium researchers puts it this way: “We believe that ecologists and managers are overlooking this widespread, crucial, and vulnerable seasonal refugium, which is rapidly deteriorating due to global climate change.” Recently proposed solutions include modeling and identifying areas of particularly important subnivean habitat and prioritizing conservation action in the most important threatened areas.

Want to see this secret ecosystem for yourself? The best time to see the subnivium may actually be in the spring, when the snowpack has nearly melted. When just a few centimeters of snow remain on the ground, visit a field or forest and look for the remains of tunnels, which will appear as raised patterns in the snow that wind across the ground. In the meantime, as we head into winter, give a little thought to the secret world hidden under the snow next time you’re out in a winter landscape.

Caitlin Mandeville is a PhD student at the Norwegian University of Science and Technology studying conservation applications of citizen science species occurrence data. You can read about her research here and see more of her writing for Ecology for the Masses here.