Earth Matters: The sub-nivean zone: Life under the snow

Last modified: Tuesday, January 05, 2016

As I write this at the beginning of December, we in the Pioneer Valley have not yet had a flake of snow; I can only hope that, by the time this is published, the situation will have changed.

I hope this not only for those who may be hoping for a white Christmas, and not only for what a lack of snow may imply about our changing climate, but also because snow itself actually makes for better winter living conditions for our small wildlife friends, including moles, mice, voles (and their predators), various insects and an assortment of bacteria and other microscopic life. This is because snowfall causes the creation of what is known as the sub-nivean zone (from the Latin “sub” meaning “under” and “nives” meaning “snow”), which occurs after about six inches of accumulation.

The sub-nivean zone takes shape in a protected area between the upper-level snow pack and the Earth’s surface, where the temperature hovers around 32 degrees Fahrenheit. Two processes initially form the sub-nivean zone. The first process occurs during early snowfalls, when the weight of the snow bends grasses, shrubs and other vegetation, creating protected caves and hollows where small animals can reside. The second process involves sublimation, in which the heat of the Earth transforms solid snow directly into gas. Sublimation creates ice crystals, which essentially form an ice “roof” topped by a layer of snow.

The upper layers of snow provide insulation against colder air temperatures, while the lowest level provides a moist environment and unfrozen food sources for the creatures that live there.

Indeed, a whole winter ecosystem exists in this realm, a fact that was not widely understood until relatively recently. Rodents living in the sub-nivean zone create tunnels, ventilation shafts, latrines and nesting areas while feeding on grasses, insect eggs and whatever else they can find during the winter months.

The upper layers of snow also hide rodents from the sight of predators, although research indicates that owls can hear the scamper of little rodent feet from at least 30 yards away, and foxes and coyotes can smell their quarries even if they can’t see them. These large predators will pounce or strike directly through the snow, attempting to catch their prey with claws or mouth. (An interesting side note: If a fox is facing north when hunting (at least in North America), he/she is 75 percent more likely to be successful in catching prey through the snow than if facing any other direction. The reason for this is not fully understood.)

Weasels — called ermines when they turn white in the winter — more sneakily slide down rodents’ entrance tunnels (often located next to a tree, rock, or shrub) and chase their victims through their own homes. Don’t feel too bad for the little critters, though; if it weren’t for predators like weasels, our world would shortly be overrun with mice, voles and other rodents, given how fast they tend to breed.

Next time you are out walking in deepish snow, keep an eye out for traces of predation, such as fox jumps or owl/hawk punches. For instance, look at the photo and note the marks left by the wing feathers and the hole punched through the snow by the bird’s feet.

Also look near the base of trees or rocks for air holes leading to the sub-nivean world. Please don’t disturb the holes, however; the creatures that made them have enough to contend with without our interference.

Snowpack has a strong impact at the microscopic level as well. A deep snowpack causes a larger population of microbes to flourish, according to research done by Paul Brooks of the University of Colorado. Microbes release nitrogen into the soil, which fertilizes plant life in the spring. In years with lower amounts of snowpack, plants receive less nitrogen and consequently are less productive during the growing season.

While many of us humans find snow to be beautiful to look at and entertaining for exercise and play, for many forms of life in northern climates, a healthy winter snowpack is essential for survival.

The next time you are shoveling off your car and running late for work, just remember all the animals, plants and microbes whose wellbeing depends on that snowpack, and take a moment to ponder the amazing cycle of life.

Reeve Gutsell is a volunteer at the Hitchcock Center for the Environment. She has an MS in Resource Management and Conservation from Antioch University, and an MFA in Creative Writing from Naropa University. She enjoys spending as much time as possible exploring the Pioneer Valley.

Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week. For more information go to, call 256-6006 or write to


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