A secret world is revealed as the snowpack melts in spring. Walk across a snow-covered field as the weather warms and you may encounter what look like, at first glance, cracks in the crust or (as a friend insisted) snake tracks. In fact, they are networks of tiny highways built in and under the snowpack by industrious mice and voles.
This mysterious region under the snow is the subnivean layer, from the Latin sub (under) and nives (snow). It is created in two ways. Frozen vegetation and leaf debris physically support the snow, leaving an open space above the ground. And, snow closest to the ground warms and becomes water vapor which rises and refreezes, creating an insulating roof above the open space. The end result is a humid winter habitat with relatively stable temperatures around 32 degrees F.
This habitat is a boon to mice and voles. The subnivean layer provides protection from harsh winter weather and functions as an ‘invisibility cloak’ for small mammals that are easy targets in the white winter landscape.
Mice and voles build extensive transportation networks in the subnivean layer. These networks have entrance holes – often located at a tree trunk, large rock, or dense shrub – which double as the HVAC system. Entrances lead to sleeping areas, ‘bathrooms,’ and sources of food. Seeds, grasses, and the bark of woody plants and roots are plentiful in the subnivean layer. (Who amongst us has not been dismayed to discover a favorite young tree or shrub girdled when the snow melts in spring. Voles and mice are the culprits!)
Dismayed landscapers may be pleased to know that mice and voles are not entirely safe from predation in their winter tunnels. Ermine, the small weasels that turn white in winter, burrow along these tunnels until they find lunch. Fox, coyote, and owls have less success than the ermine, but hunger is a powerful motivator. Fox and coyotes dig down to the subnivean layer, and owls have been known to crash through the crust, feet first, in search of prey. (These scenarios conjure up frightful images of a rodent’s The Temple of Doom – tiny creatures suddenly snatched up in the jaws of an evil weasel or talons of a barred owl. )
All of this reminds us that life, mystery, and even drama can lie just beneath the surface of a seemingly banal landscape. So next time you’re crunching through the melting spring snow, look for and appreciate these elaborate transportation networks. All traces of this vital winter infrastructure will soon vanish!