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The Subnivean Layer - Ever Heard of it?

Few things are more magical than a Northwoods winter. The sun sparkles on the snowy white blanket making everything shimmer. Conifer branches bow low to the ground under their heavy burdens of snow. The woods seem silent and still with only animal tracks and the occasional “chicka-dee-dee-dee” call to let you know that life is carrying on around you.

 

But little did you know, life is likely carrying on below you as well! One of the magical secrets of winter is definitely the subnivean layer- sub meaning below and nivean meaning snow, this layer or “zone” is exactly what it sounds like. When the snow falls gently to the ground and creates it’s beautiful white blanket, it does not fill in every nook and cranny of forest floor. I like to think of it like when you’re laying down and cover yourself in a blanket. There are gaps between the bed and the blanket in areas where your body holds the blanket aloft. Other than vegetation and debris holding the snow gently above the earth like a blanket, the subnivean can also be created through a complex process called “sublimation” in which the snow nearest the earth is warmed gently enough that it travels straight from a solid phase (snow) to a gas phase, skipping the liquid phase all together. As this gas rises up through the snow above it, the humidity of the gas creates almost a crunchy ceiling to the subnivean zone.

 

 

Small mammals such as mice and voles utilize this subnivean layer to travel around shielded from view of predators as they move between food caches and their sleeping areas. This zone does not only afford them protection from predators, but from the weather as well. Snow has insulating properties and by traveling through tunnels created below the snow these critters are protected from extreme cold and wind in a world all their own that stays approximately 32 degrees through out the winter. Food is also more plentiful below the snow. The mice and voles are able to dig in the leaf litter in search of seeds and plants or chew the bark off of young saplings and shrubs as well as navigate to stashes/caches of food they may have stored before the snow flew.

How then do predators that rely on eating these mice and voles manage to hunt in the winter? Well, ermine (slender bodied, small weasels) are able to simply slip down the subnivean tunnels and follow them right to a tasty treat. Fox us their huge, satellite dish ears to listen for the tiny footsteps below the snow and then pounce in the hopes of catching a mouse mid travel. Owls, similarly, use their astounding hearing abilities to locate prey in the subnivean zone as well. Did you know that owl ears are not symmetrical? One ear is located further back and slightly down on the head while the other is located up and forward. (See photo to the right that depicts an owl skull and its ear placement.) By having this odd ear arrangement owls ears detect sound milliseconds apart and this allows the owl’s brain to triangulate exactly where it’s prey is. This helps owls hunt in the dark year round, but is also extremely useful when mice are traveling below the snow.

 

 

Thank you to Pepin Wisconsin resident Jennifer Baker Fetzer for allowing us to use thing very cool photo (left) depicting owl wing prints in the snow (photo on left). This is clear evidence of an owl striking at a prey animal traveling through the subnivean zone.

 

 

What evidence can you find of the subnivean life in your backyard? Well, if you go out and explore you may notice tiny little mice tracks across the snow that mysteriously vanish into a hole- this is the entrance to a subnivean tunnel. Additionally, the constant breathing and body heat of critters in these tunnels creates a slightly iced wall to the tunnel. This means that often in the spring melt you can see these trails melting last. You may also see smashed down or chewed up trails through the grass after the snow has completely melted!

 

As I sit here and write this blog on a cold and windy day that might not even reach 10 degrees, I wish I was able to travel in the subnivean layer as well! 😉

 

 

 

 

 

Visible in this photo (right) are both the entrance and exit to a subnivean tunnel.

Harder to see in the foreground are tiny mouse footprints leading out of the tunnel.

 

 

 

 

 

 

Author: Bethany Heft, Trees For Tomorrow Environmental Educator