Before the January thaw hit two weeks ago, that trail in the photo above was the bottom of a tunnel in the subnivean zone. Subnivean zone. The name alone conjures an image of a different world. Only a few inches high, this is a habitat where voles and shrews can escape the howling winter wind and the sharp eyes of predators while feasting in peace on plants and seeds.

Subnivean means “under the snow,” and this zone is located at the very bottom of the snowpack. It forms when the warm earth heats the snow closest to it, turning it into water vapor, which is then replaced by a layer of air. Except for sheltered places under fallen tree limbs or other forest debris, it’s a shallow habitat, but it’s large enough for small mammals to create a network of tunnels and sleeping areas. And it stays at just about freezing—positively balmy compared to the air temperatures in my backyard, which routinely drop to single digits (in degrees F) at night.

Still, it’s not a perfect system. Weasels are little enough to stalk a rodent through its own passage. Owls and other predators listen for the sound of a scurrying animal and then pounce into the snow to grab it. And, when the weather warms, the zone’s protective ceiling melts away. On a hike through the woods one March, I spotted a vole flinging itself across a bare spot of ground where the sun had created a gap in its throughway. It reached the other side of the opening before I could even move, but if I had been a coyote or a fox, the story would have ended differently.

If you’re interested in learning more about subnivean habitats, check out the following sources:

Candace Ward’s “The Subnivean—the world beneath the snow” in Refuge Notebook 6:4, Jan. 23, 2004. (https://www.fws.gov/uploadedFiles/Region_7/NWRS/Zone_2/Kenai/Sections/What_We_Do/In_The_Community/Refuge_Notebooks/2004_Articles/Refuge_Notebook_v6_n4.pdf)

Julie Crick’s “The Subnivean Zone, life under the snow: Part 1” at Michigan State University Extension’s website, February 3, 2014. (https://www.canr.msu.edu/news/the_subnivean_zone_life_under_the_snow_part_1)

Julie Crick’s “The Subnivean Zone, life under the snow: Part 2” at Michigan State University Extension’s website, February 3, 2104. (https://www.canr.msu.edu/news/the_subnivean_zone_life_under_the_snow_part_2)