Summit Outside: Subnivean zone: life under the snow
Ryan Summerlin January 1, 2013
How do the little animals that don’t hibernate or migrate survive in the winter? The winter alpine environment has bitter cold temperatures and biting wind. Extended cold and no snow can cause serious problems for survival for many small animals, but ample snowfall provides a safe haven. Think of an igloo or a snow cave and how it can protect and insulate from the cold.
There are little animals that are called subnivean animals which include small mammals such as mice, voles, shrews and ground squirrels that must rely on winter snow cover for survival. “Subnivean climate” refers to the zone in and underneath the snowpack. The name comes from the Latin for “under” (sub) and “snow” (nives).
The subnivean climate is formed by three different types of snow stages or conditions called “metamorphosis.” There is destructive metamorphosis, constructive metamorphosis and melt metamorphosis. These three types of metamorphosis transform individual snowflakes into ice crystals and create spaces under the snow where small animals can move.
Pukak is the Inuit (Eskimo) word subnivean region: for the thin layer of snow under which small mammals spend the winter.
Deconstructive metamorphosis begins as the snow falls to the ground – often melting, refreezing and settling. The melting snowflakes fuse with others around them becoming larger until uniform in size.
Freshly fallen snow has very good insulating properties, but with time the insulating property of the snowpack decreases because the air spaces between snowflakes disappear. Once a base of over one foot of snow has accumulated, temperatures under the snow remain relatively constant.
Destructive metamorphosis occurs most rapidly during larger temperature extremes such as warm days followed by cold nights, and on slopes that receive large amounts of sun. As time goes on snow settles, compacting air spaces and reduces the penetration of the sun’s heat.
Constructive metamorphosis is caused by the upward movement of water vapor within the snowpack.
Warmer temperatures are found closer to the ground because it receives heat from the core of the earth. Snow retains heat, creating a temperature gradient between the air underneath the snowpack and the air above it.
Warmer air holds more water vapor. When the water vapor reaches the top of the snowpack and meets much colder air it condenses and refreezes. The ice crystals at the top of the snowpack form a crust.
Melt metamorphism is the deterioration of snow by melting. Melting can be increased by warmer ambient temperatures, rain and fog.
As snow melts, water is formed and these molecules move downward. They refreeze, thickening the middle of the snowpack. During this refreezing process heat is released, and as more water comes down from the surface, it creates more heat and brings the entire snowpack to near equal temperature. This process strengthens the snowpack, due to the bonding of grains of snow.
These processes continue throughout the winter forming the subnivean zone. For animals, the subnivean zone becomes a blanket that provides insulation from the elements and protection from predators.
For snow found on steep mountain slopes however, the development of the subnivean zone leads to a weaker bottom layer and a dense top layer, creating avalanche conditions.
The formation of the subnivean zone or “pukak” as the Eskimos call it, begins with the first snowfall that covers the ground vegetation. Where there is little or no vegetation, such as on the ice surface of ponds and lakes, no pukak layer forms. Small plants keep some of the snow from coming in contact with the ground and this causes small openings or cavities to form. In these spaces, small creatures scurry in a relatively balmy comfort, although it is damp and dark.
When the snow reaches a depth of about a foot, even the mid-day sun causes only a faint glow to reach the pukak. Because of the dark, animals in the pukak have to rely mainly on their hearing and sense of smell.
The subnivean zone maintains a temperature of close to 32 °F (0 °C) regardless of the temperature above the snow cover, once the snow cover has reached a depth of 6 inches or more.
Seeds keep well at pukak temperatures and some vegetation stays succulent and green all winter long.
A variety of insects also dwell in the pukak, and insects and plants provide food for these small creatures. These little rodents: mice, moles and voles maintain pathways in the pukak, moving about in their tunnels. They sometimes chew the bark of trees that are under the snow.
Where the winter is long the pukak dwellers may run low on food and will venture out in great numbers on warm, sunny days. This could be a dangerous activity because they themselves could turn into prey.
Actually, predators like hawks can hear the sound of these pukak dwellers munching on insects and have been observed to dive into the snow, emerging with a vole in their talons.
Long, skinny animals with short legs like ermine and weasels can enter the tunnels and hunt.
Snowmobiles and ATVs can collapse the subnivean space. Skis and snow shoes are less likely to collapse subnivean space if the snowpack is deep enough.
When you snowshoe or ski through the woods in mid-winter, you see the tracks of a variety of animals. You often see the large tracks of moose, deer, coyote and fox. In some areas, the tracks of snowshoe hares and squirrels leave intricate patterns in the snow. The small tracks of a mouse, vole or shrew are seldom seen. When their tracks are found, they usually skitter across the snow for a short distance and then disappear into the snow, back to the relative safety of the subnivean zone.
Snow it is “liquid gold” for us sliders in Summit County and it helps animals too!
Joanne Stolen is retired from teaching microbiology at Rutgers University.