Subnivean Zone

See those slithery, tunnel-like and bumpy shapes on top of the snow? You are witness to the creatures of…The Subnivean Zone! There are many ways that animals handle Winter’s cold. Some actually rely on the snow of winter for survival. Here’s how they do it.

During the first lingering snowfall, hardy, stiff vegetation and rocks block snow from accumulating under them. Lowering temperatures stiffen more vegetation. Subsequent snows cover these areas, bending the stiff vegetation, creating small tunnels and pockets of space. The earth’s warmth is captured by the overlying snow cover, which also insulates the soil against the freezing temperatures of winter from above. The snow laying on the soil sublimates, changing from a solid directly into a gas, creating more space. These sublimating gasses re-freeze within the snowpack, creating a dense layer of round, snowy ice crystals; we see them during the spring thaw. 6” of snow creates this snowy ice canopy; another 2” keeps this space within a degree or so of 32°. 

This winter habitat provides protection for the voles, moles, and shrews…creatures of the subnivean zone. Winter’s cold and wind are held at bay, and some predators are foiled. Tunnels are created and used for travel and fresh air between between food caches, and for foraging along the forest floor for bushes, bark, grass, leaves, seeds, and insects…all of which remain unfrozen. The system of tunnels can be quite elaborate, leading to food stores, sleeping quarters, dining areas, and waste areas. Most mazes begin at a tree, rock, or bush, absorbing solar heat and aiding temperature moderation of the surrounding soil and underlying vegetation. This action is evident at the base of the trees in the forest, where the snowpack has receded from the trunks.  

Here’s a short video of a little critter making its tunnel:

But some predators are not fooled by this little world. Owls, with their extremely keen hearing and location abilities, will zero in on these guys as they run around…making the subnivean zone a lucrative hunting zone! Foxes and other canids aren’t fooled either! The occasional miss by these predators will result in the tunnel being crushed and the unfortunate critter underneath suffocating…providing yet more food for another hungry critter. Ermine will squeeze through the tunnels, feasting on the rodents, making a nest from the fur and commandeering the living space for a short while.

Dangers are present, however, and can be catastrophic. Spring thaws and rains  can flood the tunnel systems over the entire landscape, drowning hoards of critters or chasing them out of their protected zone to become easy prey. Also, the spring freeze/thaw cycles weaken the subnivean roof structure, sometimes causing a wholesale collapse of tunnel areas. As the snow recedes, the once bustling habitat is revealed, as bumpy little humps from tree to rock and along the remaining few inches snow appear…giving us yet another reason to ponder the wonders of our wood walks…The Subnivean Zone! 

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