Let it Snow at Eastman!
Critters Create Subways in Snowpack
Text and Photos by Laura Nagy
Those of us who enjoy skiing or snowshoeing are not the only ones to benefit from a hefty snowfall. For some forest critters, a substantial snowpack is crucial to their survival.
The white blanket so peacefully covering your yard is surprisingly dynamic. When airborne flakes make landfall, their lovely filigree spokes immediately break off, leaving crystalline spheres. The accumulating ice particles cozy up, and wind, weight, and moisture amalgamate individual grains into a firmer, denser mechanical structure. This process of snow metamorphism is what turns fluffy powder into the heavier, harder-to-shovel mass on your deck or sidewalk.
When snow cover reaches about 12 inches, it crosses a threshold that some ecologists call “critical depth.” At that point, a complex interplay of energy release and recapture kicks in, and the snowpack truly does become a blanket that wards off cold from above. It also holds in warmth rising from the ground below, which under the frost line remains about 50 degrees year-round. The result is melting at the soil surface, which releases water vapor that condenses on granules in the bottom two or three inches of the pack. This creates a layer of large, detached grains, called depth hoar or, more familiarly, sugar snow.
Depth hoar is a vital component of a unique environment called the subnivean zone, where a surprising number of animals go about their lives even in the depths (pun intended) of winter. The loosely packed crystals are easy to tunnel through, and the snowpack shields these movements from predators topside. Critical depth also minimizes temperature variations. In fact, the temperature within the subnivean zone remains at a fairly constant 32 degrees, even when the air above the snowpack hovers at 40 below.
Within the subnivean environment, microbes and fungi busily break down organic matter, and these detrivores are food for grubs, worms, flies, and beetles adapted to cold-weather activity. Some, like springtails and snow fleas, even breed within the snowpack and hatch on warm days. Other invertebrates like centipedes, millipedes, and wooly bears forage in leaf litter or nibble ground-hugging plants in the subnivean zone. In turn, they’re fed on by predatory bugs like spiders, wasps, and mites.
Probably the most common mammals ranging about the bottom of the snowpack are the voles, those hamster look-alikes that in warmer seasons keep a low profile in burrows and woody or grassy tangles. Common in our area are red-backed voles, nocturnal types who prefer boggy spruce and fir forests, and the slightly larger meadow vole, which forages day and night in marshes and fields.
Unlike other rodents–squirrels and some mice, for example—voles rarely cache food, nor can they ratchet back their metabolism, like chipmunks. No, voles need to feed, so they’re almost obsessive excavators in the subnivean, searching out herbaceous forage like cinquefoils, goldthread, and bunchberry, or tubers, or seeking seeds, tree roots, bark, and the occasional unlucky insect. Spring melting frequently reveals an extensive network of vole tunnels in both the remnant snowpack and the newly exposed duff.
Mice also dig subnivean subways between downed trees, root niches, or brush piles. White-footed and deer mice are very active, while jumping mice sandwich long periods of torpor around short forage bouts, Mice do emerge from the snowpack, leaving quarter-sized exit holes and tiny footprints that bound across the surface like miniature squirrels.
Shrews add to the subnivean traffic, small but mighty hunters that forage through depth hoar for insects, worms, and grubs. Shrews also burrow and, like mice, may emerge to traverse the snow and plunge back down elsewhere. Their trench-like tracks often have a straight-line tail drag between parallel pairs of footprints.
Larger prey for shrews include wood frogs, spring peepers, spotted salamanders, and red efts, which hole up in cracks in logs or rocks. These cold-blooded amphibians have a unique cryogenic survival strategy also practiced by many insects. Falling ambient temperatures stimulate overproduction of glycogen in their bodies, a sugar that floods their bloodstream and functions as a kind of antifreeze within their cells. Snow at critical depth insulates these living popsicles and keeps them from freezing solid.
Finally, a snowpack is important to the Bigfoot of our woods—the snowshoe hare, which opts for surface roads to avoid taking the subway. Its massive hind kickers spread out foot-loading so it can move easily on the snow surface to browse exposed twigs. Walking atop a couple feet of snow also boosts the hare’s height, putting additional coniferous foliage within reach. In addition, those natural snowshoes provide an escape advantage. Hares can dash off over the top of the snowpack, while a hungry fox or fisher, equipped with smaller feet, sinks in to its elbows and watches a bunny bum recede in the distance.
So, as you contemplate—and perhaps curse—the snowpack enveloping Eastman for the next couple of months, consider that for many creatures it’s not a nuisance. It’s a life jacket that gets them through to spring.
Laura Nagy is a writer and college educator who enjoys learning about wild things. She is currently working on a book about the exterminations of wolves in the American West. She is a member of Sustainable Eastman and the Grantham Conservation Commission.