In a winter such as this, optimism is often the most important thing to carry in your daily rucksack. Abnormality has become the norm for our current season's weather and conditions as occasional snowfalls fight to create a legitimate snowpack that will not be destroyed by rain, fog or scouring winds. I have watched entire snow layers disappear from Huntington Ravine's steep and rocky fans, transforming easily traversable terrain into scratchy scrambles (in human terms, at least). Conditions such as these occasionally make us winter enthusiasts sour or frustrated, but we at least have the opportunity of retreat into the luxury of central heating to wait for better days. What about our small, furry friends who depend on snowpack for winter survival? This week, I want to dig a little deeper into the intriguing and often dangerous world of subnivean life.
As we explored in the previous blog entry, the story of subnivean existence begins in late Autumn, when the daily ground temperature becomes warmer than daily air temperature. During a normal winter season here in the Whites, a snowpack would begin to form around this time. When this snowpack reaches a stable depth of approximately 6-10 inches, small ground mammals such as voles, moles, ground mice and chipmunks will no longer be seen moving about on top of the snow. Tunnels will be established for the purpose of foraging in the relatively warm space directly above the ground and some of these small mammal species will nest and reproduce. In general, this overwintering strategy results in very little overall mortality among subnivean populations, but the smallest irregularities in weather can affect their insulated, snowy lair in potentially lethal ways.
Paradoxically, the deepest parts of winter may be the safest for mammals living underneath the snow. These are the times when dramatic changes within the snowpack are typically few and far between. It is during the snowpack's formation in late Autumn, and destruction in Spring, that the worst dangers are present. If the ideal snow depth of 6-10 inches (called the hiemal threshold) is not reached before extremely cold temperatures occur, then exposed plants and ground mammals may experience a high-mortality event. The dangers do not end here; when the snowpack finally does form, rain or thawing can cause water to percolate deeper into the snow, where it has the potential to refreeze and create "ice lenses." These impenetrable barriers may cut off an animal's subnivean nest from its foraging source. And if all goes well for the mammal throughout the winter, they must still survive spring flooding, when the snowpack begins to melt and percolating water soaks subnivean nests. Newborn ground mammals can experience severe hypthermia in this situation, just like a soaked hiker high on the Presidential ridge! Gore-Tex suits for voles and mice are unfortunately not being considered for production at this time.
This winter may have its unsatisfying moments, but abnormal rains and low snowfall may be downright dangerous to our subnivean friends. Keep performing your daily ritual snow-dance and let's hope that February and March bring the New Hampshire winter that we all know and love!
- Robert Rives, Pinkham Notch Winter Naturalist Guide
Photo Credit: AMC Photo File