Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [11]
Within this space, temperatures are physically “regulated” within a degree or two of the freezing point of water, all winter long. Several factors are involved. First, as already mentioned, snow affords remarkable insulation and, even at -50°C, heat rising from the earth generally keeps the temperatures near the ground close to 0°C. When both ice and water are balanced at near 0°C in the subnivian zone, the temperature is stabilized since whenever heat is lost through the snowpack to cool this space slightly below 0°C, there is then a water-to-ice-crystal conversion, which releases heat. Similarly, whenever ice turns to water, the process requires or uses up heat. Thus, as long as both ice and water exist side by side, they constitute a thermostat keeping temperatures constant. Only the amounts of ice and water vary, depending on the amount of heat loss or input.
In New England, the subnivian zone is the home of voles (a type of short-tailed mouse): principally the meadow vole (Microtus pennsylvanicus) and the red-backed vole (Clethrionomys gapperi) as well as the masked shrew (Sorex cinerius), smoky shrew (Sorex fumeus), pygmy shrew (Microsorex hoyi), and short-tailed shrew (Blarina brevicauda). Every spring, right after the snow melts, or just as the last inch or two is melting, I see the labyrinth of the Microtus tunnels fully exposed on the surface of the ground. Also fully exposed lie the grass nests of these rodents, many of which will soon be occupied by bumblebee queens starting new colonies.
Mice in the subnivian zone, eating bark.
The spring of 2001 provided an especially impressive demonstration of the importance of the subnivian world to meadow voles. Record amounts of snow had fallen in March in Vermont, and the voles appeared to be having a population explosion. Like lemmings, their close relatives, meadow voles have an awesome reproductive potential. One well-fed captive vole produced seventeen litters in one year, averaging five babies each after twenty-one-day gestation periods. The young females, in turn, can produce their own litters in one month. At such reproductive potential, it would not take long for them to carpet the earth. Luckily, such horrors of exponential growth are seldom realized. Instead, the voles’ role in the economy of nature is, like that of hares, to convert vegetation into the protein-rich dietary staple of many predators that rely on them in winter, principally foxes, weasels, fishers, coyotes, and bobcats. The summer shift includes hawks and snakes.
In some areas all of the young sugar maple trees, box elder, and white ash trees were debarked right up to the snow line, but never above it. The tree damage caused by voles in winter is well known by orchardists, who would lose their young fruit trees every winter if they did not in the fall cover every one of their young trees in an artificial barklike commercial plastic stripping up to the level the winter snows are expected to reach. I learned this the hard way when I planted apple trees in the field by my cabin; by spring, every single one was stripped of bark for a foot above the ground, in what had been the subnivian zone in the winter. Older trees, once they have developed a thick layer of their own bark, are protected. The cambium, or inner live layer of bark of trees, is a favorite food of many herbivores, and the thick outer dead layer is essential armor. Like most armor, its usefulness is only apparent in time of need. For trees, the greatest need for thick bark is in the winter when more easy-to-eat foliage is not available.
Due to the protection of the snowpack and the cozy subnivian zone under it, voles are able to get a jump on spring and reproduce sometimes two to three months before the snowpack has melted. Wild spring flowers of many kinds also get an early start in the relative warmth of the subnivian zone under the snow. Some, like the snowdrops in our gardens, grow in March under the snow and send their flowers directly through the snow.
Peter Marchand, a winter ecologist who