Snowy Catacombs: A Closer Look at Subnivean Life

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

Snow is Here! An Introduction to the Subnivean Zone

Here in the ever-erratic White Mountains, the slopes are finally living up to their name! Snow has arrived and it looks like it's here to stay. Throughout our first month of winter, we have watched layers of snow, that should have been the beginnings of winter snowpack, disappear before our eyes. To celebrate this first true breath of winter, lets take an introductory look at a unique feature of forests in the northern latitudes: the subnivean environment.

The term "subnivean" refers to the area beneath a formed snowpack, and the term is sometimes inclusive of the intranivean environment as well (the area within the snowpack). The subnivean zone is an insulated environment that remains at relatively stable temperatures of approximately 0 degrees Celsius, despite large shifts in ambient temperature above the snowpack. An area of relative hospitality, subnivean environments are one of the only places that small overwintering organisms can survive.

The phenomenon of subnivean microclimate is caused by a specific set of snow metamorphisms, or changes in the nature of the snowpack as it forms over the course of winter. In late Autumn, destructive metamorphism takes place - this is the initial snowfall that builds, forms the first ice crystals, and remains throughout the winter. As winter deepens, a temperature gradient in this forming snow layer causes water vapor to rise to the surface, resulting in a denser upper portion and a less dense area of "depth hoar" ice beneath. This process of constructive metamorphism allows for the first movements of small ground mammals beneath the snow surface. Finally, the snowpack reaches maximum strength and insulating ability during melt metamorphism, when freeze-thaw cycles further solidify ice structure.

Many organisms overwinter in this layer directly above the ground, including several species of insects, fungi, mammals and even plants. Life in the subnivean environment is so unique that entirely new food webs have been proposed to accompany the changing relationships and increasing energetic needs of subnivean organisms. In the coming weeks, as our snowpack here in Pinkham Notch builds higher, we will be taking a closer look at the fascinating microclimate of the subnivean zone and at a few specific species that exemplify winter hardiness. Come back soon for more!

- Robert Rives, Pinkham Notch Winter Naturalist Guide
Photo Credits: Eric Pederson; AMC Photo File

The Evergreen Advantage

One is an icon of the holiday season. Another releases a sticky sap that was used to make America's first chewing gum. Still another spreads majestically throughout the Appalachian mountains as far south as Alabama, and due to its common name, is sometimes confused with a classical poison. These "usual suspects" of our winter landscape here in the White Mountains (Balsam fir, red spruce and eastern hemlock, respectively) are all tree species that are members of a fascinating plant group: the evergreen conifers. Among many other defining characteristics, these trees are easily identified by green to dark-green needles protruding from slender branches that are not dropped in winter. Because of this, evergreen conifers are the poster children for a northern New Hampshire forest; when you conjure up an image in your mind of the north woods in winter, what do you see?

What is most fascinating about trees such as the Balsam fir and red spruce is simply their puzzling presence in the winter landscape. What makes these trees so hardy? And why on earth would they keep their leaves in winter, when it is almost always too cold to photosynthesize? And wait....why are they not covered in several inches of snow following a storm?

The answers to these questions can be found in a wide array of peculiar and subtle adaptions of the evergreen conifers. Remember those slender, flexible branches to which the needles are attached? When a significant weight of snow builds on top of them, they will eventually bend downward enough to shed nearly all of the snow! This adaptation helps to reduce snow and ice damage to the branches through a rough winter. Another crucial adaptation of these trees to winter survival is their expansive growth range; evergreen conifers can live and thrive in very nutrient-poor soils, such as those on the higher slopes of the Presidentials.

Ultimately, their most important adaptation of all, and the explanation for retaining their needles, comes down to a basic lesson in plant economics. During an average winter in New Hampshire, the temperatures stay well below those needed for effective photosynthesis. Because of this, the tree must use stored energy to keep the needles alive, even though they are producing nothing! It seems like a recipe for disaster, right? The true evergreen advantage comes in the late fall and early spring: after deciduous trees have dropped their leaves, and before deciduous trees have had time to develop new leaves, the evergreen conifers will have already been photosynthesizing for quite a while. And after a few years, the amount of extra energy created in those late fall and early spring times will balance out and exceed the energy lost during the winter.

So while you are outside this winter, enjoying all that it has to offer, take a moment to stop at one of these fascinating evergreen specimens and appreciate it for it's genius. The great north woods would be quite barren without them!

- R. Rives, AMC Pinkham Notch Winter Naturalist
Photo credits: AMC Photo File; Mount Washington Observatory Photo Journal (J. Gemmiti)

A Long Nap

When considering winter survival, the American Black Bear is commonly used as an example of hibernation, when in fact their winter rest is simply a long nap. True hibernators, such as bats, are able to drop their body temperature down to near freezing (or ambient temperature) and slow their heart rate for reduced metabolic costs. A bat’s body temperature drops to 32^oF and heart rate drops from 210 beats per minute to 8 beats per minute. Shivering is the only mechanism that stands between survival and freezing to death. Another true hibernator, the jumping mouse is able to raise its body temperature every several weeks to prevent the cells from rupturing, and then resume torpor. Chipmunks spend nights or several days in torpor, waking in between to eat food reserves. This mechanism unfortunately uses up most of their fat reserves as they raise their heart rate from 4 beats per minute to 350 beats per minute when “waking up”.

What separates black bears from the mammals presented above is the surface area to volume ratio. A bear’s low surface area to volume ratio makes it challenging to drop body temperature, whereas smaller animals are able to dissipate the heat. Its body temperature drops only 10^oF, from 100^oF to 90^oF, and heart rate drops from 50 beats per minute to 8 beats per minute. The black bear’s sleep is not deep, and they are easily aroused throughout the winter. They tend not to consume food, nor relieve themselves of waste; however, females do give birth to 2-3 young in January every other year, and will produce milk throughout the remainder of the winter using their fat reserves.

To prepare for this season, black bears double their weight in the fall season by gorging themselves on berries, seeds, and nuts. A good mast year tends to bode well for the bears, as it not only provides sufficient fat reserves, but also makes for a shorter nap season. The bears den later in the season if food sources are available. This year marked an excellent mast year, so one can expect the bears to be active into December.

To follow bear den webcams throughout the winter season go to http://www.bear.org/website/ and click on the live cameras link. Several bear dens are monitored each year and it is an incredible resource for learning about bear winter survival!

-H. Cowan

AMC Naturalist Guide

Winter Camouflage

As the end of October approached, people of all ages donned outfits to change their appearance for the evening. Halloween is one of the few holidays that allow individuals to embrace their creative and unusual sides. For humans, the chance to change appearance is celebrated, for some forest critters this change is crucial for survival. As the fall temperatures drop, the ermine (mustela erminea) and snowshoe hare (Lepus americanus) summer coats are shed for a snowy white fur. Shortened daylight hours and lower temperatures trigger the pineal gland and a hormonal response in these animals. This results in the production of altered biochromes. These biochromes are found either at the surface of the skin or in deeper level cells, where they are considered chromatopores (pigment-containing and light reflecting cells). Mammals have only one kind of chromatopore, the melanocyte. You may recognize these cells as melanin. As hair is dead tissue, the brown summer fur must be shed to grow the white winter coat. Although the colored coat provides camouflage in the summer and early fall months, its color is stark against a wintery landscape.

The ermine coat is almost entirely white in the winter season with the exception of a black tipped tail. In the middle ages, the ermine’s white coat was popular in clothing, and was commonly seen lining the King’s robes. Today, the ermine coat is rarely used and its population is neither threatened nor endangered.

The snowshoe hare’s light brown coat is also shed for long white guard hairs, starting with the ears and feet. This transformation takes roughly 10 weeks to become entirely white. They depend on their cryptic coloration to hide from predators that include lynx, coyotes, foxes, wolves, pine martens and birds of prey. In summer, they shed white for mostly rusty brown coats to blend with trees and soil. Recent climate changes result in fewer days with snow on the ground. This means that white hares are hiding in a brown forest. A hare that is the wrong color stands out like a King in his new clothes.

As the seasons are changing, keep an eye out for the white ermine and snowshoe hare. They are easiest to spot on bare ground, but will soon blend in with the white landscape.

H. Cowan AMC Naturalist Guide

Mountain Ash Tree v. Moose

The entrance to the Joe Dodge Lodge is decorated with a variety of native plants from high bush blueberries to Balsam Fir. One of the more outstanding flora is the mountain ash tree (Sorbus americana) that is now stripped of all leaves and berries. A once flourishing and beautiful icon of the garden, the mountain ash became a moose’s (Alces alces) breakfast. Noticed first in August, and slowly stripped of all vegetation throughout September, the October stage of this ash tree is quite depressing.

Moose (Algonquin for “eater of twigs”) lack upper incisors, so they tear rather than snip the bark, leaves, and twigs off trees. Their skull contains 32 teeth consisting of 12 ridged molars, 12 premolars, 6 incisors, and 2 canines. Weighing anywhere from 800 to 1200 lbs, a moose can consume 40 to 60 lbs of vegetation a day. This is the equivalent to a 150lb human eating 7.5 lbs of salad a day! To fully absorb nutrients, moose have four-chambered stomachs coupled with regurgitation and the chewing of cud to break down cellulose. A moose may chew its cud up to 8 hours a day! In preparation for winter, they increase their body weight by as much as 25%. As they do not hibernate, moose spend the winter in quiet solitude, stripping the bark and twigs off of trees and eating conifer needles. As this does not provide as much nutrition as leaves and aquatic vegetation, the winter is a time of rest and hunger.

If you come upon a moose this fall, observe from a distance. Moose are going into the rut, or mating, season and the males tend to be irritable as they attempt to win the affections of a lady!

-H. Cowan
AMC Naturalist Guide

Credit: S. Kennedy