Snow: More complicated than you thought

Posted: Friday, December 10, 2010

It is finally the season to store the riding lawnmower and start up the snowmachine. With the opening of the Kenai National Wildlife Refuge to snowmachining on Dec. 1, it is evident that we have accumulated enough snow to start our winter fun. Whether you are out snowmachining, dog sledding, snowshoeing, cross-country skiing, or building a snowman with your kids, we all realize that snow plays an important part in our outdoor activities this time of year. But what is snow and how does it affect plants and wildlife?

Snow is the type of precipitation in the Earth's atmosphere consisting of an unfathomable amount of ice formed in a conglomeration of crystals that fall from clouds. Snow crystals form when tiny cloud droplets freeze. Once a droplet hits its freezing point (typically less than 32F) a small nucleus is formed. Water droplets surrounding this nucleus begin to condense around it and freeze. This results in the formation of the intricate crystalline structures we call snowflakes. Eventually, the snowflakes become too heavy to be suspended in air and fall from the clouds.

Once on the ground, snow under goes many forms of metamorphosis. In the beginning, the intricate design of snowflakes begins to deteriorate into round grains of ice that adhere to one another in a process referred to as "destructive metamorphosis." This results in a firmer, denser snow.

After the destructive process occurs, snow then undergoes changes in its vertical structure known as "constructive metamorphosis." This process occurs when water vapor migrates upward through the snow. As this water vapor from the bottom of the snow continues its path upwards, small cavities form below the snow surface. The constructive process reduces snow's supporting capacity which leads to an increase in avalanche danger in mountainous regions.

The third process of snow alteration is "melt metamorphosis." Melting occurs when the surface temperature of the snow reaches above-freezing temperatures resulting in the conversion of solidified (frozen) water molecules into liquid molecules (water) or vapor. Vapor water molecules then return to the atmosphere to complete the water cycle.

Aside from the fascinating process of snow formation and melting, snow also possesses many interesting characteristics important to plants and wildlife.

Because of the low thermal conductivity of snow, a vertical temperature gradient exists. The upper part of snow is affected by cold air above, while the bottom of the snow is influenced by the ground. This creates colder temperatures at the top of the snow pack and warmer temperatures at the bottom.

However, these environmental conditions are dependent on the density of snow. Snow density changes with the processes of destructive and constructive metamorphosis. When snow accumulates on the ground it is full of air pockets. The amount of air trapped in the snow is dependent on the types of snowflakes that were formed. Fewer air pockets result in denser snow conditions, the type of which allows us to build snowmen. The opposite is true when there are more numerous air pockets, creating powdery snow conditions.

Much like insulation, when there are more air pockets in the snow, the temperature gradient is more pronounced and can result in a temperature difference of six degrees. Plants living beneath the snow surface where conditions are warmer are, therefore, protected from the more severe temperatures near the top, especially if the snow is fluffy.

Additionally, the warmer temperatures beneath the snow and the cavities that are produced by constructive metamorphosis provide an excellent environment for small mammals, such as shrews, voles, ermine, and martens.

Conditions above the snow, however, produce different challenges for large mammals that are active at this time of year. Large mammal movements over the snow are typically dependant on snow depth and density. Large mammals such as caribou, moose, and wolves, will usually select areas with shallower or denser snow that make travel easier. Easier travel means better access to food resources and escape from enemies.

Human activities during winter also make a difference. Activities such as cross-country skiing, snow grooming, and snowmachining change the depth and density of snow by means of snow compaction. Some of these activities have a higher impact than others. Regardless, plants and wildlife are affected by these changes.

For example, snow compaction decreases snow depth and increases snow density which causes lower temperatures and a reduction of cavities beneath the snow. These changes in snow condition subject plants and small mammals to more severe environmental conditions and difficulties in movement that have been found to reduce plant productivity and small mammal populations.

On the other hand, snow compaction has also been found to make travel easier for large mammals. This can readily be seen on snowmachine trails by encounters with moose and the presence of wolf and coyote tracks. Some studies have shown, however, that even though snow-compacted trails facilitate wildlife movements, they also change predator-prey relationships. In other words, wolves can more efficiently access moose than they would without snow-compacted trails.

The Kenai National Wildlife Refuge is currently monitoring snow depth and plans to monitor snow-compacted trails and large mammal movements to see how these variables influence large mammal distributions during winter. It is the mission of the Kenai Refuge to protect and preserve wildlife and their habitats while providing recreational opportunities. Since its establishment in 1980, under the Alaska National Interest Lands Conservation Act, the Kenai Refuge has been successful in balancing these two objectives. Continued research into how wildlife responds to human activity will provide resource managers information to continue this success.

Tim Mullet is a Ph.D. student in the Department of Biology and Wildlife at the University of Alaska Fairbanks working for the Kenai National Wildlife Refuge. His dissertation work is addressing the cumulative ecological effects of snowmachines.

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Previous Refuge Notebook articles can be viewed on the refuge website. You can check on local birds or report your bird sighting on the Kenai National Wildlife Refuge Birding Hotline 907-262-2300.

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