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During the frigid season$\dots$it's often necessary to nestle under a blanket to try to stay warm. The temperature difference between the blanket and the air outside is so palpable that we often have trouble leaving our warm refuge. Many plants and animals similarly hunker down, relying on snow cover for safety from winter's harsh conditions. The small area between the snow pack and the ground, called the subnivium$\dots$might be the most important ecosystem that you have never heard of.

The subnivium is so well-insulated and stable that its temperature holds steady at around $32$ degree Fahrenheit ($0$ degree Celsius). Although that might still sound cold, a constant temperature of $32$ degree Fahrenheit can often be $30$ to $40$ degrees warmer than the air temperature during the peak of winter. Because of this large temperature difference, a wide variety of species$\dots$depend on the subnivium for winter protection.

For many organisms living in temperate and Arctic regions, the difference between being under the snow or outside it is a matter of life and death. Consequently, disruptions to the subnivium brought about by climate change will affect everything from population dynamics to nutrient cycling through the ecosystem.

The formation and stability of the subnivium requires more than a few flurries. Winter ecologists have suggested that eight inches of snow is necessary to develop a stable layer of insulation. Depth is not the only factor, however. More accurately, the stability of the subnivium depends on the interaction between show depth and snow density. Image beng under a stack of blankets that are all flattened and pressed together. When compressed, the blankets essentially form one compacted layer. In contrast, when they are lightly placed on top of one another, their insulative capacity increases because he air pockets between them trap heat. Greater depths of low density snow are therefore better at insulating the ground.

Both depth and density of snow are sensitive to temperature. Scientists are now beginning to explore how climate change will affect the subnivium, as well as the species that depend on it. At first glance, warmer winters seem beneficial for species that have difficulty surviving subzero temperatures; however, as with most ecological phenomena, the consequences are not so straightforward. Research has shown that the snow season (the period when snow is more likely than rain) has become shorter since $1970$. when rain falls on snow, it increases the density of the snow and reduces its insulative capacity. Therefore, even though winters are expected to become warmer overall from future climate change, the subnivium will tend to become colder and more variable with less protection from the above-ground temperatures.

The effects of a colder subnivium are complex...For example, shrubs such as crowberry and alpine azalea that grow along the forest floor tend to block the wind and so retain higher depths of snow around them. This captured snow helps to keep soils insulated and in turn increases plant decomposition and nutrient release. In field experiments, researchers removed a portion of the snow cover to investigate the importance of the subnivium's insulation. They found that soil frost in the snow-free area resulted in damage to plant roots and sometimes even the death of the plant.

In paragraph $6$, the author provides the examples of crowberry and alpine azalea to demonstrate that

  1. Despite frigid temperatures, several species survive in temperate and Arctic regions.
  2. Due to frigid temperatures in the temperate and Arctic regions, plant species that survive tend to be shrubs rather than trees
  3. The crowberry and alpine azalea are abundant in temperate and Arctic regions.
  4. The stability of the subnivium depends on several interrelated factors, including shrubs on the forest floor.
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