At first glance, the winter landscape in Wisconsin can appear relatively lifeless. But life is out there — in many places. For example, beneath the snowpack hides an insulated space called the subnivium (from the Latin words for below, sub, and snow, nivea). Here are some of its secrets.

1. No matter how cold it is above the snow, it’s always right around freezing in the subnivium. The temperature there remains stable because snowpack traps heat released from the soil. As this heat slowly migrates upward, it creates a vertical gradient of decreasing temperature. Larger amounts of snow and fluffier, less-dense snow provide greater insulation. At around a half-foot or more of snow, temperatures in the subnivium will stay just around freezing (or above).

2. A diversity of winter-adapted species live and thrive in the subnivium. Most plants persist through winter by lying dormant as seed or root stock, and the moister, warmer subnivium helps them avoid drying out or freezing. Some plants even germinate or remain photosynthetically active in the subnivium. Microbes actively cycle nutrients in the subnivium, and some types of fungi (e.g., snow molds) proliferate there; they create extensive mats beneath the snow or grow vertically into the snow cover. Some invertebrates remain active in the subnivium during the winter, as well, and a few species will sporadically emerge, especially towards the end of the season. Most vertebrates inhabiting the subnivium are small (less than 9 ounces, or 250 grams), although larger- bodied organisms enter the subnivium, in part to access their dens. Many rodents hibernate within the subnivium, and many amphibians and reptiles exhibit complex physiological responses to facilitate freeze-tolerance. All of these species, though, depend on the fact that the subnivium is a stable, predictable, and warmer environment.

3. Warming winters threaten the subnivium. Rising temperatures brought about by climate change can result in thinner snowpacks or more frequent melting and refreezing. This reduces the insulating effect of the snowpack and the temperature stability of the subnivium environment, which leads to freeze-and-thaw cycles that make it more challenging for organisms to survive. Future warmer winters will decrease the extent and duration of the subnivium throughout the Great Lakes region.

4. Human activities, often linked to climate change and habitat disruption, can pose threats to the subnivium. Urban expansion creates more impervious surfaces (roads and buildings) that alter local hydrology. Precipitation runoff becomes more rapid and snow cover decreases, all of which degrades the subnivium’s stability. Land use changes, such as agriculture and deforestation, disrupt natural habitats and fragment ecosystems. This can reduce the availability of suitable subnivium habitats for various organisms. Winter recreation activities, such as skiing and snowmobiling, can compact the snow, reducing its quality and even collapsing the subnivium.

5. As the subnivium becomes less stable, certain species may struggle to adapt or find alternative habitats. During the winter, many living organisms depend on the subnivium as a food source; but warming temperatures disrupt its availability, which jeopardizes the ecosystem’s entire food web. Decreased snow cover lets predators access the subnivium more easily, making it a less reliable refuge for prey. These changes can produce cascading effects on ecosystems and the species that rely on a stable subnivium. Conservation efforts, such as habitat protection and climate change mitigation, are crucial for minimizing these impacts and protecting the delicate balance of winter ecosystems.

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Jonathan N. Pauli and Benjamin Zuckerberg are professors in the Department of Forest and Wildlife Ecology.

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This article was posted in Changing Climate, Front List, Healthy Ecosystems, Spring 2024 and tagged Benjamin Zuckerberg, climate change, ecosystem, Forest and Wildlife Ecology, Great Lakes Region, Jonathan N. Pauli, snowpack, subnivium, winter.