A soft, thick coat of snow makes a lot of the world seem to slow down or even stop — at least temporarily. The fluffy piles absorb sound and make the world quiet and still.
But deep underneath, in pockets between the snow and the ground, life goes on. This is the subnivium — a tiny ecosystem all its own.
Here under the white stuff, roots, small mammals, microbes, insects and even birds thrive. They use the subnivium to make the most of the winter months — hunting, breeding, breaking down leaves and more. All those cold-weather activities help determine which plants and animals (including insects) thrive during the snow-free seasons.
But this seasonal ecosystem is in danger. Climate change is making winters warmer. Much of the precipitation that used to fall as snow now pours out of the sky as rain. Snow covers less ground and for less time. No snow means no subnivium. And as this snowy ecosystem shrinks, a host of organisms might pay the price.
Their loss could change the way forests function all year round, scientists have found. Several groups are working to understand what is going on in the world below the snow and how it’s responding to our warming world. To save the subnivium, not only should we fight climate change — we need to save the snow itself.
A natural igloo
In places like the northern United States and Canada, a snowfall doesn’t always melt right away. Winter weather can stay cold enough for snow to stick around for days, weeks or more. It piles up, forming a snowpack. Some spots are deep enough for people to ski on. Once that snowpack gets deep enough, a new ecosystem forms beneath it.
“About 15 centimeters [or six inches of snow] is when you start to see the subnivium emerge,” explains Jonathan Pauli. He studies community ecology — how members of an ecosystem interact with each other — at the University of Wisconsin–Madison.
Snow doesn’t fall in even layers. It drifts, heaping up around fallen trees or rocks. Between the snow and the ground, it leaves open spaces, just a few centimeters (about an inch). Those little pockets link up “like a maze,” Pauli says, to form the subnivium.
AMY SHIPLEY
Life beneath the snow is very different than at the surface. “Like a natural igloo,” Pauli says, the thick snowpack insulates what lies beneath. Above the snow, the temperature might be –20° Celsius (–4° Fahrenheit). A few warm days might bring it up to 4 °C (40 °F). But beneath the snow, the ground is constantly just above freezing — never lower, never higher.
It hovers just one degree above the freezing point of water. But that one degree makes all the difference, explains Alix Contosta. She’s an ecosystem ecologist at the University of New Hampshire in Durham. “In general, you don’t see a lot of activity below freezing, because most life needs liquid water,” she says. But when the snow is deep enough, “it doesn’t matter how cold the air is, the soil will remain above freezing.”
That fact has changed the way scientists think about life in cold winter environments. When Contosta began her research, most believed “winter was a dormant season and there wasn’t a whole lot happening,” she says. But with soil warm enough for liquid water, life could go on. “That means roots and microbes can be active,” Contosta notes. “Even at very low temperatures.”
The subniveal neighborhood
During autumn, many trees in northern regions lose their leaves. This carpets forest floors with dead plant material. Those dead leaves are a buffet for bacteria and fungi. If they can stay comfortably warm in the subnivium, they can munch away all winter. “Most of what they do is either decompose leaf litter or decompose organic matter from the soil,” Contosta explains. By spring, when the snow disappears, the activity of all those tiny organisms will have transformed a carpet of leaves into rich, fertile soil.
Those tiny critters attract larger ones, including predators such as arthropods. These invertebrates include springtails, centipedes, rove beetles and more. They’re exothermic. That means their body temperature is the same as the environment around them. Above-freezing temps can allow them to stay active throughout winter.
“They’re down there feeding, moving around, searching for mates, breeding,” says Chris Ziadeh. He’s an ecologist at the University of New Hampshire in Durham and the U.S. Department of Agriculture.
Some species munch on microbes. Others hunt arthropods that go dormant under the leaves. “They’re just hanging out waiting for the snow to disappear and for summer to come back,” Ziadeh says. “That makes them really easy prey for all these predators.”
Ziadeh and Contosta are part of a team that wanted to find out exactly which arthropods were calling the subnivium home. To do that, they set out pitfall traps. These cups are partially buried in the ground. Filled with a hot pink liquid, they capture whatever wanders along and falls in. And that pink liquid? It’s a preservative. The researchers compared their subnivium catch to pitfall traps set in summer at the same sites.
Winter traps collected only one-sixth as many arthropods each day as summer ones. Some groups of arthropods went dormant in the winter. Others remained active. “It was predator-heavy in the subnivium relative to this summer,” Ziadeh says. And a few species were mostly — or only — present in the winter. These subnivium specialists included a spider (Cicurina brevis) and three types of rove beetles (Arpedium cribratum, Lesteva pallipes and Porrhodites inflatus). The scientists shared their findings last March in Environmental Entomology.
Those specialists could have an impact on the ecosystem all year, Ziadeh says. By munching on leaf litter, microbes and more, they put nutrients back into the soil. And dining on other, less active bugs, he says, “could keep down certain pest populations.”
The hungry arthropods are themselves important prey for larger animals. Small mammals like voles and lemmings hide under the snow, making a meal of the arthropods there. And they, in turn, attract their own predators.
Pauli became interested in the subnivium while studying American pine martens (Martes americana). These fluffy, ferret-like predators are about the size of a housecat. “They’re slinking in and out of that subnivium space,” Pauli says. “They’ll find an opening, and they’ll kind of go down and disappear and presumably hunt … then pop up at another spot.”
Even birds use the subnivium. Benjamin Zuckerberg spent a lot of time in New York’s Adirondack Mountains while in graduate school. “Grouse would kind of explode out of the snow,” he recalls. “Just out of nowhere, this big bird suddenly appears!”
Now he studies how ecosystems adapt to climate change at the University of Wisconsin–Madison. Ruffed grouse (Bonasa umbellus) and willow ptarmigans (Lagopus lagopus) live above the snow, he says. But they dig — or even dive — into drifts to roost, using the subnivium to stay warm.
An ecosystem gone cold
The subnivium is a temporary place, melting away in the spring. But its winter presence has a year-round impact on other ecosystems. Animals that find food and warmth here can live to see the spring.
Plants benefit, too. Tiny, delicate roots might burst if the ground froze. The warmth of the subnivium helps them make it through winter. By preventing root damage, plants “can be more successful during growing season,” Zuckerberg explains.
Decomposers have even broader effects. As microbes and fungi break down plant matter, they build nutrient-rich soil. “And while they do that, they breathe,” Contosta says. They take in oxygen and pump out carbon dioxide — a process called soil respiration. Some of the carbon from leaf litter gets stashed in their cells. “As long as those microbes stay alive, the carbon that’s in their biomass is part of soil,” Contosta explains.
A 2020 study examined how much snowpack affected the activity of microbes. Scientists dug some snow away from some areas and piled it up in others. A deeper snowpack led to a larger and more diverse population of microbes. Those microbes were more active with deeper snow, too. More microbes mean more respiration.
As the snow melts and spring arrives, those microbes die and release nutrients from their bodies into the soil — right when plants start to resume growing. “All of these nutrients, all of these carbon molecules, it’s ready for them when they wake up,” explains Kaizad Patel. “So in that sense, the microbes help regulate that.” Patel is a soil scientist at Pacific Northwest National Lab. It’s in Richland, Wash.
Hotter and colder
Climate change, however, is coming for the subnivium. A warmer Earth creates less snow.
Between 1985 and 2005, New Hampshire, for instance, had about two months of snowpack per winter, Contosta and her colleagues showed in 2022. But if carbon-dioxide emissions continue to increase, by 2100 the state will only have three weeks of snowpack per year that’s cold enough to build a subnivium. The team published its findings in the journal Northeastern Naturalist.
Oddly, warming that leads to less snow can actually make the ground colder. Snowpack acts like a blanket. The heat it traps keeps the subnivium just above freezing. If carbon emissions keep rising at the current rate, the presence of the subnivium might drop around the world from 126 days per year on average now to just 110 days by 2100. But there would be 10 more days every winter where the ground was frozen and bare. That’s according to a 2019 study in Nature Climate Change.
That frozen ground is bad news for subnivium dwellers. If soil microbes freeze, they’ll explode. That spills their nutrients into the soil during the winter, instead of spring. “That means the carbon and nitrogen that has been released in May is now being released in March instead,” Patel notes — “two months before the plants need it.”
Tiny roots can burst in frozen ground, too. Dying roots and fewer nutrients add up to a “double whammy” for trees, Patel says. Now weakened, trees may grow poorly or be more likely to fall prey to new diseases or insects.
Arthropods will suffer too, especially those most adapted to the subnivium. “They’re probably going to become locally extinct or just disappear altogether,” Ziadeh says. After all, that environment on which they depended — “it’s just gone.”
Even species that normally lie dormant through the winter might be harmed. In 2024, Pauli and Zuckerberg studied the cold tolerance of various insects. Then they used a model to see how the critters would be affected as global temps change.
If the planet warmed 3 degrees C (5.4 degrees F), the insects might be all right, the model showed. But warming of 5 degrees C (9 degrees F) would leave them exposed to dangerous cold. The researchers described this in a preprint at bioRxiv. (A preprint is a study that has not yet gone through peer review.)
Larger animals also rely on the subnivium. Hoary marmots (Marmota caligata) and pikas (Ochotona princeps) are both cold-loving rodents. Pikas scurry between boulders, nibbling grasses they stored in the fall. But, Aaron Johnston notes, “They do spend a fair amount of time moving around within the subnivium during the winter.” Johnston is a wildlife ecologist in Bozeman, Mont. He works for the U.S. Geological Survey Northern Rocky Mountain Science Center.
Pika numbers drop when there’s not enough snow. In one 2019 Ecology paper, Johnston and his colleagues looked at pika populations during years of low snow in a part of North Cascades National Park in Washington. In some areas with no snow, the pikas got too cold. Even in warmer areas, they had a tough time. Snow isn’t just insulating. It also melts to make water. Less water may have reduced the grasses that the pikas need to make it through to the next winter, Johnston says.
Unlike pikas, marmots hibernate underground in winter. But a lack of snow is stressful for them too, Johnston says. The subnivium normally keeps their burrows at around 5 °C (41 °F). At this temp, marmots have adapted stay warm using very little energy. But without snow, their burrow temperatures may may drop to a frigid 0 °C (32 °F). Now, marmots must use four times more energy to stay warm. If they run out of energy, they die.
Between 2007 and 2016, North Cascades National Park had several dry years with no snow. Marmot populations there dropped by 74 percent. Johnston and his colleagues shared their findings in a 2021 issue of Ecology and Evolution.
Finding refuge for the cold
The subnivium is disappearing, even as scientists work to better understand it.
To save it, the first thing that’s needed is to stop climate change. “It’s about keeping our winters cool,” explains Elizabeth Burakowski. She’s a climate scientists at the University of New Hampshire in Durham.
That’s something the whole world will have to tackle together. In the meantime, Burakowski is hunting for climate refuges. These are “small pockets of really unique, protected climate zones that preserve snowpack, that preserve cold,” she explains.
These colder areas could be on the north slopes of mountains. They might even lie behind large boulders. By warming slower than other areas, Burakowski says, they might allow patches of subnivium to persist.
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She’s looking for where these refuges form naturally. But she’s also interested in how we might make more.
The right number of trees in a forest seems to be key for snow buildup, for instance. “We think that there’s this Goldilocks zone,” she says. There needs to be “a thin enough forest canopy that more of the snow is reaching the forest floor, but thick enough that it’s also shading the forest floor.”
Burakowski and her colleagues are still looking for such refuges. If found, they could keep pockets of snow around for a bit longer, giving the subnivium — and its residents — a better chance at survival.