ECOLOGIST used to spend a lot of time keeping track of animals over winter – often across cold, harsh landscapes that seemed inhospitable to life. It always surprised him that as soon as the weather got warmer in early spring, insects would pop up. “Snow fleas would emerge from underneath the snow,” Pauli recalls. Where, he wondered, had they been hiding?
Eventually, he discovered some old scientific papers from the 1940s and 1960s. They revealed a secret world that Pauli, a researcher at the University of Wisconsin-Madison, has been studying ever since: a hidden ecosystem under the snow. It is found in a clandestine space between the snowpack and the soil beneath, which is sheltered from the bitter cold and is where some insects, spiders, frogs and even small mammals spend at least part of the winter. Concealed from the world above, flies buzz, plants thrive and animals forage, hunt and give birth in this so-called subnivium (from the Latin sub, meaning under, and nivis, meaning snow). But what will happen to this winter wonderland and all the creatures it shelters as the climate warms up? That’s the topic of Pauli’s most recent research.
Every winter, as the white stuff covers up to 40 million square kilometres of the northern hemisphere, the subnivium forms wherever and whenever the conditions are right. “There needs to be around 20 centimetres of snow that is not too dense,” says , also at the University of Wisconsin-Madison. Fluffy, light snow is best. As it settles on the ground, warmer air from the soil rises up, turning the snow at the bottom of the snowpack into water vapour. The resulting moisture condenses and refreezes on the cold layer above, creating a space above the soil that is usually a few centimetres high, with an icy ceiling.
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In this shallow nook, temperatures hover around freezing, even if the air above the snow drops as low as -20°C (-4°F). There is no wind, either. “It’s a refuge from ambient conditions that are much colder and more variable,” says Pauli.
This relatively mild environment allows soil-dwelling insects like springtails and beetles to stay active during winter, along with some spiders and adult flies. Frogs hibernate in the leaf litter, which is kept at a chilly but stable temperature. There is light, too, filtering through the blanket above. Often, it is bright enough for mosses and evergreen shrubs to carry on photosynthesising. Some plants are even known to flower under the snow.
Small mammals criss-cross the subnivium in domed tunnels, which allow them to roam and search for food. A few years ago, Eeva Soininen and her colleagues at the Arctic University of Norway and observed shrews, voles and other animals as they came and went.
Not only does the subnivium protect these small mammals against the worst of the cold, but it also hides them from many of the predators that prowl through the winter landscape. Although not every hungry beast is deterred. “Weasels actively hunt for mice and other rodents in the subnivium,” says Pauli. “They find an access point, maybe under a tree, where they can squeeze into it, then run underneath the snow.”
Bigger animals occasionally visit on particularly cold days or nights. “Grouses might fly up into the air, then dive-bomb into the snow,” says Zuckerberg. Often, they snuggle into the warmer base layer of the snowpack for hours, as do mammals such as foxes and wolverines, to shelter from the wind.
Melting point
As winters warm up, what will happen to the subnivium? Studies in North America show that, since the late 1980s, has decreased by 0.8 million square kilometres per decade, and the by about five days every 10 years since the early 1970s. Where snow still falls, there is often less of it, and more rain that packs it down into thinner, icier drifts. There are exceptions: parts of Scandinavia and eastern Europe have seen deeper snow in recent years.
Overall, however, the areas in which there can be a subnivium, and the time periods over which it can last, are shrinking. The subnivium itself is changing, too. Paradoxically, often make the subnivium colder, as Zuckerberg and Pauli reported in 2018. This is because there might be less snow to protect it and the snow that is there may become compacted by rain or sleet during mid-winter warm spells. Denser snow provides less effective insulation, says Pauli.
While some species will benefit from milder winters, researchers are already seeing that those animals and plants that rely on the warming effect of a thick snowy blanket will suffer. Yellow cedar trees, for example, grow all over the coastal, temperate rainforests of Alaska and British Columbia, Canada, and can live for more than 1000 years. But recently, their numbers have declined. Research has shown that these trees are dying off in areas where the snowpack has diminished. Without its insulation, the trees’ shallow roots are left exposed to any spell of frost, and they wither.
Winter-hardy amphibians will also be hit. Wood frogs, for example, survive the cold by letting their bodies freeze. They have special substances in their blood that protect their cells from damage, while their bodies turn into something “a little bit like an ice cube”, says Zuckerberg. In the stable microclimate of the subnivium, the frogs stay frozen until they thaw in the spring and resume their lives. But without this buffer, any warm spell will rev up the frogs’ metabolism, only to have them freeze anew when the temperature drops. Research suggests this will . Many of them are likely to die.
But a paper published in 2021 , too. To test how quickly future climate change will affect the subnivium, Pauli, Zuckerberg and their colleagues erected 27 greenhouses across the Upper Great Lakes region in North America. The greenhouses – some located in forests, others in open fields – had retractable roofs that the researchers could open or close to warm the air by 3°C or 5°C above the temperature outside.
They were amazed to find that 3°C of warming only shrank the spread of the subnivium by about 4 per cent and hardly diminished the duration for which it stuck around. “Contrary to our expectations, the subnivium was surprisingly resilient,” the scientists wrote in their paper.
However, this changed drastically when the researchers simulated a temperature rise of 5°C. Should this become reality, “we predict that the subnivium’s extent will decline by 45 per cent and its duration by over one month”, says Pauli. “There appears to be a tipping point that we really don’t want to cross.”
Besides stopping climate change, there are additional ways to protect this ecosystem, such as planting understorey shrubbery, which helps build airy snow cover, or reducing areas for snowmobiling and skiing. The latter quite literally eases the pressure on the subnivium.
If you live in a snowy place, you might catch a glimpse of this buried ecosystem this spring. As the snow melts, look for the markings of the tunnels as they emerge from the forest floor or fields – the remains of a seasonal refuge against the cold.
