Beneath the snow, a fascinating endangered ecosystem

This is what the Lapland Sami say “ skoavdi ». environmentalists, “ subnivium ». Get it: the thick layer of snow that many species take shelter in during the winter to survive freezing temperatures. A vital seasonal ecosystem that is little known and threatened today by climate change.

For many, the snow cover is just a sterile, homogeneous and silent space. A pile of shiny flakes to sink into a muffled snarl. Who would have thought that eagles hunt there, ermines run there, and hedgehogs hide there for months? ? “ We find extraordinary biodiversity in the subniviumJonathan Pauli, a professor at the University of Wisconsin (USA) and the author of several studies on the subject, explains. Some species, for example, lemmings [de petits rongeurs vivant notamment dans les toundras norvégiennes]it reproduces even under snow. »

A testament to the amazing vitality of this habitat, which travels 24 million km each winter² From the Northern Hemisphere: In 2015, a team of researchers hid cameras in the powder snow on the Norwegian island of Håkøya. Many mice, voles and other mustelids have been filmed frolicking in the snow. “ Small mammals build tunnels in the subniviumKimberly Thompson, a researcher at the German Center for Integrative Biodiversity Research, comments. Their cylindrical shape allows them to work efficiently. It’s like a hidden maze. »

Ermines move under a thick snow cover (“ subnivium “) Winter. Flickr/CC BY–CN 2.0/Bryant Olsen

Many insects, reptiles and amphibians also live in the subnivium kingdom. A species of plant that lives in western North America, the glacier lily even manages to grow there by catching sunlight through the ice crystals. Some birds, such as white partridges, like to hide there in very cold weather. And foxes wait for a long time on its surface to wait for the slightest noise that betrays the presence of its prey.

Why such interest ? “ When snow accumulates to a height of more than 20 cm, a network of ice crystals is formed, Kimberly Thompson explains. When they are well spaced, air pockets form. Heat released by the earth is found trapped inside. » Regardless of whether it is -5, -10 or -40 ° C outside, the temperature of the subnivium remains constant, about 0 ° C. “ This allows animals and plants to protect themselves from hostile conditions »the researcher continues.

“ It’s not snowing anymore »

Climate change threatens this balance. An article published in a scientific journal notes that since the 1980s, the extent of snow-covered surfaces in the Northern Hemisphere has decreased significantly. Environmental Research Letters Snowmelt is getting faster and faster: advancing by an average of two days per decade, according to a 2016 study. Average winter temperatures in France increased by 0.9°C between 1961 and 1990. Places located at an altitude of 500 meters have more than fifteen days of snow a year compared to two months in 1900.

“ Snowy droughts are observed in many parts of the world, Benjamin Zuckerberg, a professor at the University of Wisconsin and an expert on the effects of climate change on winter, says. The snow doesn’t fall there anymore, or when it finally does, it’s not deep enough. »

Birds such as white partridges will hide under thick snow in extreme cold. Flickr/CC BY–CN–HIM 2.0/Nick Athanas

At first glance, milder winters may seem like a boon for cold organisms. “ The problem is that there will always be severe coldthe researcher continues. But if there is little or no snow, the species will no longer have a subnivium to protect them from snow. Paradoxically, even if winters are generally warmer, species will be colder. »

Warming temperatures also lead to alternating snowy and rainy episodes in winter. “ It is especially badthe researcher continues. When the rain starts to cool again, the snow can melt and turn into ice. » Sometimes the birds stay under this frosty ceiling. When it rains, the snow gets denser. However, to maintain its insulating power, the subnivium must remain in the air. Very compact, the snow cover is colder and therefore less comfortable for its inhabitants.

It doesn’t snow as much as it used to, and the subnivium isn’t so thick that animals can hide there to protect themselves from the cold outside. Flickr/CC0/Shenandoah National Park

In a study published in 2021, Benjamin Zuckerberg, Jonathan Pauley, Kimberly Thompson and biologist Warren Porter showed that warming of more than 3°C in winter would spell the end of this habitat. At +5°C in winter—the current trajectory of temperate North America—the subnivium of the Great Lakes region would see its surface area decrease by 45%. % and its duration is one month. The authors believe that these results “ representative » the future of snow regions in the rest of the northern hemisphere. “ There is a form of resilience up to a certain pointJonathan Pauley comments. Exceeding the threshold of 3 ° C, the subnivium rapidly decreases with very serious consequences. »

Plant roots, frogs, deer, bees…

A 2017 study shows that without snow cover to protect them from the cold, plant roots can be severely damaged. Reindeer, which feed on grass hidden under snowflakes, will not be able to stay long under the tusks. Freezing the soil can also kill the microbes that live there. These play a role “ is important » in breathing mechanisms, explains Kimberly Thompson. “ This can increase the concentration of greenhouse gases in the atmosphere. »

Some animals may also suffer from reduced subnivium. They are species that have adapted to snowy winters for several million years “ often can’t keep up » current changes, observes Jonathan Pauli. Take the tree frog that lives in the forests of North America. At the first frost, it buries itself in the ground to conserve energy and becomes cryogenic. “ For this, it needs a temperature to keep it stable. », notes Benjamin Zuckerberg. Without a subnivium, this tiny amphibian risks being exposed to fluctuating temperatures and freeze-thaw cycles until exhaustion.

Unstable temperatures, for example, disturb tree frogs, who bury themselves in the ground during the cold season. Wikimedia Commons/CC BY–HIM 4.0/Ian Hodnett

Another potential victim: the bumblebee. Queens born in late summer enter the subnivium during the winter before departing to found a new colony in the spring. If the temperature of their shelter drops below -7°C, their death is certain. “ Subnivium temperatures are expected to drop below this threshold, or at least become more variable.Kimberly is afraid of Thompson. Their population is likely to decline in the Great Lakes region. »

The researcher emphasizes that in addition to climate change, winter sports are also under pressure. Like rain, snow blowers crush pockets of warm air that give the subnivium its insulating power. In the mountains covered with ski slopes, red-backed mice, martens and crows tremble more. Their stress level increases and thus energy expenditure. For several decades, scientists have feared that summer will turn into a silent day. It will be winter too ?