It's All Happenin' Below the Snow

I’ve been thinking a lot lately about a very special, ephemeral habitat.  It forms in the winter, is home to many small mammals and invertebrates, it helps plants survive the deep freeze, and, it turns out, it is also vital to carbon sequestration.  It is the Subnivean Zone.

I thought I knew enough about this microhabitat to create a pretty good introductory learning experience for our next Science Saturday (Jan. 27), but, as with all things science, it is always good to do a little research to see if anything new has been discovered since the last time I read up on the subject.  And then I thought to myself:  Self, you know about the importance of the subnivean…but with less and less snow cover, and very cold temperatures like we’ve been having these last few days of 2017 and early 2018, how are the animals that depend on the subnivean surviving?  Hmmmm.

I sent an email to a small mammal ecologist I know asking if she knew of any current studies.  She’s on vacation right now, so I also turned to the internet to see if there was any research going on.  Wow – have I been enlightened.


The subnivean layer (from the Latin “sub” meaning below, and “nives” meaning snow) has long been known by the Inuit.  They call this special snow zone Pukak:  it is the space that forms between the ground and the snow pack.  When the first snows fall, vegetation bends over creating supporting arches that hold the snow up.  The subnivean doesn’t really get created, however, until there is a minimum of six to twelve inches of snow.  Fluffy snow.  Fluffy snow is full of air pockets, and air pockets mean insulation:  the snowy layers above the ground now protect the ground from the bitter winds and temps that swirl above.  Between the heat radiating from the Earth (even when the ground is frozen), and the insulating qualities of the snow, the subnivean zone stays at, or near, a toasty 32 degrees Fahrenheit.

This may not seem balmy to you or me, but to those animals that live there, it is quite acceptable.  It is warm enough for them to remain active throughout the winter.  Mice, voles and shrews create vast networks of tunnels through the arched vegetation (surely you’ve seen these tunnels and trails after the snow melts in the spring).  A keen eye can also pick out “kitchens” and latrines – all sorts of chambers where daily life takes place.

The mice and voles are eating plants:  seeds and grains they stored over the fall, and still-green vegetation that is also hanging on in the mild climate of the subnivean.  Shrews are shrewd hunters, going mostly after invertebrates (beetles, worms, etc.) that they find hunkered down in the leaves and spaces under logs and rocks, but they will also take another small mammal if the opportunity presents itself, especially the short-tailed shrew, which is famous for its venomous saliva (that is a blog for another day).

Now, all of this I knew…no news there.  It was, however, when I started to read about the biota of the soil and leaf litter that I realized that there is more to this subnivean layer than I suspected.  Than anyone suspected. 

Let us begin by considering what happens when there is no snow cover, or the wrong kind of snow cover.  Recall that I said the snow had to be fluffy; you get fluffy snow when the temps are cold.  With climate change we are getting heavier, wetter snows more and more frequently.  These snows are much more compact, which means there are significantly fewer air spaces.  This means the snow has less insulative properties, which means the subnivean will no longer be warm.  It also means that there is less air exchange, resulting in a build-up of carbon-dioxide, which can suffocate the animals that are living there.

This also has an effect on plants:  no insulating snow means that the ground freezes harder and dries out.  Roots dry out; roots freeze.  Plants suffer the stress of these conditions and if it happens long enough, the plants will die.  We’re not just talking dandelions and ferns; we’re talking trees, shrubs…our forests and prairies.

And then there are the microorganisms, the things that live in the soil. 

Microorganisms include bacteria, fungi, and protozoans.  Toss in the small invertebrates, too.  Collectively, these are the tiny beings that, truly, make life on this planet possible. 

We tend to think of winter as a time when the “earth sleeps.”  It is easy to see why:  trees and shrubs have dropped their leaves, insects have disappeared, many birds fly south, and a handful of animals hibernate.  The world seems quiet and empty.

The thing is, it’s all happening beneath the snow!  Winter is when things really get hopping underground – great biological activity is going on and we are oblivious to it.  And all this activity is creating lots and lots of CO2.  Paul Brooks, at The University of Colorado Boulder, is one of the handful of people doing research on this, and he has discovered that the amount of CO2 that is released into the atmosphere is directly tied to the amount (and type) of snowfall.  In a nutshell, if things freeze before there is enough snow to provide insulation, microbes (et al) feast on the ruptured tissues of the plants and critters.  The more plants and organisms that die, the more tissues that are ruptured, which means the microbes go to town…and the more active they are, the more CO2 they produce.  Brooks’ research has shown that these trends can release 25-200% more CO2 than “normal.”

See that:  one more piece of evidence that climate change is doing irreparable harm.  Without adequate snow, fluffy snow, creating the subnivean zone, protecting plants, small mammals, invertebrates and microorganisms, we get yet another source of CO2 releasing into the atmosphere.

Many folks moan about the snow – they don’t like the cold, they don’t like shoveling, they don’t like driving in it.  But, y’know, it’s not about us.  It’s about this spaceship we call Earth, the only home we have.  And snow, which supposedly covers 40% of our landmasses most of the year, is vitally important for our continued survival.  It’s not just that the snowpack provides the water that keeps ecosystems from becoming deserts; the picture is so much bigger than that. 

This is why I love science.  I love that we have curious minds that ask questions and then set about trying to find the answers to those questions.  I love learning about all the intricate ways that everything on this planet is connected.  I love learning about new things that make me think “gee, I never thought of that before!”  And I love that learning these things allows us to imagine new ways to work with nature.

I hope you do, too, and that you will join us once a month this year for our Science Saturdays, where we hope to share some new science with you that opens up new realms of inquiry and fascination in your minds.  See you January 27th!