March 17, 2019: The Subnivean Zone

A friend recently mentioned that the raptors - owls and hawks included - are having a hard time getting food this year due to the deep snow which allows their primary food sources such as mice, voles, and red squirrels to take cover in a well fortified subnivean zone.  That may sound like the secret hideout of some evil creature from a Star Wars movie, but subnivean is a scientific term that comes from latin for "under" (sub) and "snow" (nives);  it refers to the narrow gap that forms between the forest floor and the bottom layer of snow.  [Much of the information here comes from the article "The Subnivean Zone:  Shelter in the snow" written by Barbara MacKay and published on December 26, 2014 in the Burlington (Vermont) Free Press .]

Subnivean Zone Illustration by Adelaide Tyrol from the above referenced article

As the early snows fall at the start of the season, the still-warm earth sublimates the bottom layer of snow which rises and condenses on the cooler surface above, then freezes, creating a sturdy ceiling that can hold up the winter's later snow.  Low plants also help to support this snow/ice roof.  It may only be an inch or so high, but that's enough to get things started, and the rodents will build intricate tunnels from there.  And it provides them everything they need - food from seeds and grasses on the ground, insulation from the cold of winter, and protection from predators.  This winter I have seen very few mice tracks up on top of the snow, which indicates to me they are fully satisfied down there under the snow.

Mouse tracks on the snow have been few and far between this winter

Usually fox can still access these animals using their strong sense of smell, and owls can track them down using incredibly sensitive hearing.  Both animals can attack through a reasonably thick layer of snow, but this year's repeated icing and deep snow have made these hideouts pretty much impregnable to all but the ermine.  (The ermine is slender enough to climb down the air holes into the subnivean zone, and eat to its heart content, sometimes taking over the area for itself.)  With all this in mind, I kept an eye out this week for signs of these secret hideouts.  I was able to find a couple, one under a large fallen limb and the other at the base of tree.

A classic entry hole to the subnivean zone

This one is a text-book subnivean den:  built on the southern side of a dark tree where the sun will help to keep things warm and melt some snow to provide a bit of water.

Plenty large enough hole for an ermine attack

I didn't dig it up to see what lies below (that seems rather cruel) but did note the location, and maybe I'll stop by again as the snow melts down to ground level to see what has been created down there.  In the mean time, we'll continue to do our part by fattening up the gray squirrels who live in the high-risk zone above the snow.

On this same walk I found a set of snowshoe hare tracks right on the lake, coming out from a hemlock stand.  This is another prey animal that all the predators still have access to, though I don't see many signs of hares around the Lakes Region.

Snowshoe hare track near the Harris Conservation Area

I also came across a set of fresh porcupine tracks;  following them for a stretch I saw that it had checked out a beech tree but decided that wasn't its cup-of-tea, so it moved on.

Mr. Porky didn't like this tree

It's next stop was a hemlock - always a favorite.

Here he found something to his liking

And there were ample signs that he dined here.

Hemlock debris dropped

Whenever you find this kind of hemlock debris on the snow you know a porcupine has been up there cutting off and chewing (not very carefully) on the tender branches.  He must have eaten his fill, as after this stop he retraced his steps right back along the same path he came in on.

Two sets of tracks, in and out

Finally, on this same trek I came across a recent blowdown with some very interesting markings on the wood inside where it split open.

I was fascinated as well as perplexed as to what could cause these markings deep inside the trunk of a tree, so I sent some pictures off to Shaun Lagueux and Andy Fast, our town and county foresters respectively, for some expert input.  They both immediately responded, saying it is spalting due to a fungus.  I'll have to do some research on this and will report what I learn next week.

Intriguing marks deep inside the trunk of a hemlock tree