Birch Trees Bending

Published in the York Weekly, Portsmouth Herald, Foster’s Daily (and more) Dec 14 2020

I went up to Blue Job Mountain State Park for a walk last weekend.  There was a lot more snow up there than where I live in North Berwick.  Lining the parking lot were the birch trees, bent over, touching the ground, with their heavy loads of icy snow.   Hiking up to the summit of Little Blue Job was an obstacle course as we worked our way around all different types of trees, some were flexible like the birch and bent by the snow, some had snapped.  

Birch and beech saplings on a snowy afternoon

And they seem not to break; though once they are bowed.

This got me thinking about birches.  They are known for bending and not breaking.  Most of us read Robert Frost’s poem about boys swinging on birches in high school, but there is a part about the birches themselves  “Shattering and avalanching on the snow crust—Such heaps of broken glass to sweep away.  You’d think the inner dome of heaven had fallen.  They are dragged to the withered bracken by the load,  And they seem not to break; though once they are bowed.” where Frost talks about how birches bend and aren’t broken by the snow.  He talks about how sometimes they stay bowed after a long winter “So low for long, they never right themselves:  You may see their trunks arching in the woods.  Years afterwards, trailing their leaves on the ground.”  The sad thing was, people had made it worse by walking over their heads, cementing them into the icy trails.  Many were snapped by the weight of feet and all it would have taken was that first person on the trail to free the birches from their icy load, pull them out of the snow and let them spring back up towards the light.  

Adaptations to the North

This ability of birches to bend is an adaptation to living in the north.  We have a number of different species of birch in New England, among them, paper birch (Betula papyrifera), is one of our most widely-distributed trees, found from Newfoundland west to British Columbia and south to New York and South Dakota.  It is also one of a handful of broad-leaved trees that can live in the far north.  It  can live so far to the north because of those flexible branches.  Its northern neighbors, balsam fir and hemlocks, have a different adaptation to the same conditions and are cone-shaped; their long sloping branches help snow slide off instead of collecting on the branches and causing them to snap. 

Hemlocks conical shape lets snow slide off (eventually!)

Flexibility isn’t a paper birches’ only adaptation to northern climates.  During cold winters the thick, dark bark of an oak or ash becomes a liability, absorbing sunlight during the day and heating up, only to cool down again, usually quite rapidly, at night. This heating and cooling can kill the cells of the cambium, the layer of cells between the bark and the wood that is responsible for the growth of the trunk. Rapid temperature fluctuations can also severely injure a tree by causing frost cracks to form in the bark.

In contrast, the highly reflective, light-colored bark of a paper birch doesn’t absorb the sun’s radiation and heat on cold winter days, and so avoids the damage caused by rapid heating and cooling.

What causes this extreme whiteness? That white powder that coats the bark is primarily composed of a chemical called betulin. The cells in the outer layers of bark contain betulin crystals that are arranged in such a way as to reflect light and appear white.

Free the Trees!!

My walking partner is a tree enthusiast.  She felt sorry for all those forlorn birches bending under their heavy loads, and even more sorry for the birch tops that had been cemented into the trails by uncaring feet, or even worse, those that had snapped due to this trammeling.  She started clearing as many as she could.  Pulling the tops out of the snow and letting the trees spring free.  It was exhilarating and infectious to watch.  I joined in and we spent more of our time freeing birches than walking.  I don’t know whether this really will help the trees survive the winter, but figure it can’t hurt.   I’ve read that Frost once said “it was almost sacrilegious climbing a birch tree till it bent, till it gave and swooped to the ground, but that’s what boys did in those days”.  I agree, it is sacrilegious, and feel like now we should know better.  Trees have a hard enough time these days, if you see one struggling with the snow, why not help it out?

Stephanie Eno clearing snow from bowed maple and birch saplings

In winter’s cold, the subnivean zone is abuzz

Published January 21 2020 The York Weekly, Portsmouth Herald and Foster’s Daily and at Titled ‘Nature News’

The subnivean zone lies between the snow and the earth.

This winter has been troublesome; a few weeks ago it was so cold my pipes froze, then it we climbed to unseasonably warm temperatures, then some more snow, and now, very cold again-my pipes continue to freeze. One thing that makes me happy is that we have a nice layer of snow on the ground, insulation that is is vital to the survival of many of the plants and small mammals, even microscopic life forms, that are essential parts of our Northern ecosystem. So, for many, the recent snow was welcome relief from the cold.

While the surface seems empty, underneath, at the interface between ground and snow, a veritable city is rising. Old leaves and branches hold the snow up, creating an open space. The ground also radiates heat, warming the overlying snow, which instead of melting sublimates (when a solid turns into a gas without first becoming a liquid) directly into water vapor. Both of these processes leave a space between the snow and the ground, called the subnivean zone.

The word “subnivean” comes from the Latin for under (sub) the snow (nivean) and is a scientific term referring to the open passageways that form under deep snow.

Six or more inches of snow are all that are needed to trap the earth’s heat and allow a subnivean zone to form. This zone remains humid because of the transformation of snow into moist water vapor and is capped by a layer of ice that acts as an insulating roof. The temperature of the subnivean zone is generally a constant 32 degrees, protecting species that would otherwise freeze.

The most common inhabitants of the subnivean are mice and voles — they make tunnels under the snow connecting sleeping areas and sources of food. Red squirrels also burrow into the subnivean to stash food. Entrance holes to these networks allow carbon dioxide to escape. Carbon dioxide is released when these animals breathe and by the ground itself. Without the entrance holes to serve as ventilation shafts, carbon dioxide could build up to lethal levels.

While the subnivean zone provides protection for mice and voles, it is also the hunting ground for the short-tailed weasel (or ermine), a weasel that, except for its black-tipped tail, turns snowy white in the winter. This small weasel, just the right size to live and hunt in the tunnels under the snow, is a major predator upon small rodents.

All you need is about 6 inches of snow to allow the subnivean space to form

Recent research has unearthed a whole new category of inhabitants of the subnivean zone — microbes that are proving to be important players in the cycling of both nitrogen and carbon dioxide between the earth and the atmosphere. A deep snowpack with an active subnivean zone appears to encourage a healthy microbial population that, through respiration, releases significant amounts of carbon dioxide into the atmosphere. Studies suggest that as much as half of the carbon taken up by plants in the summer is released back into the atmosphere by microbes in winter (Paul Brooks, University of Colorado Boulder). At the same time these microbes, by processing and storing nitrogen, fertilize the soil as the snow melts.

Without a healthy snowpack, without these snowpack microbes, plants don’t do as well come spring. While these microbes may be releasing a lot of carbon into the atmosphere during the winter, they are also necessary for healthy plant growth — plants that will absorb carbon dioxide throughout the growing season.

For all these reasons, it seems to me, a nice deep snow, serving as an insulating blanket on the earth, is a welcome part of our northern winter.