A Walk Through the Winter Woods
A snowy stroll reveals some suspicious characters traveling on foot, and a secret world beneath them.
One hard swing of the axe split the log with a satisfying thunk. Loose bark fell from each half, exposing a spidery network of beetle tunnels etched into the sapwood. It's a familiar sight to anyone whose winter warmth depends on firewood, particularly if they've run short before spring and had to scrounge half-rotten dregs from a slash pile. Dead or dying timber attracts dozens of beetle species that lay their eggs in the protected and nutritious layer between bark and wood.
Previous experience told me this was the work of the Douglas fir engraver, a tiny beetle whose tracks radiate outward from a long, central chamber. Look closely and each winding trail widens as it spreads, enlarged by the growth of the constantly chewing larva inside.
I reached down to position another chunk of wood, and there lay one of the grubs, rudely knocked loose by my axe work. It twitched helplessly on the cold chopping block, a pale, maggoty thing that any non-entomologist would likely brush aside with a shudder. But I knew it held an important lesson in winter survival, and a single thought ran through my mind: Now's my chance.
It was a moment I'd been putting off for 15 years, ever since taking a graduate course from famed University of Vermont biologist Bernd Heinrich. With books like Ravens in Winter and Winter World to his credit, the unassuming, German-born Heinrich stands in the vanguard of scientists fascinated by cold. His winter ecology class explored the myriad ways animals survive frigid temperatures, from the true hibernation of black bears and painted turtles, to the controlled shivering of songbirds, to the way a wood frog simply freezes solid and waits for the weather to warm. Beetle larvae, he told us, make their own antifreeze. Lots of insects do. As temperatures drop, they produce quantities of glycerol and sugars that effectively lower the freezing point of their body fluid. "You don't believe me?" he'd asked the class. "Taste one. They're sweet!"
As a Peace Corps volunteer in Africa, I had eaten the occasional termite or fried grasshopper to be polite, but in general I don't think of insects as food. Steeling my nerve, I plucked the grub from the chopping block and popped it into my mouth. Once past the initial crunch, I knew that Bernd had been right. My taste buds registered a burst of nutty sweetness. It gave me a sudden insight into the lives of woodpeckers--in winter, apparently, every meal is dessert.
Any walk through the winter woods offers adventures that a leafy summertime hike could never provide. But getting past the silence and empty branches requires a different pace and a new way of seeing. Bernd gets there by plunging headlong into the season, living and studying in a remote cabin and doing what animals do to survive: If you get cold, run to warm up. If you're hungry, try eating grubs or a plate of fried voles. And if you want to find wildlife, track it down. Toward the end of Bernd's course, I devoted a whole day to doing just that.
Tracks in snow have a lot more to tell than merely who walked by; they let us picture who was going slowly, who was running, who was being chased, and whether or not they got away. Any snowfall tallies these dramas, but that day's conditions were perfect--a light dusting over older, compacted material. Behind Bernd's cabin I crouched down by the tiny prints of a deer mouse, its hopping gait made clear by the broad stance of the rear feet. I could even make out the faint trace where its tail had brushed down with every leap and landing. The mouse had been moving fast, and for good reason. Crossing open ground is always risky, but on snow a rodent's dark scurry stands out like a beacon to any predator. More than once I'd seen a mouse's tracks simply disappear, and then noticed the telltale brushstrokes of owl wings, framing its last step like feathered parentheses. Not that an owl is limited to prey on the surface: With asymmetrical ears tuned to differences in sound, large northern species like the great gray owls can triangulate the exact position of a tunneling rodent and snow-plunge as deep as 18 inches to make a kill.
This mouse had been lucky, dashing successfully from the base of a maple to a hole by a nearby stump, a reminder that in spite of how much goes on at the surface, an equally busy world plays out beneath the snow. Biologists call it the subnivean zone, a place that hardly anyone paid attention to until quite recently. Now studies have found that it's alive with grazing voles, seed-eating mice, predatory weasels, shrews, spiders, and vast mats of bacteria and fungi whose respiration emits enough carbon to affect climate change.
In fact, the subnivean zone can be a surprisingly cozy place. As survivalists and hibernating polar bears know well, a thick pile of snow makes excellent insulation. Winter nights in the North Woods can reach a frigid minus-20 degrees Fahrenheit, or colder. But where the snowpack is deep and long-lived, temperatures in the subnivean zone rarely stray from the freezing point, a full 50 degrees warmer than the air above. By trapping the earth's radiant heat at ground level, snow creates a stable, relatively warm environment that helps a wide range of animals, insects, and microorganisms stay active all winter. The timing and condition of the snowfall are critical.