November is a time for transitions
It’s been a while but as memory serves, the football somehow had a different smell to it in mid-November. Pungent, dovetailing with the burning curbside leaf piles the Dads were tending all along the street (this was suburbia in the early sixties), and a fitting counterpart to the half-frozen mud griming our faces and hands.
At the bottom of the pileup we scrabbled for the ball as minute crystals filtered down from a slate sky, gathering on our eyelashes.
November is steeped in traditions of transition. Putting up the garden hoses in the shed, bringing shovels up from the basement and gloves and scarves from the inner recesses of the closet. Sliding the storm windows into position. Steeling ourselves. Waiting.
But it is also an exhilarating time. Thanksgiving is just around the corner. And we know that soon our breath will steam as we pause for a moment in a forest of sleeping oaks and maples, to marvel at the immaculate white expanse fitted snuggly to the feet of each tree, shrub and sapling.
It’s true enough that many “cold-hardy” plants deal with the onset of freezing temperatures by entering a protracted period of dormancy. But that’s not to say they’ve slid into some kind of suspended animation for the winter. Battling the ice demon requires constant vigilance.
And it is all about the ice, which is a potentially lethal danger to plants in two principal ways. As the outside temperature drops below freezing, water within and between living cells (intracellular and extracellular fluids) begins to crystalize. Those crystals are blade-sharp and can pierce cell membranes as they expand.
In some ways the second problem mimics the challenges a tree would face during drought conditions. Water turning to ice in extracellular spaces and within the fluid-transporting tubules running along the trunk from roots to leaves (xylem and phloem) tends to pull too much water from inside the cells, desiccating them.
Roots are at even greater risk as surrounding groundwater freezes. Fortunately, mid-winter soil temperatures tend to be warmer and less variable than air temperatures, especially when insulated by a blanket of snow. Still, roots’ ability to send water up the trunk of a tree to its myriad leaves, if not completely halted will at least be greatly curtailed as the thermometer falls.
Water, along with carbon dioxide from the air and sunlight, is one of the three necessities leaves require to perform photosynthesis. So it’s not so much the cold itself that makes it advantageous for deciduous trees to shed their leaves in the fall, but the coming lack of water–effectively a winter-long drought.
Seems ironic, with perhaps a foot or more of water (frozen) blanketing the forest floor. But what about the ever-green conifers? Don’t they face the same problem?
Yes, but confers address the challenge of cold in a different manner. It’s not accidental that the boreal forests stretching across Canada and northern Europe primarily consist of spruce, fir, pine and larch with a smaller scattering of birch and aspen. These regions receive less direct sunlight than temperate regions further to the south and have a much shorter growing season.
Although deciduous trees do a good job of pulling nutrients (primarily nitrogen, magnesium and phosphates) from their leaves prior to leaf-fall, most species still need a longer growing season and more fertile soils (to replace lost nutrients) than the shallow, acidic soils the northlands typically offer.
Unlike deciduous trees that shed and regrow all their leaves each year, most conifers (the larch is an exception) only shed a proportion of their needles in a given year. They take more time to tap the nutrients required for needle production from the relatively poor boreal soils and keep them, once made, for a longer period.
But the confers’ greenness is deceiving. Their thin, fibrous needles remain in a greatly reduced state of activity throughout the cold season. On a cold day, a white pine’s photosynthetic rate will only be marginally greater than that of a leafless red oak ten feet away.
To fight the ice within their bodies, both deciduous trees and evergreens produce a suite of compounds to, as much as possible, block it from forming in the first place. Living cells accumulate a variety of solutes (principally sucrose and various organic compounds) which, like salt on a sidewalk, lowers the freezing point of water and keeps ice from forming down to about 20 degrees.
In extracellular spaces, an array of antifreeze proteins (AFPs) bind to ice crystals, inhibiting their further growth. Even the chemical composition of cell membranes is modified so as to maintain their fluidity as the winter advances.
That fluidity is essential. The best way to keep water from crystalizing within a cell is to move as much of it as possible (without damaging the cell’s functions) out of the cell. To do so requires a cell membrane stabilized against the cold by a diligently maintained mix of sugars, proteins and enzymes.