I want to know the secrets of the sugar maples in our woods. I want to know how they survive the winter, naked except for their thin robes of grooved bark. I want to know how they survive the winter when the earth turns to stone and the wind drives needles of ice and snow. And I want to know how next spring’s sensitive new buds, encased in a coif of soft brown scales, survive more than three months of on-again, off-again sub-zero weather.
Like a sweater, bark is our maples’ first line of defense against the cold. Bark consists of two distinct groups of cells: those alive (the inner bark) and those not (the outer bark). The secondary phloem, conduit for transporting the maple’s food, is synonymous with inner bark. Each year, as our maples adds a new growth ring, a foamy layer of old phloem is pushed to the outside. These cells die. The foam hardens into fatty, waxy substances filled with capsules of air and becomes the bark we see that is exposed to the weather. This outer bark provides limited protection from sudden changes in temperature.
But not much. A tree, after all, is not heated from within, and since trees are more than 60 percent water, avoiding internal ice damage is a maple’s biggest winter challenge. How do they do it? The answer is that they rely on water’s ability to supercool.
At -36.6 degrees Fahrenheit, ice will form spontaneously in pure, distilled water. As long as there are no dust particles to serve as nuclei for ice crystals, pure water will remain ice-free until that very low temperature. Our northern trees make a point of keeping their internal water as pure as possible, and hence they can reach -36.6 degrees before freezing. In fact, the northern ranges of sugar maple, American beech, and yellow birch correspond roughly with the isotherm that indicates a winter minimum temperature of -40 degrees Fahrenheit. Beech actually endures a low of -42, sugar maple -45, and yellow birch -49.
These slight differences in cold tolerance are attributed to the antifreeze properties of the respective trees’ intracellular sugars. A hike up Mount Washington or Mount Mansfield quickly confirms this, as first beech and then sugar maple cease to grow. Yellow birch, the hardiest of the three, grows all the way up into the lower end of the spruce and fir zone, an ambassador from another realm.
But pure water and antifreeze aren’t the only tricks up our maples’ sleeves. In August, maples produce abscisic acid, a growth-inhibiting hormone that also increases the permeability of their cell membranes. With the first light frosts of September, the trees’ living cells begin to release water into the empty spaces between cells. When a hard freeze hits, ice crystals will form in these intercellular spaces, while the unfrozen water inside the cells’ cytoplasm continues to migrate toward the frozen water outside the cell wall. As more water leaves, the living cells shrivel like a deflated balloon, further increasing the concentration of dissolved solids within the cell walls. And the higher the concentration of dissolved solids, the lower the freezing point.
Ice crystals sometimes rupture the outer, stiff cell wall, indenting but not puncturing the inner, elastic plasma membrane. If the bottom should fall out of the mercury, however, and temperatures drop below the maple’s -45 degree threshold, as often happens on the cutting edge of the tree’s northern range, ice will form inside the cells. When the plasma membrane ruptures, the cell dies. When too many cells die, the tree dies.
Acid rain is making it more difficult for some trees to withstand cold because acidity strips calcium from the soil and calcium is essential for the proper functioning of cell membranes. Studies on red spruce have found a reduced cold hardiness among trees growing on calcium-depleted soils and among trees whose needles are exposed to acidic fog and clouds throughout the winter.
Some northern trees go one step further and nearly freeze-dry their cells by removing almost all their cellular water until huge crystalline masses of ice crowd the intercellular spaces. The paper birch and red osier dogwood that pepper our wetlands can survive submersion in liquid nitrogen at -321 degrees Fahrenheit and still recover upon thawing.
With the unlocking of spring, intercellular ice melts, water is sucked back into the maples’ cells (and, perhaps, into a waiting sap bucket) and normal metabolic processes resume. At this point, surprisingly, the trees have lost their cold hardiness, and even a minor spring freeze can damage them. Trees that withstood twenty five below in January can be severely damaged at twenty five above in May.