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Trees in Winter

01/07/04 12:00AM By Ted Levin
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(Host) How do trees survive severe cold? Recently commentator Ted Levin decided to find out.

(Levin) I wanted to know the wintering secrets of the sugar maples in our woods. How, for instance, do they survive the winter naked except for their thin robes of grooved bark?

So I settled down with a plant anatomy book and discovered that there is no such thing as "bark." Bark is a nontechnical term that applies to all the tissue outside the vascular cambium - the principal conducting tissue of a tree, a thin mesh of liing cells that surrounds the heartwood.

What I call Bark, I discovered, is made up of two distinct groups of cells: those alive and those not. The inner, living cells transport the maples' food. Each year as each of our maples adds a new growth ring, a foamy layer of old cells is pushed to the outside. These cells die. The foam hardens into fatty, waxy substances filled with pockets of air. Botanists call this distinctive outer covering the rhytidome. It is the surface of the tree exposed to weather. As lifeless as a sweater, it is our maples' first line of defense against the cold.

Avoiding internal ice damage is a maple's biggest winter challenge. How does a tree, any tree, which is more than 60 percent water, survive prolonged exposure to subzero temperatures? The answer: the trees supercool.

At -40 degrees Fahrenheit, ice forms spontaneously. As long as there are no dust particles to serve as nuclei for ice crystals, no ice will form until that very low temperature. So the nearly pure water in the cells of deciduous trees is supercooled to -40 degrees before freezing. In fact, the ranges of sugar maple, American beech, and yellow birch are determined by their cold tolerance, as a hike up Mount Mansfield confirms. First beech, than sugar maple cease to grow. Yellow birch, the hardiest of the three, enters the lower end of the spruce and fir zone, an ambassador from another realm.

All northern trees drain water from their cells into the intercellular spaces, so that ice forms between the cells - not in them. Some northern trees freeze-dry their cells by removing almost all of the water until huge crystalline masses of ice crowd the intercellular spaces. Paper birch and the red osier dogwood that pepper our wetlands can survive submersion in liquid nitrogen at -321 degrees Fahrenheit.

By March or April, with the unlocking of spring, intercellular ice melts, water is sucked back into the tree's cells and the normal metabolic processes resume. But when the trees are no longer cold hardy, they can be vulnerable even to a minor spring freeze.

This is Ted Levin of Coyote Hollow in Thetford Ctr.

Ted Levin is a writer and photographer specializing in hatural history. He spoke to us from our studio in Norwich.
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