A bright, sunny day in mid-winter in the northern U.S. makes it look like a good time for a walk in the backyard — but, one step out the door and I know it won’t be fun at all. The air temperature is -7 F, and there is a stiff wind blowing. This makes for a very short walk, snap a couple of photos, retreat indoors again. Why bother? Because I got to wondering how trees manage these sub-freezing conditions. Obviously, standing still in this kind of weather would be lethal for any warm-blooded creature, so how can trees withstand freezing solid for six months of the year? Or do they?
And what happens on warm sunny days when half of the tree is subjected to bright sunlight while the other side remains in the dark? Is there freeze and thaw going on?
The short answer is that they don’t actually “freeze solid”, because the same changing light (decreased photoperiod) and fluctuating temperature conditions in the fall that bring on that wonderful display of fall color also induce physiological changes in plants called “cold hardening” that prevent freezing.
The strikingly white bark of birch trees serves a useful purpose in the winter by reflecting a lot of the sun’s radiation on bright, sunny days, and thus preventing the unequal heating of the exposed and unexposed sides of the tree.
Photoperiod and temperature signals in the fall cause plant cell membranes to become more permeable and flexible. Sugars produced by the leaves move down to storage in the roots, and water follows the sugar movement, so cellular contents become much more concentrated. So concentrated in fact, that they lower the threshold for freezing dramatically, to -30 F or more. In addition, cells produce protective cryoproteins that act like potent antifreeze agents. Residual water trapped between cells may freeze, but the now shrunken and flexible cells remain uninjured, and ready to restart their metabolic engines when spring weather thaws the ground, the roots take up water from the soil, and the sap rises in the plants’ fluid transport vessels (xylem and phloem).
Rough textured bark also serves a useful purpose, beyond providing a foot-hold for the squirrels. It can’t serve as an insulative blanket like a warm coat of feathers or fur does, but corrugations of bark absorb the radiant heat of winter sun, expanding and contracting in heat and cold, without affecting the underlying layers that might crack with exposure to the sun.
In effect, trees and other plants that survive the sub-freezing conditions of northern winters are in a static state of super-cooled dormancy, still liquid and viable, although metabolically quiescent. Waiting…