Trees That Can Withstand the Winter Cold

In the Interior of Alaska, the forests at this time of year show that few kinds of trees thrive here. The forests contain only a handful of families, white spruce, black spruce, poplar, birch, willow, with the odd tamarack or over-grown alder thicket providing scant variety. There was a little research on why so few tree types grow naturally in Alaska.

Winter's extreme cold easily eliminates some tree species hardy elsewhere. Oak, ash, and elm endure occasional severely cold temperatures in the contiguous forty-eight states because they can produce chemicals that serve as natural antifreeze. Thus, the fluids in their cells stay liquid down to forty below, that bitter temperature that is the same on both Fahrenheit and Celsius scales. However, at lower temperatures their sap will freeze, expanding, crystallizing, and rupturing the cells containing them. Thus, any typical Interior winter would kill trees that rely on antifreeze alone to survive deep cold.

The hardiest trees rely on physics more than on chemistry to make it through the winter. When the seasonal chill begins to reach black or white spruce, for example, the sap leaves their living cells and flows into intercellular spaces. There, ice crystals can form without damaging anything vital for the tree's survival.

Conditions underfoot matter, too. Few tree seeds can sprout in the wet and cold of muskeg, the naturally refrigerated swamps common in the lowland portions of the far north. Tamarack and black spruce are among the few species that can spring up and grow on this uncompromising ground. Then, too, trees living in the far north first had to get here. It wasn't that long ago, compared to the life of a species, that Alaska was covered with ice at the edges and was too dry for much tree growth in the middle.

White spruce, on the other hand, raced northward 10 times as quickly into the western Arctic, where they could live on raw mineral soil, the sort first uncovered as the ice departed. Their small, winged seeds traveled splendidly on the winds whipping off the shrinking Laurentide Ice Sheet that had occupied the middle of North America.

Summer temperatures also are important for determining which kinds of trees grow where, one reason why the mild-winter Aleutians aren't forested. After the ice sheets withdrew from what is now New England, spruces soon covered the land. Over a span of some 500 years, the average July temperature increased about two degrees Celsius, and pines replaced the spruces.

Nature is not yet done with Alaska's forest. Slowly but surely the pines seem to be advancing north and west through the Yukon. Jack pine and lodgepole pine can handle any winter temperature Alaska can offer, but they need a little more summer warmth to set seeds than spruces do. Thus, so far, individual pines can survive if people plant them in Alaska. but they haven't settled down and raised families here.

Yet the pine species have raced our way, at a couple hundred meters a year, for the last 12,000 years. They have adapted and evolved as they've come, so even without human- induced climate warming, they should be along shortly.

It is even more important to consider cold hardiness with trees and shrubs than it is with herbaceous perennials. For one thing, they don't die to the ground in winter. Whatever's going on in the frigid months, they have to deal with it. Also, they take longer to mature. Who wants to wait five years for a tree's first bloom, only to have it die the winter before it was to have been spectacular? And, these plants are more expensive, so if you gamble, there's more at risk.

As one becomes more familiar with trees in their winter aspect, the number that can be recognized at a distance becomes greatly enlarged. We come to know trees by hardly definable traits, much as we recognize our friends at a distance by some peculiarity of form or gait. Watching the trees from a distance is a great help in acquiring this familiarity with tree characters. The method of branching and other features do not furnish such precise marks as do the twigs, and cannot therefore be of much value in a descriptive key. In fact the habit varies considerably among individual trees of the same species, no two trees having exactly the same method of branching.

Trees grown in woods in company with other trees are prevented by shading from the side from developing their normal form and produce tall trunks with only a little branching. On the other hand trees apart from other trees have usually been planted for ornament or have originally grown in woods, but have been left isolated by the cutting down of their neighbors. In the latter case the habit will be more or less that of a forest-grown tree depending on the age at which the conditions of light and shade were altered. In the former case the top of the young tree may have been cut in the process of transplanting, causing an increased branching at the point of cutting and the lower limbs may have been trimmed off, giving a greater show of trunk.

These mutilations, however, have less influence upon the outline of the head or crown than might be imagined since the tree is generally able to accommodate itself to such accidents and express its individuality despite them. The age of the tree is also an important factor in the outline, young specimens being in general narrower and more conical than in later life, while those in old age may have lost shape through ice storms, high winds and the attacks of fungi.