Trees for a Cold Climate

Peering out the window of a high-flying airplane recently, I was able to study parts of Alaska under winter's simplifying mantle. Especially in the Interior, 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. Back on the ground, I did a little research on why so few tree types grow naturally in the neighborhood.

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.

When the ice sheets withdrew, trees that had continued living south of the glaciers could expand their ranges onto the newly exposed territory. The speed at which they moved depended partly on their tolerances for soil and weather conditions and partly on the mobility of their seeds. Maples, for example, dawdled northward from their refuge around the mouth of the Mississippi River at an average pace of about 200 meters a year. 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 isn't 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. Perhaps they'll make it over the border in time for Alaska's bicentennial---if they hurry.