Retreating Glaciers, Advancing Plants
Last year’s birthday present from my husband was a trip to Glacier Bay—a glorious present, not nearly as fattening as chocolates, much better than new clothes. Probably that trip was on my mind today because it’s looking a little glacial around the house, with crusty, icy new snow covering the trees.
More likely, it’s on my mind because snow emphasizes that the trees and other plants near our ridge top home are different from what they were when we first came to Alaska, just as are the plants in Glacier Bay. Forty years ago, the north-facing slopes high on our hill were more tundra than taiga, with much moss and lichen, low wind-battered shrubs and brush, and almost no trees. Now willow and alder obscure the view, and spruce forms occasional clumps twenty feet tall. Thirty years ago, the forest creeping north along the shores of Glacier Bay had almost obscured the cabin John Muir had built there, on nearly bare rock, well before the Klondike gold rush. Last year, even Muir’s substantial stone chimney had vanished in the greenery.
“It’s right around here somewhere,” my husband said, remembering his visit to the bay aboard the U.S. Geological Survey vessel in the early 1960s. But it was gone beyond our ability to find.
For immobile creatures, trees can move swiftly. When European explorers first sailed by Glacier Bay, less than three hundred years ago, there was no bay to speak of. Glaciers filled all the channels reaching down from the mountains, and it looked as if they had always done so. The Tlingit living in the neighborhood had stories of the days
when the ice was not there, but the explorers suspected the stories were myths or legends, perhaps poetically but not literally true. Then the new people had a couple of centuries in which to watch the ice pull back, and the trees race forward.
With the ice gone, alders deserve some credit for the changes in Glacier Bay. These tenacious plants fix nitrogen—that is, they can pull this vital nutrient out of the air, where it is abundant, and transport it to their roots and the helpful bacteria that enable them to use it in the soil, where it is scarce. In fact, because of this ability, alders can even grow where soil itself is scarce. They can invade cracked rock and pebbly surfaces, building their own soil—which other plants can then use—with shed leaves and broken bits that molder into nitrogen-enriched dirt.
Yet clearly, the chief reason for the return of the rain forest to the bay’s shorelines is –in effect--that the climate changed. What else can you call the comparatively sudden absence of the ice? That trees can return swiftly when glaciers retreat has been appreciated only relatively recently in Europe. (Retreating glaciers there, at least during historical times, lie high in the mountains where forests would not soon grow.) Studies published last year documented that some tree species returned north from their refuges on the Mediterranean at a good clip, up to a kilometer—better than half a mile—a year. Even oaks managed to return to England at the stately but swift (for a tree) speed of 500 meters a year.
I think Alaska’s forests advance even faster, given the right conditions. For example, if the present estimates that the local climate is about a centigrade degree warmer than it was at the turn of the century are correct, then this hilltop has in effect been moved miles southward, as far as the plants are concerned, or shifted a few hundred feet lower. Apparently, that’s sufficient to let trees move into the tundra over the span of forty years.
