How Fast Can a Spruce Tree Travel?

We don't usually think of trees as being mobile. They grow where their seeds sprout, putting down roots in a way that has become a metaphor for stability. On rare occasions, a landslide may move a block of trees downslope, and the trees may survive the move and continue to grow, but this is about the most drastic move a tree would ever make without human help.

Yet thanks to their seeds, trees as species do move. Eleven thousand years ago, apparently no spruce grew in Alaska. The great continental ice sheets were melting away, but the landscapes emerging from the ice, as well as those poleward of where the ice sheets had reached their northern and western limits, were covered with herbs, shrubs, sedges, grasses and a few poplars. There is no evidence that spruce trees had survived the cold, dry climate that had dominated the area for almost twenty thousand years.

South of the ice sheets, however, spruce had covered much of what is now the Great Plains. As the ice retreated and the climate warmed, spruce trees died of summer heat, drought and fire in the southern and western part of their ice age range, and at the same time new seedlings grew into trees in the lands emerging from the ice in southern Canada. By around ten thousand years ago, spruce had reached central Alberta.

The next step was a truly explosive spread of spruce northward to the Arctic Ocean. Within a thousand years, spruce trees advanced over two thousand kilometers (1200 miles) to the MacKenzie Delta. The continued movement westward into Alaska was somewhat slower. Spruce reached the Tanana Valley around 9500 years ago, but continued westward rather slowly, reaching its current western limits only around five thousand years ago. Spruce is still moving in Kodiak.

We have to consider the advance of the spruce forests in terms of averages and of the tree's biology. Suppose you moved your household five hundred miles westward over twenty years. Your average rate of movement is only twenty-five miles a year, and if you were packing a tent and moving a little every day, you would need to move less than a tenth of a mile a day. But if you move only once in twenty years, you must travel the five hundred miles in one trip.

Spruce trees have to travel in jumps measured by how far from its parent tree a seed germinates. Since a spruce has to be about twenty years old to produce healthy seed , the distance traveled by at least a few seeds must be twenty times the distance that the forest boundary moves in an average year.

For most of the eastern and central United States, spruce was moving north at two to three hundred meters a year, so a few cones or seeds must regularly have traveled 4 to 6 kilometers. Since the speed of advance of spruce was fairly consistent over this wide area, and matches fairly closely the rate of spread of spruce across Alaska, 4 to 6 kilometers is probably a fairly common distance to which live spruce seeds were transported by wind and animals.

Up the corridor between the dwindling ice sheets over the Rocky Mountains and eastern Canada, however, individual cones or seeds must have traveled forty to sixty kilometers with some regularity, and we are not sure how this happened.

One theory is that spruce actually survived in the MacKenzie Delta area (which was not completely ice-covered) or in ice-free enclaves along the junction of the two ice sheets. But none of the many pollen records from this period north of the joined ice sheets shows spruce pollen.

Another is that seeds were blown along the surface of the winter snow cover by strong south winds. There is independent evidence of strong south winds, in the form of sand dunes and from climate models. But could seeds be transported regularly for distances of twenty-five miles in this way?

A third possibility is that the seeds traveled by water, and I find this one the most believable. The fastest part of the northward spread was down the MacKenzie Valley. It is fairly easy to visualize an undercut bank, with perhaps a whole clump of spruce falling into the MacKenzie, swollen by glacial meltwaters and possibly even by the outbursts of ice-dammed lakes. A hundred miles or more to the north one of the uprooted trees, with ripe seed in the cones at its top, might come aground on a bar below another bank. A sand or gravel bar is not the best place for a spruce tree, but a rodent collecting a winter food supply might easily enough have been responsible for the final few hundred feet uphill to a good germination site. Perhaps spontaneous teamwork between floods and ground squirrels hastened the northward march of spruce forests.