Migration of Vegetation during Ice Ages

Most People are familiar with maps of the major vegetation types of North America, showing a strip of tundra in the far north, a strip of coniferous forest immediately south of the tundra, and so on. It is easy to envisage similar maps showing the vegetation zones as they must have been at different times in the past, when great ice sheets covered much of the north. And it is not so difficult to visualize a motion picture version, with the zones creeping southward as the ice sheets grew and then creeping northward again when the climate warmed and the ice sheets slowly melted. It is so easy to visualize such scenarios that one is apt to forget that the southward shift of the vegetation zones as the ice expanded, and their northward shift as the ice contracted, were radically different phenomena. Obviously, vegetation does not "creep" in the way that strips of color on a motion picture map do; likewise plants cannot "migrate" or "advance" or "retreat" in the way animals can. To use these words in connection with plants is to speak in metaphors that may be useful as shorthand but are apt to disguise the true nature of what they represent.

The responses of vegetation to growing ice sheets on the one hand, and to shrinking ice sheets on the other, were entirely different. When an ice sheet expanded on of two things happened, depending on the cause of the expansion. If the ice spread because of climactic cooling, then the cooling would also have affected the vegetation ahead of the ice front. The less hardy plants gradually died off, and permafrost (perennially frozen ground...) formed, seriously inhibiting the growth of trees. In other words, a strip of tundra would have developed or, if one was already there, it would have widened, ahead of the ice. But if surging ice lobes caused the expansion, full grown forests would have been overrun by the ice, which crushed and buried them.

In either case, plants did not and do not "migrate." What does migrate is only an abstraction, a line on a map representing the margin of a vegetation zone. The plants themselves die.

From After the Ice Age: The Return of Life to Glaciated North America. By E. C. Pielou (University of Chicago, 1991) pp. 81-82