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The Ice Age Roots of American Forests
| Article
# : |
17906 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
3 / 1990 |
2,848 Words |
| Author
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Paul A. Colinvaux
Paul A. Colinvaux is a professor in the departments of zoology
and anthropology at Ohio State University in Columbus. His
specialty is reconstructing the history of plant communities
and climate of the Ice Age. |
The latest work of pollen analysts suggests that all plant communities are in continual change. None are permanent, not even the tropical rain forests.
The most striking evidence for this view comes from reconstructing the history of the broad-leaved forests of eastern North America during and since the last Ice Age, which ended just 14,000 years ago. These American forests are as rich in variety as any of their kind on earth, being equaled only by those of eastern China. Both have oaks and maples of many kinds; beeches, chestnuts, hickories, walnuts, black gum, sweetgum, sassafras, and the glories Lyriodendron, the tulip poplar.
All these trees are ancient, because we find impressions of their leaves and fruits in Tertiary rocks (65-2 million years B.P.) made out of sediments that collected millions of years ago. In those days, they grew in forests all over the north-temperate belt, not just in their present American and Chinese homelands. A Soviet paleobotanist on a field trip in America was heard to mutter in wonder, " I am walking in the Miocene (25-10 million years B.P) of the Russian Plains."
The Russian plains have few of these species now; nor indeed has all of Europe. Instead, what is left are simple forests of one kind of oak or beech; and a landscape with elms or line or sycamore. Tulip trees, the gums, sassafras, and most of the oaks and maples are unknown, all apparently lost during the climatic changes of the last few million years.
But in eastern North America the great array of species lives on, coming together at their greatest mix in a forest of rich diversity. When modern ecology began in America early this century, botanists concluded that these complex communities must have been built through long spans of time as species were adapted one to another by natural selection. The whole forest community was thought to be an ancient entity in its own right.
But we now know that this conclusion is wrong. The trees are indeed ancient, but the forest communities in which they live are not. Temperate forests are shifting alliances of species that slowly shuffle as climates change, or as fresh immigrants arrive. The forest communities look permanent only because trees live much longer than we do, or perhaps because climate can be roughly constant for centuries at a
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