Not In Kansas Anymore

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According to physicist Forrest Hall of NASA’s Goddard Space Flight Center, vegetation at the height of the Pleistocene Ice Age about 20,000 years ago was profoundly different from today’s. “For example,” says Hall, “we know that spruce—a species that we think of as belonging to the boreal forest—was common in the central United States, including Kansas, Oklahoma, and even reaching as far south as Texas.”


comparison of pleistocene and modern ice cover


The last glacial advance of the Pleistocene Epoch was also one of its most severe. At its peak about 20,000 years ago, a vast ice sheet called the Laurentide covered much of North America, blanketing Canada and parts of the U.S. with a wall of ice as much as 3.2 kilometers (two miles) thick. While the continents had roughly the same size and shape that they do now, terrestrial biomes were compressed in the remaining ice-free terrain. Vegetation colonized newly exposed areas of coastline that appeared as glaciers consumed more and more of the Earth’s moisture and sea levels began to drop. Many scientists believe that precipitation dropped dramatically during this period—perhaps by as much as 50 percent or more in some regions. The differences between ice age forests and modern forests were probably not just because the ice age was cold, but because it was simultaneously cold and dry.

  At the height of the Wisconsin glaciation during the Pleistocene Ice Age, the Laurentide ice sheet covered nearly half of North America (left). Today, the polar ice cap is greatly diminished (right). (Image courtesy National Oceanic and Atmospheric Administration Paleoclimatology Program.)

Map of Ancient and Modern Sea Level

Margaret Davis, an ecologist at the University of Minnesota, studies fossilized tree pollen in order to describe North American vegetation since the end of the last glacial maximum. Most of her work is based on fossilized pollen buried in lake sediments. By comparing modern forests and the pollen records they leave behind to pollen records from thousands of years ago, Davis has created a picture of ancient forests. Her meticulous studies of North America’s fossil pollen record show that although trees associated with modern forests existed many thousands of years ago, forests as we know them today—dense, continuous stands of trees whose branches form a closed canopy overhead— were likely very rare at the last glacial maximum.


As temperatures dropped and more and more of the Earth’s water began to be tied up in the massive polar ice sheets, sea levels dropped. The map of North America (left) shows the familiar outline of modern sea levels (light green) as well as sea levels at the peak of the Pleistocene ice age (blue) and the sea level that would result if the polar ice caps melted (dark green). (Images by Robert Simmon, NASA GSFC)


Pollen Micrographs


Stephen Jackson, a botanist at the University of Wyoming, agrees. Jackson has used both pollen and macrofossils to describe the North American landscape since the height of the Wisconsin glaciation. According to Jackson, “It appears that in upland regions, woody vegetation was indeed sparse, and canopies were relatively open. Whether this was savanna-like, with clumps of trees separated by open, non-woody vegetation, or parkland, with low tree density, remains unresolved. But I suspect, based on the pollen data, that ‘forests’ as we think of them today were restricted to riparian areas along rivers and other sources of water.”

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Botanists use fossil pollen to map the past distribution of forests. These images show pollen from modern boreal tree species. From left to right: pine, aspen birch, and alder. (Micrographs courtesy USDA Pollen Lab)