Evolving tallgrass prairie
By PAUL G. JANTZEN
Contributing writer
Recently I had occasion to review David Costello's book, The Prairie World. On page one, he describes his boyhood visit to a southeast Nebraska pasture and a quartz boulder that stood high above the prairie grasses. His elders had various explanations for the rock: a meteorite from outer space, a cover for the grave of a great Indian chief, or it was heaved from underground by frost.
Costellos' boulder reminded me of a quartzite boulder on my grandparents' farm in southeast Nebraska's Jefferson County. It had been pulled by four horses to its resting place near the garage from the field where cultivation and erosion had exposed the rock. Painted on the boulder was "October 29, 1889," the day my great-grandfather Heinrich took possession of the farm.
My older brothers suggested to Uncle Ernest that the boulder and many smaller ones scattered in the fields may have been carried to southeast Nebraska by one of the great ice sheets that once flowed from north; but he dismissed that as unlikely.
Geologists agree that the quartzite rocks now found in eastern Nebraska and northeast Kansas were, indeed, carried by glaciers and dropped as the ice melted over a million to 12,000 years ago.
After about 500 million years of alternately advancing and receding seas over much of the Great Plains, the seas receded, the land dried, and the Rocky Mountains pushed up from the land surface casting their rain shadow over the Midwest. And then came the ice ages.
During the ice ages, snow and ice accumulated to depths of thousands of feet. When reaching a thickness of just over 200 feet, the weight of glacial ice caused it to melt where it pressed on the ground underneath — like the ice under an ice skate. The film of meltwater acted as a lubricant allowing the ice sheet to slide southward.
There were numerous glacial advances and retreats but four major glacial events in North America, named for the states of their southernmost extension. The Nebraskan ice sheet began over one million years ago. Next, the Kansan glacier reached to roughly the present route of the Kansas River. Then came the Illinoian phase, and the Wisconsinan phase which began receding less that 12,000 years ago.
From this information, we know that, as the local environment became cooler and less moist, Nebraskan glaciation brought oak woodlands to Kansas. Kansas glaciation brought northern spruce forests to northeast Kansas along with associated plants and animals. The more distant Illinoian episode probably fostered oak woodlands in this area. Then, the Wisonsinan glaciation brought spruce forests to Kansas again.
Since the ice ages, the climate is warmer and rainfall is reduced. Instead of spruce forests or oak woodlands, our dominant community today is the tallgrass prairie. The retreat of glaciers has increased the temperature and the raising of the Rocky Mountains about 63 million years ago cut off the flow of moist air from the Pacific Ocean.
Analysis of pollen from core samples indicate that the total time of succession from one glacial retreat, to spruce forest, to oak woodland, and, finally to grassland took less than 600 years.
As the last glaciers receded, mammoth-hunting humans invaded North America. They walked from Siberia and Europe across the Bering Strait to Alaska possibly as early as 30,000 years ago — certainly 10,000 years ago. The glaciers had tied up enough water to lower the sea level to allow this dry-land trek. Human hunters may have contributed to the extinction of large mammals such as mammoths.
O.J. Reichman, in his natural history of Konza Prairie, notes that few plant species that make up the present tallgrass prairie "have been around for many centuries," but nearly all have migrated to our area from elsewhere, especially from southwestern and southeastern United States. Even big bluestem grass, symbol of the tallgrass prairie, originated in Appalachian valleys of eastern North America.
Continued interaction of current grassland residents with additional immigrants will develop new interrelationships. And human intervention, which began 12,000 to 30,000 years ago, may add to the dynamics of the even evolving tallgrass prairie.
