The absence of trees on the Prairie Peninsula

StoryImage( ‘/Images/Story//Auto-img-112308950419187.jpg’, ‘Photo by Dr. Robert A. Hedeen’, ‘A remnant of the Prairie Peninsula in Illinois remains in the Harlem Hills Natural Area in Loves Park.’);

When the first pioneers heading West emerged from the forests of the East and beheld the Prairie Peninsula for the first time, they must have been startled and wondered what happened to the trees. The forests had been dramatically replaced by a seemingly unending sea of grass.

The Great Plains physiographic region of the United States extends over the middle part of the country, and the Prairie Peninsula is a huge slice of the plains that extends south and eastward through Iowa and Illinois, and over parts of Minnesota, Wisconsin, Missouri, western Indiana, with isolated patches in Ohio and Michigan.

Of course, trees grow in the Prairie Peninsula, with the Illinois prairie being a classical example. Originally, the prairie of our state was about 30 percent trees, mainly growing along river and stream banks and in scattered groves. The rest of the area consisted of grasses, some of which reached a height of 9 to 12 feet, and because of this our general region is classified by ecologists as a tall grass prairie.

Undoubtedly, the early settlers speculated on what happened to the forests and what caused this dramatic change in the landscape. They were not alone in this respect, as for more than 100 years biologists also wondered why this ecological transition occurred.

Some of the speculations to explain this phenomenon were:

Not enough rainfall. This certainly plays a part in the explanation.

The soil will not support trees in great numbers. This explanation is entirely wrong. It did, however, cause many of the pioneers to pass over some of the most fertile ground in the world and push on west to the forests of Oregon and Washington. It is true that the prairie was difficult to cultivate until John Deere invented the steel plow in 1837.

Trees could not withstand the wind blowing over the prairie. This force of nature has little to do with the matter.

Indians burned the original forest. This conjecture was a very small part of the truth. It was not until 1935 that the question was properly addressed and answered in a paper published by Dr. Edgar Nelson Transeau (1876 – 1960; chairman of the Botany Department of Ohio State University for many years) in the journal Ecology. Transeau gathered all of the facts and presented a plausible explanation why the unforested grasslands in the central United States extended so far eastward to be only explained by climatological factors alone. He concluded many factors were involved, the most important of which are as follows:

Drought: Not withstanding a plentiful average rainfall, the Prairie Peninsula suffers from severe drought much more often than do the forested areas to the east.

Dry season: Forested regions have relatively uniform precipitation throughout the year, and the prairie is noticeably drier in the fall and winter.

Drying factor: Despite adequate precipitation, plants dry out faster on the prairie due to wind, temperature, and other factors.

Flat terrain: The prairie offered few natural firebreaks

Lightning: After Florida and the Gulf Coast, the Prairie Peninsula has more electrical storms than any other part of the U. S.

Fire: Fire was the central cause in Transeau’s theory for the formation of the Prairie Peninsula. Native Americans periodically torched the prairie to create more desirable grazing for game animals. Lightning started many fires as it strikes the highest thing around, namely trees. Fires raced across the prairie at incredible speeds, incinerating all in their paths. Forests had difficulty recovering, but the seeds of grasses were protected by the earth and sprouted anew the next year. The Peninsula Prairie ecosystem evolved because the grasses were better adapted to the harsh prairie environment than trees—a prime example of survival of the best suited. The grasses were able to survive and to thrive in spite of the obstacles nature threw at them. Their resiliency could not, however, cope with man’s insatiable and unholy desire for development and altering the environment to suit his wishes.

A small fraction of the Peninsula Prairie still exists in Illinois, and it is incumbent on those of us interested in the natural history of our state to do everything possible to preserve what little is left of what once was a great sea of grass.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the Aug. 3-9, 2005, issue

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