Summer 2003

Prairie Plants in Reconstructions Taller Than in Original Prairie

	Photo courtesy of Rebecca Ammann

In the summer of 2002, I set out to measure a feature of prairie structure not often considered by plant ecologists — its height. Land managers and naturalists in the field have reported that vegetation at prairie reconstructions (prairies planted on plowed ground) is taller than vegetation at remnants of original prairies, but few formal studies have been done. I wanted to test the truth of these observations.

Why does prairie height matter? For one, many area naturalists are finding that grassland birds are not using prairie reconstructions as much as expected. If reconstructions are truly much taller than original prairie remnants, the differences in vegetation height could help explain why grassland birds are not returning to reconstructed sites in the numbers that these prairies should be able to support.

I sampled six original prairies, six seeded reconstructions, and two planted reconstructions (seedlings planted one by one) in Cook, Lake, DuPage, and Kane Counties in Illinois, and Newton County in Indiana. To quantify vegetation height structure, I measured stem density (the number and thickness of stems in a given space) at different heights and locations and counted the tallest individuals at selected spots. Among other techniques, I used a Robel pole (pictured). Observing the pole from a set height and distance away, I recorded the lowest point on the pole completely obscured by vegetation. This "visual obstruction" height is the best single measure of vegetation height.

I found that reconstructed prairies dominated by classic prairie grasses had an average visual obstruction height of 30 inches, significantly taller than the 18 inches of original prairies. However, planted reconstructions were significantly shorter than seeded reconstructions. There were significantly more native species contributing to visual obstruction at original prairies and planted reconstructions than at seeded prairies, suggesting that shorter prairies tend to have greater species diversity.

My findings indicated that vegetation height structure in seeded prairie reconstructions is not mimicking that of original prairies. Reconstructions tend to be taller and have fewer species, the majority of which are taller grasses like big bluestem, switch grass, and Indian grass. Some planted reconstructions such as Schulenberg Prairie have vegetation height structure and species richness closer to that of original prairies.

Some recent prairie reconstructions in McHenry and Cook Counties and at The Nature Conservancy's Kankakee Sands project have already tried to create shorter prairies by introducing the seeds of shorter species. However, my study found that some normally short species grow taller than normal in seeded areas, suggesting that there is more to vegetation height than species composition.

I hope that my research will help people realize that vast expanses of grasses nine feet tall are probably not what original prairies looked like. Instead, most original prairies likely had shorter vegetation — rich in flowers and rich in variety.

— Rebecca Ammann

Ammann will present this study, her masters thesis, at the University of Illinois at Chicago this summer. For more details, contact the author at ramman1@uic.edu.