Tom's Blog

Friday, October 2, 2015

Prairie plants on rock outcrops

In the Driftless Area there are many rock outcrops on which plants appear to be "struggling" for survival. If such an area is fairly large it might be called a "barren" or "bald", depending on its location.

The plants that are growing on rock outcrops are sometimes called "disturbance species", because they may also grow in areas that have suffered considerable human interference.

At Pleasant Valley Conservancy we have several rock outcrop areas, and they are never devoid of plants, despite the thin, almost nonexistent soil. The area that we call the East Overlook is the largest rock outcrop, and is an interesting area to visit. (The Mid Savanna Trail passes below and around the outcrop, continuing onto the Saddle and South Firebreak.)

When we first started restoration work, the East Overlook was heavily colonized with buckthorn, reasonable since this calciphile does well in dolomitic habitats. Years of work have eradicated the buckthorn, and now the site has only native species.

In the spring, small populations of pasque flower and prairie smoke are present. Later columbine can be seen, followed by hairbell and whorled milkweed.

A survey I made yesterday was interesting because there was a large population of hoary vervain (Verbena stricta). Kathie and I counted over 50 stems of this species in an area of about 10 X 20 feet.This beautiful plant is a classic example of a "disturbance species", since it is often found in areas highly disturbed by humans. The best population we have found was on an abandoned commercial site at the corner of County I and County V, just off Interstate 90/94. There was still lots of concrete remaining, and around the edges in the thin soil was a huge population of V. stricta. (Some people might ignore such a species, but it is a true prairie species and not highly invasive, since it probably cannot compete with the "nondisturbance" species.)

Verbena stricta shoots growing out of the very thin soil surrounding the bedrock of the East Overlook

V. stricta in a prairie. It usually thrives best at the edge,
perhaps adjacent to a path.
 Yesterday Kathie and I did a species list for this small rock outcrop. In addition hoary vervain, we found big blue stem, side oats, little blue stem, prairie dropseed, hairbell, whorled milkweed, Kuhnia, Carex eburnii, cream gentian, silky rye, and woodland brome. Also present was a single large eastern red cedar (carefully protected from fire) and several bur oak grubs. Thankfully, there was no buckthorn!

Anyone interested in rock outcrop communities would be advised to access the valuable book edited by Roger C. Anderson, James S. Fralish, and Jerry M. Baskin, 1999. "Savannas, Barrens, and Rock Outcrop Communities of North America." Cambridge University Press. It has 36 chapters that cover the whole of North America, from southern Florida to subarctic Canada. Although I bought this book because of its extensive treatment of North America savannas, I found the chapters on barrens and rock outcrops also interesting. If you are a "plant" person and do much travelling in North America, this is an excellent book to take along. Here are few examples: New Jersey Pine Barrens, Serpentine Rock barrens of Eastern and Western North America, Granite Outcrops of Southeastern  U.S.,  the Cliff Ecosystem of the Niagara Escarpment, Alvars of the Great Lakes Region, Southern Ontario Granite Barrens.

One of the areas Kathie and I visited was Mount Arabia, a site in the southeastern Piedmont region of DeKalb County, Georgia, where there was a large granite outcrop that had tiny islands of soil where primarily endemic species grew. Because of its endemic plants, this area has been protected in a state park.


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