Biogeography of prairie dock in Wisconsin

Prairie dock (Silphium terebinthinaceum) is near its western distribution limit in our part of  Wisconsin, although the details are not very well established. This Silphium is distinguished by its very large and conspicuous basal leaves and its tall flower stalks (over 10 feet). It is typically found in deep-soil wet-mesic prairies, where its roots can penetrate 10 feet or more.

On the map below, from the UW-Madison Herbarium, the dots show the locations in Wisconsin of all prairie dock collections. The westernmost site in Dane County is a railroad prairie on U.S. 14 just west of the Village of Cross Plains. This population has been studied by generations of botanists (including Curtis).

However, shown on the map below are two farther west locations, not represented in Herbarium collections. A large population known to many prairie people is found along County JJ at the edge of Vermont Creek 4.47 miles farther west (see photo). Finally, 2.54 miles still farther west is a large population at Pleasant Valley Conservancy. Although the PVC population was planted, the Vermont Creek population is native to its site.

A very large population of prairie dock along the edge of Vermont Creek, County JJ, in western Dane County.

The unanswered question is why there are no populations farther west in Wisconsin. Prairie dock is fairly common in Missouri (all the way to the Ozarks), and populations are known in Dubuque and Jackson Counties in Iowa, and Albert Lea (Freeborn County), Minnesota.

One possibility is that the preferred wet-mesic habitat of prairie dock is mostly absent from the Driftless Area of Wisconsin. However, although the main populations at Pleasant Valley Conservancy are in wet-mesic areas, we do have two small populations growing (and flowering) on a savanna and a prairie on the ridge-top, a long way from wet-mesic conditions. Although we did not intentionally plant prairie dock seed in these two areas, there is always the possibility that some seed contaminated our mixes. Either way, these plants are thriving.

NOTE ADDED Nov 27, 2013!!
I recently discovered that in a post of August 31, 2008 I had described a single prairie dock plant flourishing in an Iowa County prairie remnant. This is 10 miles west of the last native population at County JJ.