Sunday, August 10, 2008

Prairie Fen Rarities

This summer, I had the opportunity to revisit several high quality occurrences of prairie fen in southern Lower Michigan. These fens are widespread but spatially limited in the state, typically occurring in association with lakes and streams, where groundwater rich in calcium and magnesium ions discharges to the surface. Most fens occur where end moraines, eskers, or kames meet outwash channels, and where groundwater is forced to the surface upon reaching impermeable sediments. "Classic" fen meadow often develops on slopes above lower, flatter, inundated wetland or on peat "domes" where groundwater upwelling favors peat accumulation above the source. These slopes and domes are typically dominated by Carex sterilis, often associated with Andropogon gerardii, Schizachyrium scoparium, Sorghastrum nutans, and, locally, Sporobolus heterolepis and Muhlenbergia richardsonis. The presence and codominance of these "prairie" grasses, in addition to the presence of several forbs characteristic of upland prairie, are responsible for the controversial "prairie fen" term used for this community type.

I was lucky enough this July to observe two of Michigan's rarest butterflies, the Mitchell's satyr (Neonympha mitchellii var. mitchellii), and powesheik skipperling (Oarisma powesheik). Jim McCormac's blog (linked here) discusses the Mitchell's satyr in some detail. It is extant at just over a dozen sites in Michigan, where populations are typically small.

The powesheik skipperling is just as interesting. This butterfly is typical of upland prairies in the Great Plains, but is restricted to prairie fens in Michigan. Interestingly, almost all of the sites for this species also contain mat muhly, although there is no documentation the butterfly uses this species as a caterpillar food plant. Powesheik skipperling is apparently extremely susceptible to fire, and populations have disappeared in nature preserves where prescribed fire was used to manage prairie and fen. Anecdotally, the species was absent from burned portions of the two fens I observed it in this summer, and was locally abundant in adjacent patches that were unburned.

Lest I forget this is a botany blog, I was out with MNFI, DNR, and TNC colleagues surveying a new prairie fen site in SE Michigan when we stumbled across a colony of this species:

It quickly became apparent that none of us had ever encountered this shrub before, despite collectively having visited probably over 100 prairie fens in the state. The unusual fruits and fragrant leaves had me thinking Myricaceae, which was exciting, because only two species are listed for Michigan in Voss: Sweet-fern (Comptonia peregrina), a distinctive shrub of dry, acidic sands, and sweet gale (Myrica gale), a characteristic low shrub in wetlands in northern Michigan, also a distinctive species that didn't match our unknown. On returning to Lansing, I pored over my Ohio Nature Preserves Directory, seeming to recall the presence of bayberry, Myrica pensylvanica, in that state. Sure enough, the description for Gott Fen in Portage County (NE Ohio) included the following statement: "Gott Fen is one of only two known sites for bayberry in Ohio and the most western location for this species in the United States." Our botanist keyed a specimen, and, sure enough, our unknown was M. pensylvanica! Excitement was somewhat subdued upon learning this species had previously been discovered in Michigan at two or three sites, all since the publication of Voss in 1985. Still, the species is proposed as "threatened" on the revised list of endangered, threatened, and special concern plant species in Michigan, and this find added to the value of this interesting fen complex.

Fens are truly special habitats, and each footstep has the potential to uncover something new and exciting. Just remember to keep an eye out for massasauga rattlesnakes!

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