Tuesday, August 24, 2010

Ohio River buttonweeds

A view of the banks of the Ohio River from a lofty perspective. This is the site of the great Amorpha Borer Expedition covered in the last post, the marina at Shawnee State Park in Scioto County, Ohio. The host plant of that most gorgeous of beetles, False Indigo, Amorpha fruticosa, reaches the northern limits of its natural range along the Ohio River. So too, I suspect, does the Amorpha Borer beetle.

They aren't unique. A whole host of animals and plants run up against their northward limits in the Ohio River Valley. There are probably two primary reasons. One, the climate. The valley of the mighty Ohio has its own microclimate, remaining warmer on average than even just a short distance north, over the first set of hills. Two, the river serves as a migratory corridor, and flora and fauna have dispersed along the stream for many thousands of years. Many of them need the regular flood-scouring disturbance cycles that are associated with large rivers, and thus must grow or live within the flood zone.

We saw two of these Ohio River specialties while stalking the beetle, and I took the opportunity to make some photos. This one is Virginia Buttonweed, Diodia virginiana. It is a small member of the enormous Madder Family, Rubiaceae, which sports some 13,000 species, most of which are tropical. Coffee is probably its best known representative.

Virginia Buttonweed is small, nearly prostrate plant, and easily missed by those seeking larger objects. Like most things wee, it is quite showy upon inspection. The ciliate margins of the flowers lend them an almost crystalline look, like botanical confectionaries. Don't be fooled by the delicate appearance - this buttonweed is one tough cookie.

Range map courtesy of USDA Plant Database.

Virginia Buttonweed grows right along the river banks, where floods regularly scour the soil. It is a pioneer species; one of the first plants to occupy a freshly disturbed habitat set back to ecological ground zero by massive flooding. Ironically, mowing seems to duplicate this disturbance and the plant thrives in cropped grassy area along the river's banks. Note how it occurs only in Ohio's river counties, and then only right next to the river in most cases.

This plant is an even better find. It's Smooth Buttonweed, Spermacoce glabra, another Madder Family constituent. It tends to be more rare and local than Virginia Buttonweed, but occurs in the same habitat and often in association with it.

Smooth Buttonweed is listed as Potentially Threatened in Ohio, which is the botanical watch list category, a notch below Threatened or Endangered on the rarity scale.

We've been very hard on Ohio River habitats, and lots of animals and plants have become rare or vanished as a result. The river itself has been largely tamed by the U.S. Army Corps of Engineers. They've built a series of locks and dams which have essentially transformed the once free-flowing river into a linear series of pools to accommodate shipping. Terrestrial habitats adjacent to the stream have been cultivated, built on, become the site of massive chemical and power plants, and all manner of other development disastrous to our natural resources.

The spotty and local Ohio River distribution of Smooth Buttonweed, one of our more interesting little plants.

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