Cahaba Indian paintbrush can be found only in one county in Alabama, Bibb County. Since 1992, botanist James Allison has explored the area where Castilleja kraliana grows and exclaims, “Bibb County is blessed with an even greater number of rarities than anyone had imagined. It appears, in fact, to support the most significant diversity of rare plant species of any county in the temperate Southeast!”
Why so biodiverse? The area where Castilleja kraliana is found (along with at least sixty other taxa of conservation concern) has a set of characteristics which foster high levels of biodiversity: 1) it is mostly rural; 2) it is geologically diverse (it is the intersection of 3 geographic regions, each with its own associated flora and fauna); and 3) it contains rocky outcrops with unusual soil chemistry. The rocky outcrops are known as glades; nearly treeless, open areas with little soil. What soil does exist is the result of breakdown of the underlying Ketona Dolomite. Dolomite is a rock composed of carbonates of calcium and magnesium, but it is often filled with impurities (roughly 40%). Ketona Dolomite is special in that it is nearly pure, with impurities ranging in the 2% range. Without the impurities to balance out the magnesium carbonates, the soil that is formed from the dolomite is consequently high in magnesium concentration. Although important to the growth of plants in small amounts, high concentrations of magnesium prevent the uptake of other nutrients, i.e., it becomes toxic to plants. Only specially adapted plant species can withstand the combination of high magnesium and little soil, leaving, as Allison states, “a community of drought- and magnesium-tolerant plants able to evolve in the absence of competition from more generally adapted types. The presence of multiple newly discovered species, several of them with seemingly primitive features, as well as the occurrence of others whose nearest known locations are hundreds of miles distant, suggest that this plant community is an ancient one.”
Thanks to the discoveries of Allison and others, The Nature Conservancy has helped create a preserve, the Kathy Stiles Freeland Bibb County Glades Preserve, to protect the most significant areas of biodiversity. Similarly, in 2002, the adjacent Cahaba River National Wildlife Refuge was established.
To read more about this intriguing area, visit Allison’s web site to read A Botanical Lost World in Bibb County, Alabama or Vascular Flora of Ketona Dolomite Outcrops in Bibb County, Alabama (published originally in Castanea, 66:1-2 (154-205)).