INTRODUCTION

The eastern United States has been well explored botanically. New species continue to be described every year, but mostly in difficult groups like Carex L. and Isoëtes L., and are usually "split" out of recognized species. The discovery of endemic plant communities with multiple undescribed species mostly occurs in remote regions of South America, Africa, or Southeast Asia.

In 1992 the United States Fish and Wildlife Service contracted with the first author to conduct a status survey in Alabama for Arabis georgiana Harper. As a part of efforts to explore comparatively inaccessible habitats, Allison organized a canoe trip in Bibb County, Alabama, with three friends: the second author and Jim and Debi Rodgers of Senoia, Georgia. By the end of the first morning (May 30), canoe travel had led to the discovery of several previously undocumented populations of Marshallia mohrii Beadle & F. Boynt., a federal Threatened species, on rocky places along the banks of the Little Cahaba River.

At midday, we noticed a strongly sloping, rocky area dominated by herbaceous vegetation, above the right bank of the Little Cahaba River. As we explored the site, it soon became apparent that it supported plant communities distinct from the well studied ones on flat, limestone outcrops ("cedar glades") in Tennessee, Kentucky, Alabama, and Georgia [see ASB Bulletin, vol. 33, no. 4 (1986), for a collection of articles on cedar glades, and Castanea, vol. 59, no. 3 (1994), on the closely related topic of "barrens" vegetation]. Although some of the woody species we saw on glades near the Little Cahaba were typical of calcareous glades and barrens (e.g., Juniperus virginiana L., Quercus muehlenbergii Engelm.), there were others that would have marked this as a distinctive glade community even in winter, such as Pinus palustris P. Mill. and Sabal minor (Jacq.) Pers., and particularly two shrubs in the Euphorbiaceae, Leptopus phyllanthoides (Nutt.) G. L. Webster and Croton alabamensis E. A. Sm. ex Chapman var. alabamensis. The herbaceous component, moreover, included a surprising number of unfamiliar taxa, along with recognizable rarities such as Marshallia mohrii. It was clear that this was a natural community that deserved further study.

Upon consulting the most recent statewide Alabama geological map (Szabo et al. 1988) we found that the locations of these glades all fell within a particular mapping unit, Ketona Dolomite. We discovered most of the localities that were not visible from a canoe by studying 1:24,000 topographic maps (on which glades often show as irregular white blotches within green areas) or through examination of infrared aerial photographs. The latter were available for inspection at the USDA Natural Resources Conservation Service office in Centreville, the county seat. Within regions on the photographs that corresponded to the Ketona Formation, as mapped by Szabo et al. (1988), we identified dozens of localities that had a similar appearance to that of known glades. By the time we completed the ground checking of these, we had found approximately 40 sites, defined as outcrops with characteristic flora and separated from each other by at least 0.2 km. For each glade we concocted a name and recorded the endemic, rare, or characteristic Ketona Glade species present.

PHYSICAL CHARACTERISTICS OF HABITAT

This ecosystem is apparently restricted to outcrops in Bibb County, Alabama, of the Ketona Formation, an unusually pure dolomite of Upper Cambrian age (Rheams 1992). These are all within a zone about 18 km long and at most about 0.8 km wide, within the southern "fringe" of the Ridge and Valley Physiographic Province (Figure 1).

Glades developing on the Ketona Dolomite (Ketona Glades) vary in size from about 0.1 hectare to at least 5 hectares and have a general appearance (Figure 2) resembling the cedar glades developed over limestone or dolomite in Alabama, Georgia, Kentucky, and Tennessee, though they are seldom as level as "classic" cedar glades. The terrain is mostly gently sloping or rolling, but varies from flat to sometimes very strongly sloping. There are patches of exposed bedrock, thin-soiled areas dominated by grasses and other herbaceous vegetation, variously sized islands and peninsulas of woody vegetation where soil has accumulated to greater depth, and marginal ecotones where the glade grades into the surrounding forest. In many areas the bedrock projects above the surrounding surface as low boulders or ledges.



