Ecology of two geographically restricted Astragalus species (Fabaceae), A. bibullatus and A. tennesseensis, of the eastern United States

Astragalus bibullatus is endemic to limestone glades in the Central Basin of Tennessee, and except for one population in Illinois, A. tennesseensis is endemic to limestone glades in the Central Basin and in the Moulton Valley of Alabama. However, A. tennesseensis has been extirpated from four counties in Illinois, from its only known site in Indiana, from one county in Tennessee, and from one county in Alabama. Astragalus bibullatus is closely related to the geographically widespread Great Plains taxon A. crassicarpus var. crassicarpus (section Sarcocarpi), whereas A. tennesseensis is the only taxon in section Tennesseensis. Both species are shallow-rooted, hemicryptophyte perennials without vegetative reproduction, have no effective means of seed dispersal, form long-lived seed banks, have similar life cycle phenologies, are intolerant of heavy shade, and have moderate amounts of genetic diversity. Much additional information is available on the autecology of A. tennesseensis. Its primary habitat is the transition zone between open glades and glade woods, where physical environmental factors are intermediate between those of the adjacent zones. Seedling-juvenile survival is low. Plants flower first in their second to fifth year and only a few times before dying, are self-incompatible, respond to drought by shedding leaves and by accumulating large amounts of proline, and compete poorly. Populations exhibit high fluctuations in number of individuals and have high turnover rates. A greenhouse study of A. bibullatus and A. crassicarpus var. crassicarpus did not identify any difference in responses of these species to light or soil moisture that could account for the great differences in geographic ranges. Thus, historical factors were also considered to explain the narrow endemism of A. bibullatus. Finally, we present previously unpublished data on the effect of light level and of competition with the cedar glade dominant Sporobolus vaginiflorus in several watering regimes on growth of A. tennesseensis. The role of these factors in restriction of this species to its transition zone microhabitat is discussed.