Scale-Dependent Classification of Xeric Limestone Prairies: Annual or Perennial Grasslands?

Xeric limestone prairies (XLPs) are herbaceous plant communities that occur on shallow rocky calcareous soils from Missouri and Pennsylvania south to Arkansas and Georgia (U.S.A.). There has been considerable confusion regarding the classification of XLPs due to their similarity with limestone cedar glades (LCGs), an edaphic climax community type restricted to unglaciated southeastern United States. Although C₄ perennial grasses typically are the dominant taxa in XLPs, portions of sites with extremely shallow soils (≤ 0.1 m) are typified by local dominance of Sporobolus vaginiflorus (Torr. ex Gray) Alf. Wood, a C₄ summer annual grass that is the characteristic dominant in LCGs. In this study, we assess the relative importance of C₄ perennial grasses versus S. vaginiflorus in XLP community types identified in Kentucky over a range of scales. We then use these data to analyze variability in XLP vegetation and physical environmental conditions and to further compare XLPs with LCGs. Sporobolus vaginiflorus had high frequencies in many of the fine-scale community types (0.01 and 0.1 m²) identified in this study. However, dominance of C₄ perennial grasses (particularly Schizachyrium scoparium (Michx.) Nash) in all 12 of the community types identified at the largest scale (100 m²) is indicative of the relatively small areal extent of extremely shallow-soiled regions in XLPs. XLPs also differ from LCGs in that the former generally lack early-stage primary successional community types dominated by cryptograms and/or C₃ annual forbs. Thus, the results of our study suggest that the total range of environmental conditions and the variety and extent of plant community types present in a site should be considered when classifying the vegetation of calcareous rock outcrop communities in the eastern United States.