Abstract: High-elevation rock outcrops of the Southern Appalachians support numerous rare plant species and are among the most distinctive of Southern mountain habitats. The presence of species disjunct from northern alpine and arctic areas suggests that the flora is partly a relict of a Pleistocene alpine flora. Vegetation samples were collected on the outcrops to (1) describe composition in relation to environmental and geographic gradients, (2) determine if rare species occurrences are predictable from environment, and (3) compare composition and rare species habitat preferences with those observed in treeless and alpine habitats of the White Mountains, New Hampshire.
Composition of Southern Appalachian high-elevation rock outcrop vegetation is strongly correlated with elevation, potential solar radiation, geology, and the amount of seepage. Within bedrock types, vegetation varies consistently with elevation and potential solar radiation, but relationships with soil parameters vary with bedrock type. Geographic position alone explains much of the variation in species composition, which suggests the importance of chance establishment and extinction on this insular habitat.
Occurrence of individual rare species is generally predicted by a combination of potential solar radiation or aspect, and soil parameters, factors that also relate to compositional variation. No two species are predicted by the same set of parameters or by similar functions of those parameters, despite their frequent co-occurrence. At the microhabitat scale, species are more individualistic in their relationships to environment.
Among the southern outcrop communities studied, those above 1600 m on felsic bedrock or above 1350 m on mafic bedrock are most similar to their counterparts in the White Mountains. Northern communities on outcrops below treeline are more similar to southern outcrops than are northern alpine communities. Vegetation in the two areas shows a similar relationship to elevation and soil chemistry, but potential solar radiation is more important in the south. Co-occurrence of more than three alpine disjuncts is more likely in the north where suitable habitats are less fragmented.
