High-Elevation Cliffs and Outcrops of the Southern Appalachians: Vascular Plants and Biogeography

Vascular plants of high-elevation cliffs and rock outcrops were sampled on 42 peaks in the Southern Appalachians. Species found were predominantly native, summer-flowering, herbaceous perennials. Species endemic to the Southern and Central Appalachians comprise over 25% of the total flora; species restricted to eastern North America include nearly 50% of the flora. The diversity of biogeographic affinities shows that the northern alpine element (those species found above treeline in the Northern Appalachians) is only a small subset of the total flora. At least 17% of species were likely components of the Pleistocene alpine flora. An annotated checklist of the 288 species is provided. INTRODUCTION The flora of Southern Appalachian (latitude 35?-37?) high-elevation rock outcrops has numerous rare species and is one of the most distinctive of any habitat in the mountains. Forty species are considered rare in North Carolina and Tennessee including five very rare endemics: Geum radiatum, Liatris helleri, Solidago spithamaea, Calamagrostis cainii, and Houstonia purpurea var. montana (Weakley 1993, Tennessee Rare Plant Protection Program 1992). The presence of seven species found in the alpine of New England and farther north in the arctic (Minuartia groenlandica, Juncus trifidus, Scirpus cespitosus, Sibbaldiopsis tridentata, Huperzia appalachiana, Agrostis borealis, Trisetum spicatum) suggests that these isolated communities represent relicts of an alpine flora that was widespread in the Southern Appalachians during the last ice age, until 12,000 years ago (e.g., Ramseur 1960, White et al. 1984, Billings 1988). Harshberger (1903) went as far as to consider these communities part of a distinct 'sub-alpine treeless formation.' Ramseur (1960) suggested that the ancient flora, of which these communities are relicts, moved south ahead of the Wisconsin ice sheet and later moved into the northern region when the ice retreated. Paleoecological evidence suggests that the high peaks of the Southern Appalachians JUNE 1994 85 * Present address and address for reprint requests: 6 Wentworth St., Ilam, Christchurch, New Zealand. had a treeline at about 1,500 m and permafrost in very exposed areas during the last glaciation (Delcourt and Delcourt 1985, 1988). During the current warm period this flora presumably has been eliminated in all but small, isolated pockets (refugia) on outcrops where trees are largely absent with the consequence that competition for sunlight is minimal. A second component of the outcrop flora consists of species that were probably not part of the Pleistocene alpine flora but migrated into these habitats with post-glacial climatic warming. Despite long-time interest in high-elevation outcrops and a wealth of botanical and ecological surveys of the Southern Appalachian flora, there have been no thorough, quantitative studies of these areas. Because these unusual plant communities are threatened by human impact (e.g., air pollution, trampling), documentation of their vegetation and flora is essential. An understanding of the floristic variation among high-elevation outcrops throughout the Southern Appalachians is a necessary prerequisite to their adequate representation in reserves. Several questions regarding these unique areas are addressed here. 1) What is the typical life form spectrum of high-elevation cliff vegetation, and is this spectrum more similar to the flora of Piedmont rock outcrops or alpine/arctic areas to the north? 2) Does the high-elevation outcrop flora have an abundance of late-spring flowering plants like Piedmont outcrops or summer flowering like alpine areas? Life history features of the high-elevation outcrop flora may be more comparable to Piedmont outcrops because of their geographic proximity, but floristic affinities suggest a similarity to northern alpine. The first two questions attempt to clarify this. 3) Are 'weedy' species an important component of the high-elevation outcrop flora? Open, sunny outcrop habitats may be suitable to 'weedy' species and may have harbored ancestors of some of our current native weeds (Griggs 1940, Marks 1983). The presence of individual species considered to be alpine relicts on high-elevation outcrop communities (White et al. 1984) suggests the final question. 4) Does the rock outcrop flora have particular biogeographic affinities? In particular, is the flora a relictual, alpine flora persisting from the Pleistocene? STUDY AREA The rock outcrop vegetation above 1,200 m was studied on 42 peaks in western North Carolina and eastern Tennessee during the summers of 1989 and 1990 (Figure 1). The study area spans a geographical region of 20,000 km2 and encompasses much variation in climate and geology. Mean annual precipitation ranges from 127 to above 200 cm with highs in the vicinities of Highlands, North Carolina and the Great Smoky Mountains (Ruffner 1985). Bedrock types range from mafic amphibolites and metagabbros in the north to felsic granites, gneisses, and metasandstones in the southern and central part of the study area. There is a southeast to northwest gradient of decreasing metamorphism (King 1955, McConnell and Costello 1980). Areas sampled encompass four communities described by Schafale and Weakley (1990): High Elevation Rocky Summit, High Elevation Granitic Dome, Montane Mafic Cliff, and High Elevation Seep. 86 CASTANEA VOLUME 59 \LT' / \ [ \< BLUFF MT., THREE-TOP MT., HANGING ROCK PHOENIX MT. / ~~~~~~~ROAN MT. MASSIF \