Introduction

Despite the lack of a climatic tree line, the southern Appalachian mountains support several treeless, high-elevation (> 1,200 m) communities. The three principal open communities are heath balds, grassy balds, and rock outcrops. Other open communities include frequent, but small, rocky, steep streamsides and seeps, and, very rarely, mountain bogs (Schafale and Weakley 1990). The information on heath and grassy balds will be briefly reviewed, but the focus will be on outcrops because they support a highly distinctive flora and are the least studied of the three primary open communities. Notably, they support one of the richest floras of rare species of any regional habitat, including both rare endemics and northern alpine disjuncts.

Heath Balds

Heath balds are species-poor communities with only 10–20 vascular plant species on a site (White and Renfro 1984) and are dominated by ericaceous evergreen shrubs. They occur within a restricted elevation range (1,220–1,525 m) on narrow ridges and adjacent south and west slopes, and become larger and more frequent in localities disturbed by logging activity (White, Wilds and Stratton, unpublished data). In Great Smoky Mountains National Park, heath balds are more frequent (>400) than grassy balds (~30) but dominate less than 50% of apparently suitable topographic sites.

Because of dense shade, thick leaf litter, and high soil acidity (aluminum may reach levels toxic to tree roots), heath balds are stable or only slowly invaded by trees. Some heath balds created by logging remain treeless one hundred years later.