| Rare plant conservation efforts increasingly call for restoration, not just protection, of extant habitat. Restoration initiatives can be strengthened by knowledge of ecological processes that occur across a range of spatial and temporal scales. In the Southern Appalachian Mountains of the United States, 15 rare herbs are restricted to high montane rock outcrops. My studies of six species indicate that the interaction of three primary factors explains both the distribution of the rare herbs and their declines: (1) the evolutionary history of the species as arctic-alpine relicts; (2) the combined influences of physiological tolerances and competitive relationships; and (3) human impacts through direct effects of trampling and indirect effects on plant succession. At broad spatial and temporal scales, ecological processes associated with global climate change have restricted elements of a former alpine flora to rock outcrops on cool, moist, mountain summits. Visitor disturbance and woody plant succession appear to be the primary agents of recent population declines; within sites, their effects occur primarily at the scale of individual outcrops, and habitat patches are affected according to their position on the outcrops. Within and among habitat patches, a moisture gradient best differentiates the microhabitats of different rare species. Experimental plant introductions showed that the rare species can grow to reproductive maturity over a broader range of microhabitat conditions than those in which they naturally occur. The combined influences of physiological tolerances and competitive relationships appear to be the primary determinant of species distributions at both site and microhabitat scales. If these species survive in their outcrop refugia, they may be able to reoccupy the zonal vegetation during the next period of global cooling. Thus, the high montane rock outcrops may play an important ecological role in larger landscape processes occurring over longer temporal scales. The long-term ecological role of the rare herbs is not evident from their current distribution, but rather in the context of such broader spatial and temporal processes. A hierarchical approach to habitat protection and restoration offers a practical way to place human influences within the context of broader and finer scales of influence. |
