Endemism in rock outcrop plant communities of unglaciated eastern United States: an evaluation of the roles of the edaphic, genetic and light factors

Within the unglaciated E USA edaphically-controlled herbaceous plant communities occur on unshaded rock outcrops in shallow soil developed from a variety of rock types. The most distinctive features of the flora of these communities are their endemic taxa. Many endemics are not restricted to a single geological substratum, and they grow equally well, or better, in non-outcrop as in outcrop soil. Several endemics exhibit intraspecific variability in breeding system, flower colour and/or enzyme pattern, so it is unlikely that lack of genetic variation is the cause of endemism. A requirement that the endemics have in common for growth and survival is a high photosynthetic photon flux density. -from Authors