The tropical flora of rockhouse cliff formations in the eastern United States

Deep recesses in bedrock cliffs, often referred to as "rockhouses", can maintain habitats that are highly buttered from extremes in temperature and moisture, but have very low light levels. Within these habitats in the eastern United States are a suite of plants with tropical affinities, i.e. endemic species whose closest relatives are tropical, and disjunct occurrences of species whose principal range is tropical. Ferns are the most conspicuous plants of this group, but bryophytes are the most numerous. Both groups are preadapted to persistence in rockhouses by cabilities of vegetative reproduction and of net photosynthesis in habitats of very low light, the latter made possible by a high ratio of photosynthetic to non-photosynthetic tissue. Morphological, physiological, genetic, geological and geographical data support a scenario of persistence of these plant species in rockhouses since their origins in pre-Pleistocene times when tropical/subtropical climates prevailed over the eastern United States.