SANDSTONE ROCKHOUSES OF THE EASTERN UNITED-STATES, WITH PARTICULAR REFERENCE TO THE ECOLOGY AND EVOLUTION OF THE ENDEMIC PLANT TAXA

Rockhouses are semicircular recesses extending far back under cliff ov erhangs that are large enough to provide shelter for humans. The large st sandstone rockhouses in the eastern United States are at the heads of gorges, and they are in stream valleys cut during the Pleistocene; most are formed in Mississippian- and Pennsylvanian-age rocks. Compare d to the surrounding environment, the interior of rockhouses is shaded , is warmer during winter and cooler during summer, and has lower evap oration rates and higher humidities. Water enters rockhouses primarily by groundwater seepage and by dripping from the ceiling. Soil consist s mostly of sand with low pH, but high levels of some nutrients are as sociated with saltpeter earth and with ecofactual and artifactual rema ins left by human occupants during prehistoric time. Most plant taxa i n sandstone rockhouses in eastern United States are native C-3 phanero phytes or hemicryptophytes, and similarities in species composition am ong rockhouses are low. Eleven plant taxa belonging to eight families of flowering plants and ferns are endemic or nearly endemic to sandsto ne rockhouses in eastern United States. Three endemics are restricted to the gorges of a single river, and only one taxon ranges far north o f the Wisconsinan Glacial Boundary. The endemic ferns are Tertiary rel icts derived from tropical taxa. The majority of endemic flowering pla nts are derived from temperate taxa that grow in habitats in the vicin ity of rockhouses; their relative age ranges from Late Tertiary to the Recent. All the endemic taxa are perennial; two ferns occur as indepe ndent gametophytes. The endemic taxa of rockhouses are threatened prim arily by disturbances associated with recreation.