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A rockhouse microhabitat in the west cross timbers of North Central Texas
Abstract
A rockhouse microhabitat located in the xeric savannah/woodland community of the West Cross Timbers of North Central Texas is discussed and an associated disjunct population of Arisaema triphyllum is reported.
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Rockhouses are semicircular recesses extending far back under cliff overhangs that are large enough to provide shelter for
humans. The largest 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. Compared to the surrounding environment,
the interior of rockhouses is shaded, is warmer during winter and cooler during summer, and has lower evaporation rates and
higher humidities. Water enters rockhouses primarily by groundwater seepage and by dripping from the ceiling. Soil consists
mostly of sand with low pH, but high levels of some nutrients are associated with saltpeter earth and with ecofactual and
artifactual remains left by human occupants during prehistoric time. Most plant taxa in sandstone rockhouses in eastern United
States are native C3 phanerophytes or hemicryptophytes, and similarities in species composition among rockhouses are low. Eleven plant taxa belonging
to eight families of flowering plants and ferns are endemic or nearly endemic to sandstone rockhouses in eastern United States.
Three endemics are restricted to the gorges of a single river, and only one taxon ranges far north of the Wisconsinan Glacial
Boundary. The endemic ferns are Tertiary relicts derived from tropical taxa. The majority of endemic flowering plants are
derived from temperate taxa that grow in habitats in the vicinity of rockhouses; their relative age ranges from Late Tertiary
to the Recent. All the endemic taxa are perennial; two ferns occur as independent gametophytes. The endemic taxa of rockhouses
are threatened primarily by disturbances associated with recreation.
Fossil Picea pollen recovered from the Boriack Bog in central Texas are from layers radiocarbon dated as being older than 15,000 years in age. In an effort to identify the species of fossil spruce present, a comparison between modern Picea species and the fossil Picea grains was conducted with aid of a scanning electron microscope. Those studies revealed the closest correspondence between the fossil Picea pollen and modern pollen of Picea glauca. The presence of Picea glauca in the regional vegetation near the bog is further supported by macrofossil evidence from late‐glacial localities in Louisiana. These new data enable a reevaluation of the proposed east‐central Texas late‐glacial vegetation and a comparison with similar late‐glacial flora in the southeastern United States.