ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
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FIG. 123/MAP SHOWING EXTENT <strong>OF</strong> ICE AT END <strong>OF</strong> THE LAST GLACIAL MAX-<br />
IMUM (18,000 YEARS BEFORE PRESENT) WITH LOCATION <strong>OF</strong> VEGETATION TYPES<br />
AT THAT TIME. T—TUNDRA; BF—BOREAL FOREST; MF—MIXED FOREST<br />
(CONIFER-NORTHERN HARDWOOD); DF—DECIDUOUS FOREST; SE—SOUTH-<br />
<strong>EAST</strong>ERN EVERGREEN FOREST; SS—SAND DUNE SCRUB.(MODIFIED FROM DAVIS<br />
1983 AND DELCOURT & DELCOURT 1993).<br />
oaks with some open grassland areas. Bryant<br />
and Holloway (1985b) further suggested that<br />
around 1,500 years before present, the modern<br />
day oak-savannah vegetation (Post Oak<br />
Savannah) became established, reflecting “a<br />
prolonged period of drier, and perhaps<br />
warmer, climatic conditions.” However,<br />
Bousman (1998) has more recently concluded<br />
that arboreal cover reached its low<br />
point in the area about 5,000 years ago<br />
and has increased since that time. Given<br />
the difficulty of determining with confidence<br />
the changes in paleoclimates, he<br />
emphasized that there probably have<br />
been “numerous shifts between forest,<br />
woodland, and open plant communities”<br />
in the area since the end of the last glaciation<br />
(approximately 18,000–15,000<br />
years ago). The vegetation pattern<br />
present at the time of settlement, while<br />
often mistakenly viewed as static, was<br />
in a state of flux, and was thus just one point in a continuing series of changing conditions<br />
(Smeins 1984).<br />
It is interesting to consider the effects of past glaciation on the present flora. Many otherwise<br />
difficult to explain modern plant distributions may be easily accounted for by regarding<br />
them as the result of changing climatic conditions of the past. One example is the occurrence<br />
of Cladium mariscoides (smooth saw-grass, Cyperaceae) disjunct to a few bog localities in East<br />
Texas and the far southeastern U.S., but otherwise known only from southeastern Canada and<br />
the northeastern U.S. as far south as South Carolina and Tennessee (Bridges & Orzell 1989a;<br />
Tucker 2002c). Another possible example is the occurrence of jack-in-the-pulpit (Arisaema<br />
triphyllum) in Parker County in the West Cross Timbers. This species is generally limited to<br />
mesic environments in the eastern part of the state. Nonetheless, a thriving population can be<br />
found in a mesic “rockhouse” microhabitat between sandstone rock walls in Parker County,<br />
well to the west of other known locations of this species (Diggs & O’Kennon 2003). It seems<br />
likely that this population is a relict from<br />
a previously extensive forest that was<br />
largely lost as the climate warmed and<br />
dried. Such persistence in climatically<br />
moderated “rockhouse” environments<br />
has been documented for a variety of<br />
plant species, including endemics, in the<br />
eastern U.S. (e.g., Walck et al. 1996;<br />
Farrar 1998). Other eastern species that<br />
have been found surprisingly far west in<br />
Texas include eastern hop-hornbeam<br />
(Ostrya virginiana) in Tarrant County and<br />
shag-bark hickory (Carya ovata) in<br />
Parker County, both separated by more<br />
than 120 miles (193 kilometers) from<br />
their present distributions in East Texas.<br />
Numerous other examples could be<br />
given, including many species and genera<br />
ORIGIN AND DIVERSITY <strong>OF</strong> <strong>EAST</strong> <strong>TEXAS</strong> <strong>FLORA</strong>/INTRODUCTION 209<br />
FIG. 124/ MODERN DISTRIBUTION <strong>OF</strong> PICEA GLAUCA (PINACEAE) (WHITE<br />
SPRUCE) (FROM TAYLOR 1993, WITH PERMISSION <strong>OF</strong> OXFORD UNIV.PRESS).