Appendices & Glossary - Botanical Research Institute of Texas

EAST TEXAS AS A UNIQUE HABITAT/APPENDIX TEN
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It is interesting to consider the impact of such profound climatic change on where plants occur today.
Many otherwise difficult to explain plant distributions may be easily accounted for by considering
them relicts of different climatic conditions of the past. A possible example is the occurrence of
jack-in-the-pulpit (Arisaema triphyllum) in Parker County in the West Cross Timbers. This species is
generally limited to mesic environments in the eastern part of the state. Nonetheless, a thriving population
can be found in a mesic microhabitat, situated between sandstone rock walls in Parker County,
well to the west of other known locations of this species. It seems likely that this population is a relict
of a previously extensive forest that was largely lost as the climate warmed and dried. Other eastern
species that have been found in surprising places much to the west include eastern hop-hornbeam
(Ostrya virginiana) in Tarrant County and shag-bark hickory (Carya ovata) in Parker County, both
disjunct by more than 120 miles (193 kilometers) from their current day locations in East Texas.
Numerous other examples could be given, including many species and genera typical of East Texas
showing up in isolated pockets on the Edwards Plateau (e.g., groundnut (Apios americana), cross-vine
(Bignonia capreolata), witch-hazel, (Hamamelis virginiana), spice bush (Lindera benzoin), barbed
rattlesnake-root (Prenanthes barbata), Carolina rose (Rosa carolina), dwarf palmetto (Sabal minor),
American basswood (Tilia americana), and maple (Acer). An alternate explanation for these disjunctions,
long distance dispersal, seems unlikely given the distance (up to 280 miles (450 km)), the number
of species displaying the pattern, and the significant number of Edwards Plateau endemics, including
many in genera typically found in more mesic habitats farther east (e.g, Texas snow-bell,
Styrax platanifolius subsp. texanus). Further, dispersal by wind, water, or migratory birds seems unlikely
since the prevailing winds and river drainage are from west to east, and there is virtually no
migration from the east and almost no faunal overlap between the Edwards Plateau and East Texas
(Palmer 1920). Likewise, many more typically eastern species extend west in the Red River valley (e.g.,
numerous species reach their western limits in Grayson County, including beaked groovebur
(Agrimonia rostellata), black oak (Quercus velutina), may-apple (Podophyllum peltatum), and
Solomon’s-seal (Polygonatum biflorum). In addition, many herbaceous species, common farther north
and east in the U.S., occur in Texas only rarely and erratically in the Pineywoods and have extremely
limited distributions. Kral (1966c) and MacRoberts and MacRoberts (1997a) discussed a number of
these “northern woodland elements” south of their normal range (e.g., Erythronium rostratum, Lilium
michauxii, Sanguinaria canadensis, Silene stellata, Trillium recurvatum, Uvularia sessilfolia) and indicated
that they appear to be relicts of glacial times surviving in areas with particularly favorable soil
and moisture conditions (i.e., refugia). Kral (1966c) noted that he could walk for miles and then find a
large colony of one of these species, apparently reproducing predominantly via vegetative means.
These species may thus be “Ice Age holdovers hanging on precariously to the older geologic terraces in
the coolest locations in the forest” (MacRoberts & MacRoberts 1997a). Even the presence of the
drought intolerant American beech (Fagus grandifolia) in East Texas is surprising given its ecological
requirements. This species reaches its southwestern limit in the U.S. in Montgomery Co. (not far north
of Houston)—here beech appears to do best when protected from the intense Texas summer sun by a
canopy of associated trees (McLeod 1975). All of these examples may be relicts of much more widespread
distributions of deciduous forest species during glacial times when the climate in Texas was
quite different and conditions much more mesic (Kral 1966c, Palmer 1920; O’Kennon 1991; Delcourt &
Delcourt 1993). It is not surprising that isolated populations of once more widespread plant species are
able to persist in small areas of special microclimate or unusual geology—in fact, such persistence
would be expected.
5) Rich biogeographical heritage—The eastern deciduous forest, while covering only approximately
11 percent of the North American continent, “is the most diverse and species-rich component of the
North American vegetation” (Graham 1999). This diverse forest extends as far west as East Texas,
where a number of temperate genera reach the southwestern limit of their present distribution in the
continental U.S. (Graham 1999)—in other words, numerous genera and hundreds of species occur all
across the eastern U.S., in some cases only barely reaching East Texas (in a few instances only Jasper
and Newton counties—e.g., Magnolia pyramidata, pyramid magnolia; or only Newton County—e.g.,
Stewartia malacodendron, silky-camellia or Uvularia sessilifolia, sessile-leaf bellwort). From the