Appendices & Glossary - Botanical Research Institute of Texas

EAST TEXAS AS A UNIQUE HABITAT 1
George M. Diggs, Jr.
Department of Biology, Austin College, Sherman, Texas 75090, U.S.A.
& Botanical Research Institute of Texas, Fort Worth, Texas 76102, U.S.A., gdiggs@austincollege.edu
ABSTRACT
APPENDIX TEN
East Texas is a botanically unique area with information from a variety of perspectives pointing to its distinctiveness. Some of
these include: 1) high species diversity; 2) numerous habitats occurring within a small area; 3) similarities to the tropics; 4) influence
of glaciation; 5) the modern vegetation as the result of a rich biogeographical heritage; and 6) unusual geological features.
East Texas is a botanically unique area from a variety of perspectives. The following six noteworthy
aspects will be discussed: 1) high species diversity; 2) numerous habitats occurring within a small area;
3) similarities to the tropics; 4) influence of glaciation; 5) the modern vegetation as the result of a rich
biogeographical heritage; and 6) unusual geological features.
1) Biological diversity—The approximately 3,230 vascular plant species found in East Texas (nearly
66% of the Texas flora) make it particularly rich in species for its size (ca. 60,000 square miles or ca.
23% of Texas; roughly the size of Georgia). This can be put in perspective when it is realized that the
flora of the entire Great Plains, which make up one-fifth of the area of the conterminous United
States, consists of 3,067 taxa of vascular plants (Great Plains Flora Association 1977, 1986; Thorne
1993d). When the entire flora of North America north of Mexico is considered (estimated at approximately
18,000 species—Thorne 1993d), East Texas includes about one out of every six plant species
known in the continental United States and Canada. Likewise, when the number of species in three
adjacent states Arkansas (2,356—Smith 1988), Louisiana (2,952—MacRoberts 1984), and Oklahoma
(2,549 species—Taylor & Taylor 1994) is considered, East Texas again appears particularly diverse. This
diversity is the result of a number of factors, including the region’s geologic and climatic variation. Yet
another factor is the habitat diversity present—large numbers of different habitats in a relatively small
area (discussed below), each of these supporting a diverse assemblage of species. However, perhaps the
most important factors contributing to the biological diversity in East Texas are the area’s position on
the ecotone or transition zone between the eastern deciduous forests and the central North American
grasslands, and its proximity to the desert southwest and the nearly tropical area of south Texas. The
result is a flora derived from several major, and quite different, floristic provinces (Thorne 1993d). Because
of the resulting diverse floristic elements and its mid-continental position, the Big Thicket has
been referred to by some as the “Biological Crossroads of North America” (e.g., Gunter 1993). The same
description is perhaps even more fitting for East Texas as a whole. The intermingling of elements typical
of the eastern deciduous forest, southeastern swamps, central North American grasslands, southwestern
deserts, and even the tropics is striking, resulting is an extremely diverse flora.
2) Habitat richness—An unusual phenomenon seen in some portions of East Texas (particularly the
Big Thicket area) is the close proximity of numerous radically different habitats (e.g., arid sandylands,
beech-magnolia forest, baygall, cypress slough, palmetto hardwood flats, etc.). The complex vegetational
pattern is controlled by slight variations in elevation, soil type, and available water (Parent
1993), but the result is that an extraordinary number of habitats can be found within a very small area.
One of the best examples can be seen in the Big Thicket, where walking a transect over a few tens of
meters can reveal the following—beginning in an arid sandylands upland with longleaf pine, shrubby
bluejack oaks, bull-nettles, yuccas, prickly pear cacti, bracken fern, and roadrunners, proceeding down
slope to a beech-magnolia forest with Carolina lily, crane-fly orchids and beech drops, and finally arriving
at a baygall at the bottom of the slope with sweetbay magnolia, gallberry holly, titi, nodding-nixie, and
1 Reprinted from Diggs (2002).