ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

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GENERAL GEOLOGY OF EAST TEXAS/INTRODUCTION 31 INTRODUCTION TO EAST TEXAS OVERVIEW East Texas is an area of approximately 62,600 square miles (162,200 square kilometers), delimited on the east by the state border, on the north by the Red River, and extending west and south to Dallas, Austin, and nearly to San Antonio and Houston. While a small region compared to Texas as a whole (approximately 269,000 square miles = 697,000 square km), it is about the size of Georgia. It includes the Pineywoods, Post Oak Savannah, Blackland Prairie, and the Red River Area (Fig. 2). The flora includes 3,402 species, slightly more than two-thirds of the total for all of Texas (Hatch et al. 1990; Turner et al. 2003), and 3,660 taxa (species, subspecies, and varieties) overall. This biological diversity is the result of numerous factors, including the region’s geologic and climatic variation and its location on the ecotone or transition zone between the eastern deciduous forests and the central North American grasslands. Further, this biological crossroads is influenced by its proximity to the southwestern deserts, the southeastern swamps, and the nearly tropical areas of southern Texas. East Texas is thus a mixing ground for plants, with the result being high species richness. For the past two centuries, humans have had, and are continuing to have, a tremendous impact on the plants and animals of the region. Presettlement and early settlement conditions were radically different from those found today, and environmental change continues to accelerate. Given current trends, the present generation may be the last with the opportunity to preserve even small remnants of the once extensive natural ecosystems of the area. GENERAL GEOLOGY OF EAST TEXAS The geology of East Texas is a fascinating story which has unfolded over millions of years. It is also a story that is key to understanding the plant life of the area, as well as the mineral wealth (e.g., oil and gas) so important to this part of the state. The geological history of the region is much more interesting and complex than the gently rolling to flat topography of most of East Texas would suggest. It is a subtle tale that can be read in stream beds, road cuts, or other areas where the soil is removed and small areas of the underlying bedrock are exposed. In only a few places can rocks be seen at the surface (e.g., Daingerfield State Park in Morris County; Boykin Springs in Angelina County—Fig. 10). In general, the easily erodible rocks and relatively heavy rainfall have combined to create a thick soil layer obscuring the interesting geology beneath. During the Pennsylvanian Period (325–286 million years ago [mya]) (Fig. 11), what is now East Texas was under an ocean on the southern edge of a North American continent shaped very differently than it is today. Later, as a result of plate tectonic movements, North America collided with Africa and South America to become part of the supercontinent Pangaea. The outcome of this collision was the uplift and formation of an extensive mountain system including the Appalachians, Wichitas, and Ouachitas. The ancient Ouachita Mountains formed approximately 300 million years ago in a line roughly following the western edge of the current Blackland Prairie, from near Sherman and Dallas, south along the Balcones Escarpment to Austin and beyond (Fig. 12). This ancient Ouachita mountain belt also continued to the northeast; the eroded Ouachitas seen today in southeastern Oklahoma and southwestern Arkansas are remnants of this once much more extensive range. To the west of the ancient Ouachita Mountains, crustal areas sagged and low basins formed. Shallow inland seas invaded these low areas, and during the Pennsylvanian, and later the Permian (286–248 mya), western Texas served as a collection basin for the sediments that eroded from the Ouachita Mountains east of the basin. These thick sediments harbor oil-bearing layers, so
32 INTRODUCTION/GENERAL GEOLOGY OF EAST TEXAS FIG.10/ROCK OUTCROP NEAR BOYKIN SPRINGS,ANGELINA NATIONAL FOREST,ANGELINA CO.(PHOTO BY GMD). important to the Texas economy, and are also the source of the strikingly bright red, iron oxide-rich (hematite) Permian layers that easily erode and give the modern Red River its name. The salinity of the Red River and thus of Lake Texoma is also the result of erosion from salt-rich Permian-age evaporation flats, through which the river passes on its course east from the Texas Panhandle. Over tens of millions of years the Ouachita Mountains gradually eroded, until today all that is left over most of Texas are their roots, deeply buried under thousands of feet of younger sediments (Spearing 1991). During the Triassic (248–213 mya) and Jurassic (213–145 mya) periods and continuing into the Cretaceous Period (145–65 mya), Pangaea eventually split into a southern portion (Gondwana) and a northern portion (Laurasia) and then into separate continents. The western part of the East Texas region once again became very active geologically, with the ancient zone of weakness where the Ouachitas had originally formed serving as the site of continental rifting or breakup between North and South America. It was here, where the continents pulled apart and the crust sagged, that huge shallow seas, eventually retreating to become the present-day Gulf of Mexico, began to form. Relatively early in this process, during the Jurassic Period, the Gulf was shallow and not well-connected to the ocean, and sometimes it virtually dried up, leaving vast salt flats. The result over long periods of time was the deposition of tremendously thick layers of salt known now as the Louann Salt. Eventually, as sedimentation continued and more and more material was laid down over the salt, the tremendous pressure of the overlying younger sediments (from the Cretaceous and Tertiary) caused the salt layers to become distorted. Acting under pressure, almost like toothpaste being squeezed out of a tube, the salt formed upward thrusting columns and spires, which in some areas broke through the covering sediments to reach to or near the surface in the form of isolated domes of salt (e.g., near Palestine in Anderson County and Grand Saline in Van Zandt County; in the latter case, the salt is currently mined) (Jackson & Seni 1984a; Spearing 1991) (Fig. 13). These salt domes are ecologically significant because of the effect they have on plant and animal life. Coastal salt marsh plants, for example,
Page 1 and 2: ILLUSTRATED FLORA OF EAST TEXAS
Page 3 and 4: ALLDREDGE, LINDA & JACK BATTERBAE,
Page 5 and 6: We would like to recognize and expr
Page 7 and 8: ISSN 0833-1475 ISBN 1-889878-12-X S
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Page 11 and 12: Elray S. Nixon E lray Nixon was bor
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Page 15 and 16: TABLE OFCONTENTS/ PAGE APPENDICES A
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Page 21 and 22: 4 INTRODUCTION/GEOGRAPHIC AREA OF E
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Page 37 and 38: 20 INTRODUCTION/GUIDE TO ENDANGERED
Page 39 and 40: 22 INTRODUCTION/GUIDE TO GLOSSARY,
Page 41 and 42: 24 INTRODUCTION/GUIDE TO ABBREVIATI
Page 43 and 44: 26 INTRODUCTION/SUMMARY DATA ON FLO
Page 45 and 46: 28 INTRODUCTION/ACKNOWLEDGMENTS Kar
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Page 95 and 96: 78 INTRODUCTION/GEOLOGY OF PINEYWOO
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82 INTRODUCTION/PRESETTLEMENT, EARL
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88 INTRODUCTION/PRESETTLEMENT VEGET
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106 INTRODUCTION/POST OAK SAVANNAH
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110 INTRODUCTION/GEOLOGY AND SOILS
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112 INTRODUCTION/SOILS, FIRE, AND T
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114 INTRODUCTION/SOILS, FIRE, AND T
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116 INTRODUCTION/ PRESETTLEMENT, EA
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118 INTRODUCTION/VEGETATION OF POST
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128 INTRODUCTION/RED RIVER AREA are
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130 INTRODUCTION/BLACKLAND PRAIRIE
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132 INTRODUCTION/GEOLOGY OF BLACKLA
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144 INTRODUCTION/VEGETATION OF BLAC
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146 INTRODUCTION/FUTURE OF BLACKLAN
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148 INTRODUCTION/FUTURE OF BLACKLAN
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150 INTRODUCTION/CADDO LAKE LOCATIO
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152 INTRODUCTION/CADDO LAKE FIG.84/
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154 INTRODUCTION/PRESENT AND FUTURE
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156 INTRODUCTION/BIG THICKET THE BI
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158 INTRODUCTION/BIG THICKET GEOGRA
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160 INTRODUCTION/NATURAL HISTORY OF
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234 INTRODUCTION/HISTORY OF BOTANY
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244 INTRODUCTION/BOTANY IN NORTH CE
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246 INTRODUCTION/BOTANY IN NORTH CE
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248 INTRODUCTION/BOTANY AT UNIVERSI
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250 INTRODUCTION/BOTANY AT TEXAS A&
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252 INTRODUCTION/BOTANY AT TEXAS A&
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254 INTRODUCTION/OTHER CONTRIBUTION
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256 INTRODUCTION/CURRENT BOTANICAL
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258 INTRODUCTION/BOTANICAL ART IN E
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