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

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ORIGIN AND DIVERSITY OF EAST TEXAS FLORA/INTRODUCTION 211 Here, special soil (derived from the Eocene Carrizo and Reklaw geologic formations) and topographic conditions have allowed the survival of a loblolly pine-dominated community disjunct to the west from other such habitats. The Ottine Swamp at Palmetto State Park in Gonzales County is another unusual, surprisingly isolated habitat. This “boggy oasis most notable for the thick understory of dwarf palmettos” seems quite out of place in the surrounding relatively dry portion of the southern Post Oak Savannah. It occurs on a terrace in the San Marcos River valley and is fed by flooding and springs from the “Carrizo sands that are exposed at the base of the bluff that lines the river valley” (Parent 1997). The unusual physiographic situation (the confluence of the Guadalupe and San Marcos rivers—Bousman 1998), the occurrence of certain geologic strata, and the unique microhabitat created have fostered the presence of numerous disjunct species. This isolated wetland is probably the remnant of a much more extensive area of Ice Age swamp that has been able to survive to the present due to its unique hydrological circumstances. Such remnant habitats are also known from various other parts of Texas. Just one example found in the Hill Country of the Edwards Plateau just west of East Texas is the Lost Maples State Natural Area. Here in the protection of deep canyons are found a number of species unusual in the area, including Acer grandidentatum (plateau big-tooth maple), Cotinus obovatus (American smoketree), Hamamelis virginiana (common witch-hazel), Philadelphus texensis (canyon mockorange), Platanus occidentalis (American sycamore), and Styrax platanifolius (sycamore-leaf snowbell) (Parent 1997). Only because of the special microclimate provided by the canyons have such species been able to survive the drying and warming that have occurred since the end of the last Ice Age. In summary, the influence of the eastern deciduous forest on the flora of East Texas is an extremely complex and interesting story written across tens of millions of years, with untold upheavals in geology and climate and profound evolutionary change. Yet it is a story that can be deciphered using modern concepts and techniques in geology, paleobotany, paleoclimatology, and evolutionary biology. Having an understanding of the geohistorical roots of the modern day flora makes it a profoundly more interesting and rewarding subject of study. It is hoped that this type of information will encourage preservation of at least small areas of these fascinating ecosystems. While forestry practices of the past 150 years have eliminated virtually all old growth forest in East Texas, significant second or third growth stands still survive. Unfortunately, even these are now being destroyed by economic pressures and accelerating development. It is difficult not to question such actions. Should the shortsightedness and greed of humans be allowed to destroy within a span of a few hundred years an ecosystem with geohistorical roots extending back tens of millions of years? INFLUENCE OF THE NORTH AMERICAN PRAIRIES PROVINCE ON THE EAST TEXAS FLORA The second major floristic component of East Texas is derived from the grasslands comprising the North American Prairies Province (Thorne 1993d). At the time of European settlement, grassland was the most extensive North American vegetation type, covering approximately 30% of today’s U.S. and 21% of North America north of Mexico (Sims 1988; Barbour & Christensen 1993; Graham 1999; Sims & Risser 2000). According to Graham (1999), “When French explorers moved south from eastern Canada into the central plains, they encountered a vast expanse of grassland never before witnessed by western Europeans, and applied the closest French word for a community dominated by grasses and forbs— prairie (meadow).” Grassland vegetation historically covered the western portion of East Texas (Blackland Prairie), shared dominance with woody plants in the Post Oak Savannah, and occurred as isolated pockets of prairie even within the Pineywoods on areas of special soils. Allred and Mitchell (1955) viewed much of East Central Texas as prairie. In their broad classification of Texas vegetation, they considered not only the Blackland Prairie, but also the eastern Texas Post Oak belt (Post Oak Savannah) to be part of the True Prairie Association. They supported this contention by pointing out that the grasses of the True
212 INTRODUCTION/ORIGIN AND DIVERSITY OF EAST TEXAS FLORA Prairie are important components in the vegetation of the Post Oak Savannah. Barbour and Christensen (1993) included all of East Texas except the Pineywoods in their tall grass prairie vegetation type, and stated that in the southern part of the tall grass prairie—deciduous forest boundary (including Texas), the ecotone is an oak savannah 50–100 km (31–62 miles) wide. Graham (1999) also considered the savannah to be “primarily grassland— deciduous forest boundary vegetation.” Since it is clearly composed of components of both prairie and deciduous forest, viewing the Post Oak Savannah as an ecotone seems the most reasonable approach. Vankat (1979) and Graham (1999) emphasized that open savannah is fire-dependent and that fire is important to the perpetuation of this type of vegetation. Graham (1999) also noted that “because fire is now controlled in many areas, woodland or forest vegetation has invaded areas once occupied by oak savanna.” This is certainly true for East Texas where recent human activities, particularly the suppression of fire and the conversion to cropland, have greatly reduced the amount of grassland and open savannah present. Even where not completely destroyed, the Post Oak Savannah of East Texas is thus probably significantly different than in presettlement times, with tree densities much higher at present than previously. Axelrod (1985) argued that North American grasslands are geologically recent, and that the rise of extensive grasslands probably dates only to the Miocene-Pliocene transition (about 7–5 mya), the driest part of the Tertiary. Fossil evidence shows that the Great Plains were largely forested from the Middle Miocene into the Early Pliocene. As the climate dried toward the end of the Miocene, forests gradually became more restricted (e.g., confined to mesic areas such as moister valleys and stream bottoms), grasslands were able to spread rapidly, and there was an “explosive evolution of grasses and forbs” (Axelrod 1985). Fossil floras from the Great Plains area dated between about 18 to 4 million year ago indicate that prairie grasses and herbs became more abundant, widespread, and diverse throughout the period (Graham 1999). The increasing number of grazing animals seen in the fossil record also reflects the shift toward increasing aridity and drought in the region (Axelrod 1985). At least part of the climatic drying of the continental interior resulted from the continued rise of the Rocky Mountains. “The upper flow of the atmosphere was blocked…, preventing tropical moisture from the Pacific Ocean…from reaching midcontinent.…” (Van Devender 2002). The midcontinent region was thus in the “rain shadow” of the Rockies and the climatic conditions were created which resulted in the development of grasslands. Fossil evidence also suggests that subsequently, during the Pleistocene Epoch beginning 1.8 million years ago, there was great fluctuation in grassland versus forest vegetation. These changes were associated with glacial cycles (Delcourt & Delcourt 1993). According to Axelrod (1985), during the Pleistocene, “the treeless plains were reduced in area as forests and woodlands spread under moister climates.” Warmer, drier interglacial periods promoted the spread of grasslands, while moister, cooler glacial periods favored forest or woodland. During full-glacial times, boreal forest (e.g., spruce) dominated much of the Great Plains (e.g., spruce forest in Kansas—Graham 1999), with mixed conifer-northern hardwoods to the south and deciduous forest to the south of these (Delcourt & Delcourt 1993). Extensive areas of grassland probably did not exist during full-glacial conditions (Wright 1981). During interglacial periods, deciduous forest (oak-hickory), savannah, and grassland expanded as the climate warmed and dried. As late as from 15,000 to 12,000 years ago, areas now covered with grassland vegetation (e.g., parts of the Texas panhandle) supported forest or woodland (Delcourt & Delcourt 1993), with drier areas of the region having oak savannah and grassland (Hall & Valastro 1995). Central Iowa went from fir-spruce forest at 14,500 years ago to a vegetation dominated by deciduous trees at 8,300 years, to a succession of oak-grassland and then grass around 3,000 years ago. (Axelrod 1985). It is thus now clear that at the end of the last glaciation (approximately 18,000–15,000 years ago), with increasing temperatures and aridity, there was a profound shift in vegetation across much of North America. Grasslands were favored by these changes, due to a
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ILLUSTRATED FLORA OF EAST TEXAS
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ALLDREDGE, LINDA & JACK BATTERBAE,
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We would like to recognize and expr
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ISSN 0833-1475 ISBN 1-889878-12-X S
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“DEEP EAST TEXAS IS AN OLD, QUIET
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Elray S. Nixon E lray Nixon was bor
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ILLUSTRATED FLORA OF EAST TEXAS TAB
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TABLE OFCONTENTS/ PAGE APPENDICES A
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Hymenocallis liriosme
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ILLUSTRATED FLORA OF EAST TEXAS OVE
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4 INTRODUCTION/GEOGRAPHIC AREA OF E
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10 INTRODUCTION/MAJOR RIVERS OF EAS
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12 INTRODUCTION/PLANTS TREATED The
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14 INTRODUCTION/GUIDE TO ARRANGEMEN
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16 INTRODUCTION/GUIDE TO SOURCES OF
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18 INTRODUCTION/GUIDE TO GEOGRAPHIC
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20 INTRODUCTION/GUIDE TO ENDANGERED
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22 INTRODUCTION/GUIDE TO GLOSSARY,
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24 INTRODUCTION/GUIDE TO ABBREVIATI
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26 INTRODUCTION/SUMMARY DATA ON FLO
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28 INTRODUCTION/ACKNOWLEDGMENTS Kar
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30 INTRODUCTION/AUTHORS’ NOTE Pre
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32 INTRODUCTION/GENERAL GEOLOGY OF
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46 INTRODUCTION/SOILS SOIL ORDER DE
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48 INTRODUCTION/SOILS, EAST TEXAS B
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50 INTRODUCTION/SOIL-RELATED GEOLOG
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60 INTRODUCTION/SOILS OF BLACKLAND
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66 INTRODUCTION/CLIMATE OF EAST TEX
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Forests define East Texas, and to a
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78 INTRODUCTION/GEOLOGY OF PINEYWOO
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80 INTRODUCTION/PRESETTLEMENT, EARL
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88 INTRODUCTION/PRESETTLEMENT VEGET
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90 INTRODUCTION/CURRENT VEGETATION
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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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124 INTRODUCTION/CONDITIONS TODAY O
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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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134 INTRODUCTION/PRESETTLEMENT AND
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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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262 INTRODUCTION/BOTANICAL ART IN E
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