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ORIGIN AND DIVERSITY OF EAST TEXAS FLORA/INTRODUCTION 207 multiple origin scenario, with Xiang and Soltis (2001) suggesting that at least three independent events contributed to the disjunction. It thus now seems clear that while the similarities between the eastern Asian and eastern North American floras in general are explained by the widespread Tertiary distribution of North Temperate forest elements and subsequent extinctions in western North America and Europe, the pattern is the result of multiple origins and a complex series of causes. Another interesting floristic relationship is seen between some isolated forests in the mountains of Mexico and those in the eastern United States (Fig. 121). Numerous East Texas deciduous forest genera (e.g., Acer, Alnus, Carpinus, Cercis, Crataegus, Cornus, Epifagus, Fagus, Fraxinus, Juglans, Liquidambar, Magnolia, Mitchella, Myrica, Nyssa, Ostrya, Pedicularis, Platanus, Prunus, Quercus, Rhus, Smilax, Tilia, Ulmus, and Vaccinium), and even a few individual species (e.g., Carpinus caroliniana (American hornbeam), Epifagus virginiana (beechdrops), Fagus grandifolia (American beech), Nyssa sylvatica (black-gum), Liquidambar styraciflua (sweetgum), and Mitchella repens (partridge berry)) occur broadly across the eastern United States as far west as eastern Texas and then reappear in the Mexican highlands, and in some cases even in Guatemala (Miranda & Sharp 1950; Martin & Harrell 1957; Thorne 1993d; Graham 1993a, 1999). Most of the deciduous forest “temperate” genera occur in Mexico in “isolated patches of humid montane forest, typically in the Sierra Madre Oriental,” usually above 1,000 m (3281 feet) elevation (Martin & Harrall 1957). These occurrences in Mexico represent a disjunction across 500 km (311 miles) or more of arid grassland and thorn scrub (Martin & Harrall 1957). At one time, the floristic similarities were thought to be the result of the migration of vegetation zones associated with Pleistocene glaciation (Dewey 1949). However, the fact that this relationship exists primarily at the generic level implies a substantial period of isolation of the Mexican components, during which speciation occurred. This conclusion is in line with recent paleobotanical evidence showing that the relationship was established long before the glacial cycles of the Pleistocene (Graham 1999). In actuality, this relationship represents a Middle Miocene (Miocene Epoch—23.8–5.3 mya) extension of deciduous forest and associated fauna (particularly amphibians) into Mexico during a period of widespread climatic cooling (e.g., Antarctic glaciation) (Burnham & Graham 1999; Graham 1999). In the words of Graham (1999), a “major decline in temperature occurred in the Middle Miocene (15–14 mya),” and “global temperatures reached new lows, allowing the deciduous forest to reach its southernmost extent.” Subsequently, during the Pliocene (5.3–1.8 mya) and later, the climate warmed and dried, and other types of vegetation (e.g., prairie and dry area shrub communities) spread at the expense of deciduous forest. As a result, the once continuous deciduous forests became fragmented, surviving in the eastern U.S. and eastern Mexico. During the subsequent millions of years, evolution has resulted in the differentiation of some disjunct populations into separate species, while others are still similar enough to be considered one species. With continued climatic change, this forest type has been further reduced in Mexico, until at present it survives in only limited isolated pockets of appropriate microclimate in the highlands (Miranda & Sharp 1950; Graham 1993a, 1993b, 1999). More recently, during the Quaternary Period (beginning about 1.8 mya and including the Pleistocene and Recent epochs), there have been profound changes in the vegetation of the southeastern U.S., including East Texas. During the Pleistocene Epoch there was significant climatic variability and at least 20 glacial-interglacial cycles. Not surprisingly, widespread changes in vegetation were associated with these climatic fluctuations (Delcourt & Delcourt 1993). This is true even in southern areas such as Texas, even though actual glaciers were hundreds of miles to the north of Texas. During the last major period of full glaciation (100,000 to about 18,000-15,000 years ago), the vegetation of the eastern U.S. was radically different than at present. Sea level was 100–120 m lower than in modern times, and the Laurentide Ice Sheet extended south to approximately 40º N latitude (Fig. 122). It is now known that the boreal forest region “was more than 1,200 km south of its modern southern
208 INTRODUCTION/ORIGIN AND DIVERSITY OF EAST TEXAS FLORA border in Canada” (Delcourt & Delcourt 1985). In fact, boreal forest extended south to about 34º N latitude, and a narrow ecotone existed between 34º and 33º N latitude between “northern boreal and more southern temperate communities” (Delcourt & Delcourt 1993). It should be noted that this ecotone cut across the northern several tiers of East Texas counties (e.g., Bowie, Grayson, Hunt, Red River). “South of latitude 33º, across the southern Atlantic and Gulf coastal plains, floristic elements of temperate deciduous forest occurred with plant taxa characteristic today of southeastern evergreen forests” (Delcourt & Delcourt 1993). It was in far southern areas such as East Texas that some eastern deciduous forest species FIG. 122/EXTENT OF ICE IN NORTH AMERICA AT THE END OF THE LAST GLACIAL MAX- found refuge during full glacial IMUM 18,000 YEARS BEFORE PRESENT; NOTE ALSO THE DIFFERENCE IN CONTINENTAL times (Fig. 123). Indeed, data from MARGINS DUE TO LOWERING OF SEA LEVELS AS A RESULT OF LARGE AMOUNTS OF WATER East Texas bogs (e.g., Boriack Bog, BEING TIED UP IN GLACIAL ICE (ADAPTED FROM BROUILLET & WHETSTONE 1993, WITH Lee County) in the Post Oak PERMISSION OF OXFORD UNIV.PRESS). Savannah confirm the presence of such deciduous forest genera as Acer, Alnus, Carya, Castanea, Cornus, Corylus, Myrica, and Tilia more than 15,000 years ago and suggest the region was heavily forested near the end of the last glaciation (Bryant 1977; Bryant & Holloway 1985b). While these genera can still be found in East Texas today (generally further east), the profound climatic differences at that time are reflected in the presence of small amounts of Picea glauca (white spruce) pollen in late glacial deposits at Boriack Bog (radiocarbon dated as older than 15,000 years) (Bryant 1977; Holloway & Bryant 1984; Bryant & Holloway 1985b). This cold-tolerating species occurs today only much farther north (Fig. 124). Bryant and Holloway (1985b) concluded that as post-glacial warming occurred in East Texas, the glacial age forest lost certain key components such as Picea and Corylus, yet the region remained forested with a wide variety of deciduous trees. Another significant vegetational change in North America as a whole has been the great fluctuation in the amount of grassland versus forest/woodland vegetation. This fluctuation can be associated with glaciation and deglaciation—e.g., areas now covered with grassland vegetation, such as the Texas panhandle, supported forest prior to post-glacial warming (Webb 1981; Axelrod 1985). All of Texas was thus under very different conditions during the last full-glacial interval. At that time, there existed across the unglaciated parts of southwestern North America (e.g., Edwards Plateau), a cool, moist “pluvial” climate (Delcourt & Delcourt 1993) with forest species expanding their ranges. Subsequently, the climate moderated between 15,000–10,000 years ago, with interglacial conditions (i.e., warmer and drier) prevailing for the last 10,000 years (Delcourt & Delcourt 1993). According to Bryant and Holloway (1985b), since late glacial times there has been in general a warming and drying trend in the western part of East Texas. Data from Weakley Bog (Leon County) indicate that vegetation from about 2,400 to 1,500 years before present was open woodland dominated by
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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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258 INTRODUCTION/BOTANICAL ART IN E
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