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

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NATURAL HISTORY OF BIG THICKET/INTRODUCTION 169 How does this compare with other areas of similar size? While the needed comparative data are not available, this analysis seems to agree with preliminary work by MacRoberts and MacRoberts (2004a, pers. comm.) suggesting that species richness of the Big Thicket may not be any greater than in certain other parts of the generally species-rich southeastern U.S., such as parts of Louisiana. When compared with larger areas such as North Central Texas (2,223 species in 40,000 square miles—Diggs et al. 1999), Kansas (1,807 species in 82,000 square miles—McGregor 1976), Oklahoma (2,549 species in 70,000 square miles—Taylor & Taylor 1994), or all of North America north of Mexico (about 18,000 species—Thorne 1993d), the Big Thicket can justifiably be considered a species-rich area. When one contemplates that the Big Thicket has approximately 10% of all the species in North America north of Mexico, past characterizations of the area as species-rich do not seem inaccurate. However, the same can be said for a number of other areas in the southeastern U.S. It will be interesting in the future to compile similar data sets for other areas of Texas and do actual comparisons. We speculate that there may be a few other Texas regions of a similar size that have diversity levels approaching those of the Big Thicket. These probably include: 1) the area of the Edwards Plateau centered on Gillespie County or some nearby area (this region includes numerous Edwards Plateau endemics, Central Mineral Region graniteloving species, and a wealth of species found on various substrates including limestone and sand), 2) an area in the Trans-Pecos including the Big Bend (which has dramatic topographical variation, many endemics, and numerous species entering from Mexico), and 3) the region of Brazos and surrounding counties (an ecotonal area of rich habitat diversity). It is also interesting to note that of the 1,826 species in the Big Thicket, 566 (31%) are monocots—the Big Thicket thus seems relatively rich in monocots when compared to North Central Texas, whose flora has only 26% monocots. This greater monocot diversity probably relates to the high percentage of mesic to wet habitats (favored by many monocots such as sedges, rushes, and xyrids) in the Big Thicket. For example, the floras of hillside seepage bogs and wetland pine savannahs have 55% and 51% monocots, respectively (MacRoberts & MacRoberts 2001). As with East Texas as a whole (see page 226), a number of interrelated factors contribute to the species richness of the Big Thicket. These include: 1) geologic and associated hydrologic variation; 2) high habitat diversity; 3) position near the ecotone between the eastern deciduous forests and the central North American grasslands (e.g., prairie inclusions within the Big Thicket); 4) proximity to a number of other source floras; 5) rich biogeographic history (e.g., remnant northern species as the result of the last Ice Age); and 6) the present-day warm wet (humid subtropical) climate allowing many species to coexist. The last of these factors reflects the general rule that within-habitat diversity of woody plants is higher in areas with warmer, wetter climates (Marks & Harcombe 1975). In the Big Thicket, abundant precipitation is well-distributed throughout the year—the average yearly precipitation is 48 to nearly 60 inches (122 to 152 cm), with the difference between the driest and wettest months being only 5 cm (2 inches) (Griffiths & Orton 1968; Marks & Harcombe 1981; Bomar 1995). With the exception of the Pacific Northwest, the Big Thicket is the westernmost area in the U.S. that receives such large amounts of precipitation. Because of the abundant rainfall and because of edaphic variation, soils vary from excessively drained to waterlogged. Therefore, species ranging from those requiring quite dry conditions to those needing extremely moist conditions can easily coexist in a relatively small area (i.e., there is a long topographic-moisture gradient—Marks & Harcombe 1975). Further, the relatively low latitude (approximately 30˚ N) and proximity to the Gulf of Mexico result in mild temperatures, with the frost-free season averaging between 230 and 260 days from north to south (Griffiths & Orton 1968). Snowstorms and hard freezes occur only rarely (Marks & Harcombe 1981). Because the mild winters are generally favorable for photosynthesis, many evergreen species can occur. Marks and Harcombe (1975) estimated that one-third of the tree
170 INTRODUCTION/NATURAL HISTORY OF BIG THICKET and shrub species in mesic Texas forests are evergreen, and some forest types and strata of the Big Thicket can justifiably be called “semi-evergreen” (Harcombe & Marks 1977). RARE AND ENDANGERED SPECIES OF THE BIG THICKET—Five plant species of the Big Thicket, as well as numerous animals, are considered to be of national conservation concern by the Big Thicket National Preserve (2002). These include bog coneflower (Rudbeckia scabrifolia, Species of Concern), Navasota ladies’-tresses (Spiranthes parksii, Federal Endangered), slender gay feather (Liatris tenuis, Species of Concern), Texas trailing phlox (Phlox nivalis var. texensis, Federal Endangered), and white firewheel (Gaillardia aestivalis var. winkleri, Species of Concern). Numerous other plants occurring in the Big Thicket are also quite rare in the state and are of conservation concern in Texas (Appendix 12). Many of these species, much more common further east, are known in the state only from one or two Big Thicket counties. While there is significant variation in the community affiliation of rare species, many of the rare plants of the Big Thicket (and East Texas in general) are pyrogenic (fire dependent) species associated with longleaf pine woodlands, a community that has been greatly altered by human activities including logging and fire suppression. THE BIG THICKET AS A BIOLOGICAL CROSSROADS—As noted earlier, the Big Thicket has often been described as a biological crossroads where species typical of areas to the east, west, north, and south meet and intermingle. Indeed, dry area plants such as Opuntia (prickly-pear, Cactaceae) and Yucca (Agavaceae) do occur in close proximity to Taxodium (bald-cypress, Cupressaceae) and other plants typical of swamps. While the Big Thicket is near the ecotone or transition zone between the eastern deciduous forests and the central North American grasslands, MacRoberts and MacRoberts (2004a), concluded that there is no evidence to support the crossroads idea. For example, they note that over 99% of the species in the Big Thicket are endemic or eastern in affinity. Further, they note that the main source of the crossroads concept, …appears to be the presence of xeric sandylands and prairies in southeast Texas. Superficially, xeric sandylands resemble deserts and some of the genera, but not the species, found in this habitat originated under desert conditions; cactus, agave, and yucca are obvious examples. But xeric sandylands are by no means confined to southeast Texas but extend from East Texas northward to Oklahoma and eastward to North Carolina. The same is true for prairies, which are not confined to the central and western states but extend across the southern and eastern United States. There is no evidence that the Big Thicket is a floristic crossroads. Its flora is eastern, notably southeastern. Indeed, at the species level, there is relatively little evidence of the Big Thicket as a biological crossroads. However, at the generic and family levels, the Big Thicket and much of the southeastern U.S. is an area of floristic intermingling. Plants with tropical origins (e.g., Mayacaceae, Melastomataceae), and those with affinities to the deserts of southwestern North America (Opuntia, Yucca), mix with plants more typical of the central North American grasslands and with plants pushed south during the climatic shift associated with glacial times (see page 208). While the Big Thicket is not unique in this regard, it does clearly share this interesting biogeographic history with other areas of the southeast. SIMILARITIES TO TROPICAL RAIN FORESTS—A striking aspect of the Big Thicket is its resemblance in a number of respects to tropical rain forests. Similarities include: ■ the very tall trees, in some cases over 150 feet (46 m) in height (Fritz 1993), ■ the swollen buttressed bases on trees such as bald-cypress and water tupelo, ■ numerous evergreen species (e.g., southern magnolia, sweet-leaf, wax myrtle, red bay, cherry laurel, American and other holly species, dwarf palmetto, Smilax species, etc.—in some areas about “50% of the forest composition is that of evergreens” (McLeod 1971),
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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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108 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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220 INTRODUCTION/ORIGIN AND DIVERSI
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228 INTRODUCTION/CONSERVATION IN EA
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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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