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

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SOILS, FIRE, AND THE POST OAK SAVANNAH/INTRODUCTION 113 this process begins, any prairie area that becomes substantially invaded by trees would lose grass biomass and come to serve as an additional firebreak, thereby further reducing the likelihood of fire on adjacent remaining prairies. Significantly, the result of the tree invasion is that many (but certainly not all) areas of the prairies (and of the Cross Timbers and Post Oak Savannah understory) is now dominated by Juniperus virginiana (eastern red cedar) and Juniperus ashei (Ashe’s juniper), species that are sensitive to fire (easily scorched/ignited and unable to resprout from roots). At this time, measurements of grass biomass on intact remnants of the prairies and woodlands have not been made. While some biomass and fuel loading data are available in the literature (e.g., Johnson & Risser 1974; Engle & Stritzke 1995), we have been unable to find directly comparable data for the area being considered. However, substantial indirect evidence is consistent with the assumption of higher grass biomass on clay soils. First, during dry periods clay soils generally hold more water at grass rooting depths than do sandy soils. This is due to the relatively large surface areas of the individual clay particles and the large number of very small pores acting as billions of capillary tubes, which collectively hold large amounts of water (Vankat 1979). The result of this increased water-holding capacity is that plants rooted in such soils may continue active growth much later in the dry season than plants rooted in coarser soils (Daubenmire 1974; Burgess 1995; McAuliffe 1995; Tucker 1999; Greeves et al. 2000; Ball 2001). Furthermore, undisturbed Blackland soils form gilgai, microtopographical surface features that function like shallow basins, increasing water retention during heavy rains (Hayward & Yelderman 1991; Diamond & Smeins 1993). Early settler accounts and observations of existing prairie remnants (e.g., the Nature Conservancy’s Clymer Meadow preserve in Hunt County, the Matthews-Cartwright-Roberts Prairie in Kaufman County, and Austin College’s Garnett Prairie in Grayson County) suggest that these “hog wallows,” as they were known to early settlers, were apparently abundant on Vertisols of the presettlement Blackland Prairie (see page 63 for discussion of gilgai formation). Temporary water storage in the numerous gilgai depressions of one-half acre-foot of water per acre of flat prairie has been estimated. As much as six inches (15.2 cm) of rain could be temporarily trapped in these structures before runoff began (Hayward & Yelderman 1991). FIG.67/ AREA <strong>OF</strong> BLACKLAND PRAIRIE SHOWING INVASION BY WOODY VEGETATION UNDER CURRENT FIRE SUPPRESSION CONDITIONS (PHOTO BY WCW).
114 INTRODUCTION/SOILS, FIRE, AND THE POST OAK SAVANNAH Meanwhile, the high surface area and negative surface charges of clay particles give clay soils high cation exchange capacity. This allows these soils to hold more ionized minerals or nutrients, including those essential for plant growth (Foth 1990; Whitehead 2000; Harpstead et al. 2001; O’Connell 2001). It is therefore not surprising that indirect evidence, such as agricultural productivity, suggests that the Blackland clay soils were among the most fertile soils west of the Mississippi River (Haywood & Yelderman 1991). In addition, the high below-ground biomass of prairie vegetation serves to continually add organic matter to the soil, thereby functioning as a positive feedback mechanism to increase fertility and water-holding capacity (in part due to the surface area provided by the additional organic material). Conversely, sandy soils have larger pores that allow water to drain more easily. They not only dry earlier during dry periods but “the more water that percolates through the soil, the more nutrients are washed out— particularly nitrogen, potassium and sulfur” (Tucker 1999). Therefore, soils that are high in sand, like those of the woodlands, are often poor for plant growth since they are relatively infertile and often too well-drained (Vankat 1979). The soil-mediated fire frequency hypothesis is merely a special case of a generally accepted explanation for tree-grass interactions, based on mechanisms that, by reducing the frequency and intensity of fire, enable trees to grow where grass would otherwise dominate. The fireinduced state of grasslands on the prairies is therefore apparently destabilized when fire is suppressed for any of a number of reasons. In addition to the hypothesized effect of soil on grass biomass, other variables that can hinder fire and thus allow trees to invade include precipitation, grazing, and topography (Collins & Wallace 1990; McPherson 1995). Trees invade Shade High tree seedling/sapling survival Sandy or rocky soil Low fertility & low shallow moisture Low grass biomass Low potential to carry fire Low probability of frequent or intense fire Reduced grass vigor Woodland/Savannah High tree seedling/sapling mortality Clay soil High fertility & shallow moisture Low grass biomass High potential to carry fire Low probability of frequent & intense fire Indreased grass vigor Prairie Increased soil organic matter FIG.68/FLOW DIAGRAMS SHOWING POSITIVE FEEDBACK ROUTES TO TWO ALTERNATIVE STABLE STATES (FROM DIGGS & SCHULZE 2003). FIGURE PRODUCED BY BRIT/AUSTIN COLLEGE; REPRODUCTION <strong>OF</strong> THIS IMAGE DOES NOT REQUIRE COPYRIGHT PERMISSION.
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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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Page 81 and 82: 64 INTRODUCTION/SOILS OF BLACKLAND
Page 83 and 84: 66 INTRODUCTION/CLIMATE OF EAST TEX
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Page 95 and 96: 78 INTRODUCTION/GEOLOGY OF PINEYWOO
Page 97 and 98: 80 INTRODUCTION/PRESETTLEMENT, EARL
Page 105 and 106: 88 INTRODUCTION/PRESETTLEMENT VEGET
Page 107 and 108: 90 INTRODUCTION/CURRENT VEGETATION
Page 123 and 124: 106 INTRODUCTION/POST OAK SAVANNAH
Page 127 and 128: 110 INTRODUCTION/GEOLOGY AND SOILS
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Page 135 and 136: 118 INTRODUCTION/VEGETATION OF POST
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Page 147 and 148: 130 INTRODUCTION/BLACKLAND PRAIRIE
Page 149 and 150: 132 INTRODUCTION/GEOLOGY OF BLACKLA
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Page 171 and 172: 154 INTRODUCTION/PRESENT AND FUTURE
Page 173 and 174: 156 INTRODUCTION/BIG THICKET THE BI
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164 INTRODUCTION/NATURAL HISTORY OF
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172 INTRODUCTION/SUMMARY DEFINITION
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174 INTRODUCTION/PRESETTLEMENT AND
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184 INTRODUCTION/HISTORY OF BIG THI
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190 INTRODUCTION/CONSERVATION IN BI
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200 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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