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The Edwards Plateau, lying east of the continental divide, is a plateau sloping gently eastward, dropping on average about 180 cm per km, but steeper at the western margin. The southern and eastern margins, and to a lesser extent the other parts, are much dissected by the following rivers: Colorado of Texas, Guadalupe, Nueces, Rio Grande/Pecos, and tributaries thereof (Map 8). This is an ancient evolutionary arena. Most of the land surface has been exposed continuously for occupation by terrestrial biota for at least 65,000,000 years. The plateau consists of three sub-regions:
1. The north-western sub-region, with little macrorelief (i.e., flat or gently undulating), a true plateau geomorphically, comprising about half the Edwards Plateau. Elevation 700-1000 m. Soil is a dark stony clay loam where present, but much of the surface consists of outcrops of Cretaceous limestone. The part of the Edwards Plateau west of the Pecos River is sometimes called the Stockton Plateau.
2. The southern and eastern margins are dissected by canyons separated by flat or undulating divides. The dissected part is locally known as the Texas Hill Country (picture); roughly half of the total area of the Edwards Plateau. Virtually the entire surface consists of outcrops of Cretaceous limestone. Elevation: 250-800 m, or as low as 100 m in the bottom of the canyon mouths.
3. Most geomorphologists also include in this broad category a relatively small area of about 6000 kmē, known as the Granitic Central Basin, or Central Mineral Region, in the north-central margin. In this area the Cretaceous limestone has been removed by erosion. A variety of rocks crop out, ranging from Archean through upper Palaeozoic in age. The soil, where present, is thin and stony. The exposed rocks are largely granitic and gneissic, with some sandstones and limestones. Elevation: 500-800 m.
Average annual rainfall varies from about 800 mm on the eastern encanyonated margin to 500 mm at the western margin. Rainfall is erratic on the eastern margin and its dependability declines even further westward. Virtually all rain runs off rapidly in the eastern and southern parts. In the north-western quarter of the Plateau, water accumulates for some days after rains, gradually permeating the underlying strata. Some years are virtually rainless. In relatively "normal" years, rainfall is low in winter and spring through April, but a peak of rainfall occurs in May/June followed by a summer slump and then another peak in September, tapering off in October. Rainfall usually occurs as local, moving, showers or storms, originating from cumulus clouds. Rare flooding is associated with the "hurricane season", May through October. Winter snow is rare, scant and usually melts within 24 hours.
Average annual temperature for the Edwards Plateau is approximately 21°C. Monthly average temperatures vary from around 11°C in January to 30°C in August. Temperatures are lowest at the western, higher parts of the Edwards Plateau, and higher at the eastern, lower parts. But, as in the case of precipitation records, extremes are in several ways more important statistics for the biologist. July afternoon highs often range to 41° or 42°C and January night-time lows often approximate -10°C. The lowest night-time temperature ever recorded in the Plateau was about -23°C. The average last spring frost is usually in late March and the earliest frost in autumn is in mid-November.
The "original" (c. 1800) vegetation was grassland or, more commonly, a type of open savanna, with shrubs and low trees along rocky slopes (Correll and Johnston 1970; Stanford 1976; Hatch, Gandhi and Brown 1990). "Tall" prairie grasses (about 1 m tall in late September, the month of maximum height) are still common on level or nearly level rocky outcrops and protected areas having good soil moisture. Shallow or more xeric, exposed sites support "midgrasses" (maximum heights usually 20-50 cm) and those areas with much grazing have a predominance of "shortgrasses" not more than about 15 cm in height. The vegetation in 1800 was therefore not as dense or lush as would be expected in a climate with a mean annual precipitation of 500 to 800 mm, but this is because of the unpredictable rainfall. In the early 1800s and even up into the 1840s, 1850s and later, the open, grassy nature of the landscape was associated with recurrent fires which suppressed woody growth. Fires are well known to be propagated on relatively level ground, but not on steep slopes. Thus the woody brush was confined to the steeper slopes and canyon walls.
Presently not only the rocky slopes but many of the undulating uplands, especially in the eastern and southern halves, carry a dense growth, 3-8 m tall, of shrubs and small trees, mostly oaks (Quercus fusiformis and other species) and juniper (Juniperus ashei). This invasion of woody plants into former grasslands is attributed to the suppression of fires in historical times, which allowed the woody invaders to displace some of the grassland. Some writers invoke the working of cattle and other livestock on the landscape to help explain these historical trends. At the north-western margin of the Edwards Plateau, the vegetation grades into that of the "mesquite-tobosa country" of the Rolling Plains, also a short-grass savanna with mesquite (Prosopis glandulosa) dotting the short tobosa grass (Hilaria mutica) which forms vast almost mono-specific stands on flatlands with slow run-off.
The earliest professional botanical collections in the Edwards Plateau were those of Jean Louis Berlandier, who accompanied a bison-hunting party in the Hill Country late in 1828. Another notable Texas botanist, Ferdinand Lindheimer, settled at the mouth of a Hill Country canyon in 1843 and collected in the area for some years. Many of the species of the region carry his name. A few other botanists collected in the area in the nineteenth century and early twentieth centuries. From the 1920s onward, many botanists have collected here. The Edwards Plateau is well known botanically. Approximately 2300 species of native vascular plants have been recorded from the Edwards Plateau (Correll and Johnston 1970; Stanford 1976; Hatch, Gandhi and Brown 1990). An additional 200 species are introduced. A study of the distributions of the species outside of the Edwards Plateau indicates that about 500 of the gramineous and herbaceous, especially the prairie-type, species tend to be those of wide distribution in the grasslands of North America, many extending south into north-central Mexico and north into the Great Plains and even into southern Canada. Of the shrubs and small trees, most reach their northern and/or eastern limits in the Edwards Plateau, and thus can be thought of as having floristic relationships more with the calcareous mountains and plateaux of northern Mexico (the Sierra Madre Oriental, broadly construed). Many of these plants seem to "spill over" to some extent onto the edaphically similar limestone uplands just east of the Edwards Plateau, for example, the upper Cretaceous Austin Chalk outcrop, or farther south onto the calcareous hills of the Rio Grande plain, especially the Bordas Scarp.
Only a few species (about 10% of the Plateau flora) are strictly endemic, that is, they are not known to occur at all outside of the Edwards Plateau. But the list of strictly endemic species includes some truly fascinating threatened and endangered species. In the listings below a careful attempt has been made to list the rarest species first, and less and less rare species in sequence. Topping the list are the beautiful and endangered Styrax texana and the threatened Styrax platanifolia, known from a few canyons in the Hill Country (Gonsoulin 1974). The rare, beautiful and probably endangered Salvia penstemonoides deserves an early listing, followed by: Dalea sabinalis (Barneby 1977), Streptanthus bracteatus, Crataegus secreta (Phipps 1990), Philadelphus ernestii, P. texanus, Penstemon triflorus, Carex edwardsensis (Bridges and Orzell 1989), Seymeria texana (Turner 1982), Tridens buckleyanus (Gould 1975), Anemone edwardsiana, Penstemon helleri, Matelea edwardsensis, Amsonia tharpii, Ancistrocactus tobuschii, Onosmodium helleri, Erigeron mimegletes, Tragia nigricans, Berberis swaseyi, Amorpha texana (Wilbur 1975), Hesperaloë parviflora, Galactia texana, Opuntia edwardsensis (Grant and Grant 1979, 1982), Kuhnia leptophylla (Turner 1989), Perityle lindheimeri (Powell 1974), Tradescantia edwardsiana, Chaetopappa effusa (Nesom 1988), C. bellidifolia (Nesom 1988), Quercus laceyi, Vitis monticola (Moore 1991), Buddleja racemosa, Garrya lindheimeri (Dahling 1978) and Verbesina lindheimeri.
One special case is that of Muhlenbergia involuta which appears to be a series of sterile first-generation hybrids between M. lindheimeri and M. reverchonii. These hybrids are known only from the Edwards Plateau, which is the overlap-area of the ranges of the two putative parental species.
Examples of more widespread species of which endemic varieties occur in the Edwards Plateau are few. Some examples are Samolus ebracteatus var. cuneatus, a variety of a very widespread American tropical complex (Henrickson 1983) and Aesculus pavia var. flavescens, a local variety of a species widespread in the woodlands of south-eastern U.S.A. Likewise, Vitis aestivalis var. lincecumii is the Edwards Plateau race of a grape widespread in eastern North America (Moore 1991). Pediomelum hypogaeum var. scaposum on the other hand is a variety of a species widespread in the southern Great Plains and Prairie States (Grimes 1988). Croton alabamensis is a special case, with one extremely rare variety endemic to northern Alabama and an even rarer variety endemic to the eastern Edwards Plateau (Ginzbarg 1991, 1992).
If we extend our consideration of endemism to limestone uplands generally in northern Mexico and southern, central and western Texas, we find many species that occur in the Edwards Plateau and range perhaps 100 to 300 km outside of the strictly defined Edwards Plateau. Among the species that are not strictly endemic to the Edwards Plateau but also occur on the edaphically similar limestone uplands of central and southern Texas (including the Austin chalk) are Euphorbia jejuna, Brickellia dentata, Agalinis edwardsiana, Physostegia correllii (Cantino 1982), Clematis texensis, Penstemon guadalupensis, P. brevibarbatus, Yucca rupicola, Muhlenbergia reverchonii, Vernonia larsenii (King and Jones 1975), Argythamnia simulans, Dichromena nivea, Thelesperma curvicarpum, Mirabilis lindheimeri, Desmanthus reticulatus, Pediomelum cyphocalyx (Grimes 1988), Desmanthus velutinus, Hedeoma acinoides, Panicum pedicellatum, Salvia roemeriana, Tetragonotheca texana, Lespedeza texana, Salvia dolichantha, Salvia texana and Quercus buckleyi (Dorr and Nixon 1985).
Species common to the flora of limestone uplands of western Nuevo Leon and Coahuila (Mexico) and also the Edwards Plateau include: Colubrina stricta, Hesperaloë funifera, Pinus remota (Bailey and Hawksworth 1979), Bouteloua uniflora, Pistacia texana, Pavonia lasiopetala, Muhlenbergia lindheimeri, Hunzikeria texana (Hunziker and Subils 1979), Antiphytum heliotropioides, Salvia engelmannii, Lythrum ovalifolium, Passiflora affinis, Penstemon baccharifolius, Scutellaria microphylla, Forestiera reticulata, Rhus virens, Thelesperma longipes, T. simplicifolium, Chaptalia texana, Chrysactinia mexicana, Bernardia myricifolia, Galphimia angustifolia, Croton fruticulosus, Acacia roemeriana, Stillingia texana, Indigofera lindheimeri and Cassia lindheimeriana.
Many other species of the Edwards Plateau could be listed as widely distributed in the south-western U.S.A. and northern Mexico. Examples are Thamnosma texanum and Pinaropappus roseus. A compilation of species common to both the desertic and montane floras of trans-Pecos Texas and adjacent areas and the Edwards Plateau will probably total 300 species. A few species are known to be endemic to non-limestone substrate in the Central Mineral Region, e.g. Campanula reverchonii and Valerianella texana (Mahler 1981). Two widespread subtropical ferns are found in Texas only on Enchanted Rock, one of the granitic knobs of the Central Mineral Region: Blechnum occidentale (Seigler and Lockwood 1975) and Cheilanthes kaulfusii.
The region abounds, during years of normal rainfall, with grasses and herbs useful for foraging by domestic stock. In comparison to this role in ranching, the direct human values of these and other plants of the Edwards Plateau are miniscule. An industry of moderate proportion derives fence posts from the abundant Juniperus ashei of the region. These posts have the reputation of high decay-resistance, when in contact with soil, as compared to any other readily available natural posts. A small industry in the region derives, through steam-distillation, an aromatic oil from the roots, trunks and branches of the same Juniperus ashei. The aromatic oil is incorporated into germicidal bathroom cleaners, all of which have the word "pine" in the name. Exploitation of trees for timber and lumber is of negligible importance. A small industry derives firewood for sale to householders and restaurateurs, mainly from Prosopis glandulosa and Quercus fusiformis. A couple of endemic species have been taken into the horticultural trade, namely Clematis texana and more importantly Hesperaloë parviflora.
The region comprises mainly small to large ranches with a mixture of domestic stock including many cattle and slightly fewer goats. Sheep are infrequent, followed by even fewer pigs. Hunting privileges, contracted months or even years in advance of the annual season, provide significant revenues to landowners. The major species hunted is the white tailed deer. Harvests of Rio Grande turkey, wild boar, javalina (collared peccary), bobwhite and scaled quail, and mourning doves are of lesser value. Some ranchers stock exotic species, such as various African and Indian antelopes, for sport hunting. Others stock ostriches and emus, these large birds being valued principally for their skins used in making "cowboy boots". Non-hunting recreation and tourism brings in some revenue. Eco-tourism, specifically ornitho-tourism brings thousands to catch a glimpse of the increasingly uncommon endemic black-capped vireo (Vireo atricapillus) and the golden-cheeked warbler (Dendroica chrysopareia). Hill Country rivers, clear, relatively cool and lined with stately Taxodium distichum trees, attract thousands of visitors, principally from the even hotter and more humid coastal cities such as Houston. In the last 20 years the Hill Country has blossomed as a sun-belt retirement area with relatively low land values and other low living costs, compared to the traditional overcrowded areas The region comprises mainly small to large ranches with a mixture of domestic stock including many cattle and slightly fewer goats. Sheep are infrequent, followed by even fewer pigs. Hunting privileges, contracted months or even years in advance of the annual season, provide significant revenues to landowners. The major species hunted is the white tailed deer. Harvests of Rio Grande turkey, wild boar, javalina (collared peccary), bobwhite and scaled quail, and mourning doves are of lesser value. Some ranchers stock exotic species, such as various African and Indian antelopes, for sport hunting. Others stock ostriches and emus, these large birds being valued principally for their skins used in making "cowboy boots". Non-hunting recreation and tourism brings in some revenue. Eco-tourism, specifically ornitho-tourism brings thousands to catch a glimpse of the increasingly uncommon endemic black-capped vireo (Vireo atricapillus) and the golden-cheeked warbler (Dendroica chrysopareia). Hill Country rivers, clear, relatively cool and lined with stately Taxodium distichum trees, attract thousands of visitors, principally from the even hotter and more humid coastal cities such as Houston. In the last 20 years the Hill Country has blossomed as a sun-belt retirement area with relatively low land values and other low living costs, compared to the traditional overcrowded areas in California, Arizona and Florida. The population approximately doubled in the dacade 1981 through 1990, from 1,000,000 to 2,000,000 (these estimates do not include the nearby cities east and south of the Hill Country).
The current value of resources and services that the Edwards Plateau provides to the national economy on a sustainable basis is difficult to ascertain. Minor forest products are not only minor, but negligible in importance.
The Edwards Plateau serves as an important watershed and ground-water recharge zone for several of the most important cities of the region (Waco, Temple, Belton, Georgetown, Austin, San Marcos, New Braunfels, San Antonio, Uvalde, Del Rio), lined up along its eastern and southern margins where rivers disgorge onto the coastal plain. All these cities rely for their water supplies on the streams flowing out of the Edwards Plateau and the large springs at its eastern and southern edges (which are marked by the Balcones Fault Zone). Water percolates through the permeable limestone to the water-table, thence to the springs and wells. A number of streams have been dammed in the last 50 years to ensure the continuity of water-supplies and to support aquatic recreation. Watershed protection has not been, and still is not, a priority with governmental agencies of the region. As a result of the rapid population growth, the quality of the surficial and subterranean water has deteriorated in recent years. Nevertheless, the Edwards Plateau's function as a water recharge and supply zone will in the future become more and more its major economic value to the human population of the region. Vegetational integrity and maturity are known to correlate positively with the quality of underground and surficial water.
The plant resources of the region have been threatened for hundreds of years by an over-abundance of non-native and native animals. Some species have survived the intensive over-browsing only as a few individuals. For example, probably fewer than 15 individuals of Styrax texana exist in the wild. This is an extreme case of poor survival and perhaps indicates the likelihood that some species became extinct prior to botanical exploration. The threat continues and in some areas is further exacerbated by high deer populations which are no longer controlled by natural predation. The major acute new threat, accelerating in recent years, is the development of land for residential and light industrial purposes, especially in the Hill Country near the largest cities (Austin, San Antonio, Waco). Diffuse urbanization, being the result of literally millions of independent decisions to buy small parcels and settle in the area, appears to be immune from governmental regulation without a massive public awakening.
The only lasting and worthwhile effort to minimize loss of biotic and genetic diversity will be to establish large biological reserves selected for their relatively undisturbed state. While this concept is not new to existing policymakers and those with financial resources, at the present time only a miniscule area of the region has been set aside. It should be noted that, in contrast to other western states which initially were carved out of federal territorial land (and which therefore presented more opportunities for establishment of federal parks and forest reserves), Texas initially had virtually no federal lands. State and municipal parks in the Edwards Plateau total only c. 30 kmē or possibly 35 kmē. There is no National Park, save one of c. 1 kmē devoted mainly to historical interpretation. While no true wilderness presently survives in the Edwards Plateau, tracts of several hundred to several thousand hectares still exist that could be set aside. It is suggested here that a minimum 2% criterion be applied, that is that concerned governmental and private initiatives work together to set aside appropriate tracts with the minimum goal being 2% of the land area, or about 2000 kmē. That will represent an almost hundred-fold increase over present holdings. Only then can any reasonable expectation that the biotic richness of the region will be passed on to succeeding generations.
Map 8. Edwards Plateau, Texas, U.S.A. (CPD Site NA32)
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Barneby, R. (1977). Daleae imagines. Mem. N.Y. Bot. Gard. 27: 1-891.
Bridges, E.L. and Orzell, S.L. (1989). A new species of Carex (sect. Oligocarpae) from the Edwards Plateau of Texas. Phytologia 67: 148-154.
Cantino, P.D. (1982). A monograph of the genus Physostegia (Labiatae). Contr. Gray Herb. 211: 1-105.
Correll, D.S., Johnston, M.C. and collaborators (1970). Manual of the vascular plants of Texas. Texas Research Foundation, Renner. 1881 pp.
Dahling, G.V. (1978). Systematics and evolution of Garrya. Contr. Gray Herb. 20: 3-104.
Dorr, L.J. and Nixon, K. (1985). Typification of the oak (Quercus) taxa described by S.B. Buckley (18091884). Taxon 34: 211-228.
Ginzbarg, S. (1991). Croton alabamensis Chapm. (Euphorbiaceae), disjunct populations in Texas. Master's Degree Report, The University of Texas at Austin. 38 pp.
Ginzbarg, S. (1992). A new disjunct variety of Croton alabamensis (Euphorbiaceae) from Texas. Sida 15(1): 41-52.
Gonsoulin, G.J. (1974). A revision of Styrax (Styracaceae) in North America, Central America, and the Caribbean. Sida 5: 191-258.
Gould, F.W. (1975). The grasses of Texas. Texas A & M University Press, College Station. 654 pp.
Grant, V. and Grant, K.A. (1979). Systematics of the Opuntia phaeacantha group in central Texas. Bot. Gaz. 140: 205-214.
Grant, V. and Grant, K.A. (1982). Natural pentaploids in the Opuntia lindheimeri - phaeacantha group in Texas. Bot. Gaz. 143: 117-120.
Grimes, J. (1988). Systematics of the New World Psoraleae (Leguminosae: Papilionoideae). Doctoral dissertation, Univ. of Texas at Austin. 307 pp.
Hatch, S.L., Gandhi, K.N. and Brown, L.E. (1990). Checklist of the vascular plants of Texas. Texas Agricultural Experiment Station, College Station, Texas, MP-1655. 158 pp.
Henrickson, J. (1983). A revision of Samolus (Primulaceae). S.W. Nat. 28: 303-314.
Hunziker, A.T. and Subils, R. (1979). Salpiglossis, Leptoglossis and Reyesia (Solanaceae): a synoptical approach. Bot. Mus. Leafl. Harvard 27 (1-2): 1-44.
Johnston, M. (1990). The vascular plants of Texas - a list, up-dating the manual of the vascular plants of Texas. Second edition, published by the author. Austin, Texas, 108 pp.
King, B. and Jones, S.B. (1975). The Vernonia lindheimeri complex (Compositae). Brittonia 27: 74-86.
Mahler, W.F. (1981). Field studies in Texas endemics. Sida 9: 176-181.
Moore, M.O. (1991). Classification and systematics of eastern North American Vitis L. (Vitaceae) north of Mexico. Sida 14: 339-367.
Nesom, G. (1988). Synopsis of Chaetopappa (Compositae - Astereae) with new species and the inclusion of Leucelene. Phytologia 64: 448-456.
Phipps, J.B. (1990). Crataegus secreta (Rosaceae), a new species of hawthorn from the Edwards Plateau, Texas. Sida 14: 13-19.
Powell, A.M. (1974). Taxonomy of Perityle section Perityle (Compositae - Peritylinae). Rhodora 76: 229-306.
Seigler, D.F. and Lockwood, T.F. (1975). Blechnum occidentale, new to Texas. American Fern Journal 65: 96.
Stanford, J.W. (1976). Keys to the vascular plants of the Texas Edwards Plateau and adjacent areas. Published by the author. Brownwood, Texas. 366 pp.
Turner, B.L. (1982). Revisional treatment of Mexican species of Seymeria (Scrophulariaceae). Phytologia 51: 403-422.
Turner, B.L. (1989). An overview of the Brickellia (Kuhnia) eupatorioides (Asteraceae, Eupatorieae) complex. Phytologia 67: 121-131.
Wilbur, R.L. (1975). Revision of North American Amorpha (Fabaceae). Rhodora 77: 337-409.
This Data Sheet was written by Dr Marshall Johnston (The University of Texas, Botany
Department, Austin, Texas 78713-7640, U.S.A.).
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