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The Balcones Escarpment :

Plant Communities of the Edwards Plateau of Texas :
an Overview Emphasizing the Balcones Escarpment Zone Between San Antonio and Austin with Special Attention to Landscape Contrasts and Natural Diversity, p. 20-32

by David H. Riskind and David D. Diamond

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INTRODUCTION

The Edwards Plateau of west central Texas comprises about 93,240 sq. km. of territory (LBJ School of Public Affairs, 1978). It contains several distinct subregions. It is species rich and its mesic canyons harbor a number of endemic and insular species (Amos and Rowell, 1984). Although dominantly limestone, the southern margin of the Plateau is bounded by the Balcones fault system with limestone, chalk, marl, claystone, and localized outcrops of intrusive igneous features (Lonsdale 1927). Hence, the Edwards Plateau is a large distinct region that supports a diversity of habitats. The following sections will provide a description of the variation in physiography, geology, climate, soils, and vegetation that compose the Edwards Plateau, but emphasis will be on the landscape lying between San Antonio and Austin.

Plant sketches by Margaret Campbell

Figure 1 : Landforms of the Edwards Plateau and adjacent areas.

Figure 2 : Approximate delineation of the Edwards Plateau natural region showing major drainages.

Table 1 : Normal annual and growing season (April-October) precipitation based on 1951-1980 means, for stations along an east-to-west transect across the central Edwards Plateau.

Table 2 : Generalized transect of slope woodland communities on dissected uplands from east to west (mesic to xeric) across the Escarpment (=Balcones Canyonlands). Characteristic species for each community are ranked according to relative dominance of the most important woody perennials only.
a. Ft. Hood, Bell Co., TX. Lampasas Cut-Plain
b. Austin, Travis Co., TX.
c. South central edge of Escarpment. Bexar, Medina, Bandera, Kendall Counties
d. SW edge of escarpment. Uvalde, Kinney, Edwards and Real Counties.

PHYSIOGRAPHY AND TOPOGRAPHY

Hill (1892) was the first to recognize the Edwards Plateau as a distinct physiographic province, but the definition of its extent has varied. Tharp (1939) described the vegetation of Texas and included the Grand Prairie to the north and Hill Country to the south and southeast in his definition of the Plateau, but excluded the Central Mineral Region (= Llano Uplift) and the flatter, central and northwestern portions. Dice (1943) provided a map of biotic provinces of North America based primarily on faunal distributions, and included the Plateau with the Rolling Plains in his Commanchean Biotic Province. This treatment was later modified by Blair (1950), who separated the Plateau (including the Llano Uplift) as the Balconian Province. Gould (1975) included the Llano Uplift and Stockton Plateau west of the Pecos River, but not the Lampasas Cut Plain, in a widely recognized treatment of the vegetational areas of Texas. Godfrey, et al.(1973) also used a similar definition, but excluded the Llano Uplift. The Lyndon B. Johnson School of Public Affairs (1978) published a map of the natural regions of Texas that was essentially similar to one adopted by the United States Fish and Wildlife Service (1979). These treatments excluded the Llano Uplift but included the Lampasas Cut Plain in the Edwards Plateau natural region.

The Edwards Plateau, taken in broad context, is a southern extension of the Great Plains of North America (Fenneman, 1931; Hunt, 1974). To the south and east it is separated from the lower-lying West Gulf Coastal Plain by the Balcones Fault Zone, where elevations drop sharply to less than 180 m. To the north it grades gradually into the Rolling Plains, while to the northwest it grades into the High Plains (= South Sandy Plains). To the west it is separated from the Stockton Plateau by the Pecos-Devils River divide. The Stockton Plateau is geologically similar to, and has been considered by some as part of the Edwards Plateau (Gould, 1975); however, it has more often been lumped with the more desertic Trans-Pecos region (Tharp, 1939; LBJ School of Public Affairs 1978). Figure 1 provides a schematic rendering of this physiographic region.

The elevation of the Edwards Plateau generally increases from the southern and eastern margins to the northwest. Austin and San Antonio on the south are at 167 m and 213 m, respectively, while Junction near the center of the Plateau is at 521 m and Big Lake on the northwest is at 734 m.

The southern and southeastern margins of the Edwards Plateau are highly dissected, and could hardly be considered a plateau. This "Hill Country" (= Balcones Canyonlands) consists of steep canyons, narrow divides, and high-gradient drainages. These short streams originate in the Hill Country and generally flow south or southeast to the Gulf of Mexico. They include, from west to east, the Nueces, Frio, Sabinal, Medina, Guadalupe, and Blanco Rivers. The Pedernales flows eastward through the region, joining the Colorado just west of Austin.

The granitic Central Mineral region or Llano Uplift centered in Llano, Mason, and Burnet Counties is likewise not a plateau. Topographically, it is a basin with respect to the main body of the Plateau to the south and west. Its geologic origin is as an uplift, hence the name. There are numerous rounded, nearly barren, granitic outcrops and the landscape is gently rolling except near drainages such as the Llano and Colorado Rivers and their tributaries or near granite outcrops, where steep slopes and some sheer cliffs appear.

The Lampasas Cut Plain on the northeast is generally flatter than the Llano region or southeastern margins of the Plateau previously discussed. It consists of broad valleys and wide stream divides with relatively few steep, high-gradient canyons. The Lampasas and San Gabriel Rivers are the only two major streams that bisect the area.

From the central Edwards Plateau to the north and northwest, the topography is generally flat to gently rolling with rounded hills, wide stream divides, and few steep slopes. Much of the area could be described as a broad plain. Several major streams cut west to east paths across this plain, including, from north to south, the Concho, San Saba and Llano Rivers. These eventually join the Colorado, which flows southerly through the Llano Uplift and eventually to the Gulf of Mexico. The Devils River and its tributaries also bisect this plain in the southwest, but flow south to join the Rio Grande.

GEOLOGY

Most of the Edwards Plateau consists of limestone rock of Cretaceous origin. The less eroded central and western portions are dominated by Lower Cretaceous rocks within the Edwards Limestone group, while southward and eastward Edwards Limestone has largely been eroded exposing older Cretaceous material, primarily the Glen Rose formation (Sellards et al. 1932). The Lampasas Cut Plain, which represents a generally more mature landscape than the main portion of the Edwards Plateau to the south and west, is composed of strata from both the Glen Rose and Fredericksburg Divisions. Patches of limestone, dolomite, chert and marl alternately crop out at the surface across the area. Some Upper Cretaceous material, consisting primarily of chalk and marl, crops out along the southern and western margins of the Plateau.

The geology of the Central Mineral Region or Llano Uplift is strikingly different from that of the remainder of the Edwards Plateau. It is an intrusive outcrop of Precambrian rock that comprises about 1.5 million ha in the northwestern part of the Plateau. The material overlying this intrusive granite, where it has not been eroded away (around the perimeter, especially the northern border), consists of early Paleozoic sedimentary rocks including limestone, dolomite, sandstone, siltstone and shale. Minerology of the granitic material varies, with hornblende schist, graphite schist, quartz-feldspar gneiss and quartz-plagioclase-microcline rock common. In addition, local Precambrian outcrops are scattered throughout the southern and eastern margins of the Plateau.

SOILS

Variation in substrate and a generally hilly landscape have led to the development of a large number of different soil types on the Edwards Plateau. Excluding the Llano Uplift, upland soils of the Plateau have generally developed in place and occur over limestone or caliche. They are shallow and rocky or gravelly on slopes and deep in broad valleys and on flats. Most are dark colored and calcareous, although pH is variable depending on base saturation of the substrate, and the degree of soil profile development (Godfrey et al., 1973). Surface texture also varies from loamy to clayey, depending on substrate and profile development.

These upland soils are generally classified as Mollisols on flats and valleys (deeper soils) or Inceptisols on slopes (shallow soils). Many have vertic properties due to montmorillonitic clay minerology. These soils shrink and swell on wetting and drying, developing deep cracks in the dry months. Clayey Vertisols are also present, especially in the east or run-on areas in the north and northwest. Both Mollisols and Vertisols have surface layers that are high in organic matter, but nitrogen, phosphorus, potassium, iron and magnesium may still be limiting factors to plant growth when water is sufficient. Inceptisols may also have fairly high organic matter content, although they are not generally as fertile, mature, or deep as Mollisols and Vertisols. Over less alkaline parent materials, or where soil profile development has occurred for long periods over moderately or non-calcareous secondary colluvium or alluvium (for example, on old stream terraces or in former shallow depressions), loamy Alfisols have developed. They are often less fertile than Mollisols or Vertisols, although plant-soil-water relations may be good.

Soils of the Llano Uplift have generally developed over long periods from granitic materials or, around the margins of the region, from a variety of shale, limestone, dolomite or siltstone. Most have acid, loamy surface layers and are classified as Alfisols. Some deep, well-watered, sandy deposits occur around the base of major granite outcrops and in stream bottoms. These have poor profile development and are classified as Inceptisols.

CLIMATE

The climate of the Edwards Plateau becomes increasingly arid to the west and cooler to the north. The eastern and central portion is classified as sub-tropical, subhumid, while the western one-fourth is classified as sub-tropical, semi-arid (Larkin and Bomar, 1983, Fig. 2). These categories correspond to Thornthwait's (1948) dry sub-tropical and semi-arid moisture regions. The general decrease in moisture content of Gulf air as it flows northwestward across the Plateau is the controlling factor responsible for this difference in moisture regime.

Mean annual precipitation decreases from east to west, ranging from about 85 cm/yr on the eastern edge to 35 cm/yr on the western edge of the Plateau (Table 1); (Bomar 1983). There is a concomitant increase in mean lake-surface evaporation rates from east to west. July-plus-August evaporation rates increase from 46 cm in the east to 57 cm in the west, while annual rates increase from 160 cm/yr to 206 cm/yr from east to west. The July-plus-August precipitation rates also decrease from east to west, ranging from 13 cm to 9 cm (Larkin and Bomar, 1983). Hence, there is a pronounced decrease in summer precipitation and an increase in summer evapotranspiration, and this effect is increasingly severe to the west. In addition, there are periodic drought years, such as those that occurred in the mid-1950's and in 1980 that cause even more severe moisture stress on plants.

The average frost-free period ranges from approximately 260 days in the south (early March through late November) to 230 days in the north. Summer average highs and lows do not vary significantly across the Plateau and average about 35 degrees C and 22 degrees C respectively. Average January lows decrease northward, ranging from approximately 4 degrees C to O degrees C. Hence, there is little variation in environment related to north-south variation in temperature.

Along with normal summer moisture deficiencies and periodic severe drought, high-intensity rainfall events caused by tropical cyclonic disturbances are characteristic of the Edwards Plateau. These torrential storms are most common in the Hill Country along the southern and southwestern margins of the region (Baker, 1975). Flooding and erosion caused by the storms are major factors in the environment of the Edwards Plateau.

VEGETATION

The climate of the Edwards Plateau becomes markedly drier to the west, and the topography becomes less dissected. Soils of the Llano Uplift region are generally sandy and non-calcareous, in contrast to the calcareous, clayey or loamy soils of most of the remainder of the region. The southern and southwestern margins (= Hill Country; Balcones Canyonlands) are markedly more dissected, and the topography rougher than that of the Lampasas Cut Plain on the northwest. These observations have been made by early (Bray, 1906; Johnson, 1931; Tharp, 1939, 1952) as well as later (LBJ School of Public Affairs, 1978; USFWS, 1979) investigators, who have all separated these regions into separate vegetational or at least physiographic subregions. A recent map of the current vegetation of Texas based on LANDSAT data (McMahan et al., 1984) and a map of potential natural vegetation by Kuchler (1964) have noted the differences among these regions. The Balcones Canyonlands or Hill Country region is more mesic and supports more forest or woodland vegetation on slopes and in canyons; the Lampasas Cut Plain is also mesic but flatter and more open and, therefore, grassier; the central and western Plateau becomes more xeric and more open; and the Llano Uplift region contains a species composition similar to but distinct from the remainder of the Plateau. Hence, the interactions of climate, topography and soils cause major shifts in vegetation patterns evident across the region.

These factors, along with past and present disturbance regimes, also interact to cause coarse and fine-scale variations in vegetation on the Plateau. The demise of free-roaming bison, introduction of domestic livestock and exotic herbivores and the drastic change in fire regime since 1700 have led to widespread increase in density of woody species and loss of grasslands across the Plateau (see Smeins, 1980). In addition, variations in the timing and density of grazing by domestic livestock, together with mechanical and chemical brush control have led to an even more patchy landscape in which the influence of natural variation in soils, slopes and aspect are obscured.

The following will provide a general regional characterization of the contemporary and potential, late-seral vegetation of the Plateau; however, the principal focus will be on the San Antonio-Austin segment of the Balcones Canyonlands and adjacent lands east of the fault zone, including the southern extension of the Blackland Prairies.

Affinities of the Vegetation

Modern flora and fauna of the Edwards Plateau are comparatively well known. Pleistocene fauna, known primarily from caverns and sinkholes is likewise fairly well known (Lundelius 1967); however, we know almost nothing of the last 22,000 years of vegetational history on the Plateau except through inference from Quaternary pollen records to the east and west (Bryant and Schafer, 1977).

There are hints of an excitingly complex vegetational history which is manifested in the modern occurrence of certain insular woodland communities such as the temperate deciduous Acer-Tilia-Quercus or evergreen Pisatacia-Quercus or Lacey oak (Quercus glaucoides) woodlands restricted to mesic canyons; the restricted, insular Pinus remota evergreen pygmy woodlands; the insular Taxodium-Sabal grotto swamps; the tropical ferns in isolated sinkholes; and by such exciting stories as the apparently rapid colonization of Ashe juniper (Juniperus ashei) onto the Plateau from a source on the margins of the Mexican Plateau (Adams, 1977).

Plant communities of the more mesic, dissected portions of the Plateau owe much of their origin to the Sierra Madre Oriental and its outliers. One could also characterize the Balcones Canyonlands of the Plateau as northern facies of the eastern piedmont of the Sierra Madre Oriental. Mesic habitats in the protected eastern canyons are strongly influenced by floristic contributions from the eastern (Austroriparian) deciduous forests, including tall-grass prairie species.

The Plateau on the undissected uplands owes much of its influence to the Great Plains grasslands to the north. On the more xeric western plateau and its canyons, the biotic contribution is from the dry plateaus and massifs of northern Mexico and Trans-Pecos Texas where semidesert grasslands prevail. To the northwest, centered in Reagan, Irion, Schleicher and Crockett Counties, the mesquite-tobosa community seems more akin to the Rolling Plains, as does the mesquite savannah on heavy textured soils of the Llano Basin.

Other parts of the Llano basin, over lighter textured soils, are covered in an open oak-hickory woodland whose affinities are with the Cross Timbers and oak woodlands to the north and east. Oak woodlands are also widespread on limestone uplands across interfluvial divides on the eastern margins of the Plateau where Alfisols occur, usually over karstic features or Quaternary terrace deposits.

The southern segment of our treatment area, near San Antonio and environs, is influenced by yet another suite of elements whose origins are the Tamaulipan thorn woodlands/shrub of the Mexican Gulf Coastal Plain. Taxa such as spiny hackberry (Celtis pallida), catclaw acacia (Acacia gregii), fern acacia (A. berlandieri), persimmon (Diospyros texana) and mesquite (Prosopis glandulosa) tend to be more common on dry, edaphic sites or where disturbance has played a role in landscape development. Disclimax or disturbed grasslands on heavy soil usually have an abundance of huisache (Acacia smallii), while a sub-tropical component, anaqua (Ehretia anacua), is found occasionally along riparian corridors.

Balcones Canyonlands

This region of steep slopes and high-gradient streams is dominated by evergreen woodlands and deciduous forests. Grasslands are restricted primarily to drainage divides, usually in the context of open woodlands. Although more quantitative data on plant ecology are available for this region than for other subdivisions of the Plateau (Buechner, 1944; Solcher, 1927; Lynch, 1962, 1971; Van Auken et al., 1979, 1980, 1981; Ford and Van Auken, 1982; Bush and Van Auken, 1984, 1985; Fowler, 1985: Van Auken and Bush, 1985; Fowler and Dunlap, 1986) the composition and structure of the plant communities of this zone are still not well known. Community composition reflects exposure, edaphic factors and microclimate, and although vegetation changes covered by the factors are qualitatively obvious, only one study (Van Auken et al., 1981) has investigated this topic for the escarpment, and none have compared communities of similar habitats across moisture and exposure gradients in the zone. An idealized profile of the canyons contains at least three major community types.

Streamsides

Along perennial watercourses, the streamside component is dominated in our area south of the Colorado by bald cypress (Taxodium disticum), sycamore (Platanus occidentalis) and to a lesser extent black willow (Salax nigra). Buttonbush (Cephalanthus occidentalis) is often conspicuous in the shrub stratum. Quite often, bald cypress forms monodominant stands. This streamside community is always very narrow, often less than 2 m. Dwarf Palmetto (Sabal minor) occurs occasionally. This community is a western expression of eastern swamp communities, although it is adapted to periodic flooding of great magnitude, which may be essential for its maintenance (see Gehlbach, 1981). Cypress swamps are well expressed at grotto sites like Hamilton's Pool and West Cave Preserve (Travis County) and at Honey Creek and Curry Creek in Comal County.

Intermittent drainages support sycamore woodlands or in the case of very "dry" sites, cedar elm usually predominates. If deep soils accumulate, the streamside component is often indistinguishable from some mesic lower-slope or floodplain woodlands within Canyonlands.

Floodplains

Like the streamside community, floodplains are subject to periodic catastrophic flooding, and are dominated by some combination of oak-elm-hackberry gallery forests. In our area this gallery woodland also may include Arizona walnut (Juglans major), box elder (Acer negundo), chittamwood (Bumilia lanuginosa), soapberry (Sapindus), Ashe juniper, pecan (Carya illinoensis), eastern cottonwood (Populus deltoides), live oak, Texas oak, chinkapin oak (Quercus muhlenbergii), ash (Franixus pennsylvanica), American elm (Ulmus americana), cedar elm, (Q. sinuata), red mulberry (Morus rubra), and rarely basswood (Tilia caroliniana), although there is considerable east to west variation (Buechner 1944, Ford and Van Auken 1982). Species such as pecan, scalybark oak, chinkapin oak, and black walnut are more important in the east or on more mesic bottoms. Live oak, cedar elm, and sugarberry increase to the west or on more xeric bottomland sites. Floodplain forests are usually at least two-layered, with deciduous holly (Ilex decidua), roughleaf dogwood (Cornus drummondii), elderberry (Sambucus spp.), Mexican plum (Prunus mexicana), and hoptree (Ptelea trifoliata) often present. Sugarberry and cedar elm increase in disturbed floodplains. The lower Devils River along the southwestern margin of the Edwards Plateau is a mesic outlier with a riparian forest of live oak, pecan and sycamore (Smith and Butterwick 1975a, Gehlbach, 1981). Elevated, Quaternary gravel terraces occasionally support post oak (Quercus stellata) woodlands. Early descriptive accounts for the eastern portions can be found in Bray (1906) and Palmer (1920).

Riparian vegetation changes in response to an east-west moisture gradient, as well as available riparian water and soil depth. Most eastern deciduous species such as pecan, chinkapin oak, bur oak (Q. macrocarpa), elms (Ulmus spp.), and ashes (Fraxinus spp.), extend no further west than on a line through Tom Green, San Saba, Menard, Kimble and Real Counties.

Steep Slopes

The steep slopes of the Balcones Canyonlands support short-stature woodlands which vary from evergreen juniper and juniper-oak on south and west exposures to deciduous mixed-oak hardwood woodlands on north and east exposures (see Table 2a.-d.). Texas oak (Quercus texana) is usually dominant in the east, but westward to the Nueces River on the southern margins of the Plateau, lacey oak may dominate. Farther west to the Pecos, vasey oak (Q. vaseyana) is dominant. Some northern exposures are dominated by Texas ash (Fraxinus texensis) or locally big-tooth maple (Acer grandidentatum). In our treatment area, these forests often contain a distinct understory shrub layer, with yaupon (Ilex vomitoria), American beautyberry (Callicarpa americana), hoptree, Mexican buckeye (Ungnadia speciosa), red or yellow buckeye (Aesculus), deciduous holly, and rough-leaf dogwood are variously present. A few of these communities have been documented by Van Auken et al. (1979, 1980). These studies, as well as earlier works (Anderson, 1904; Cuyler, 1931) noted that substrate has an effect on the vegetation. Texas madrone (Arbutus xalapensis) and pinyon pine (Pinus remota) are Sierra Madrean elements that occur in this community but are restricted to favorable exposures and elevations west of the Colorado River.

Slope communities on dry southern and eastern exposures are primarily evergreen and dominated by Mexican juniper, often in nearly pure stands called cedar breaks. Live oaks, Mexican persimmon (Diospyros texana), shin or scalybark oak (Quercus sinuata var. sinuata), evergreen sumac (Rhus virens), skunkbush sumac (R. aromatica), elbow bush (Forestiera pubescens), and Texas mountain laurel (Sophora secundiflora) may also be present. Scrub oak (Quercus pungens) is important in the west. These xeric woodlands usually contain no understory woody layer and are less diverse in woody species than deciduous woodlands on mesic north and west slopes previously discussed.

Slopes of the dissected portions of the Lampasas Cut Plain support communities like those of the Balcones Canyonlands in the escarpment zone between Bexar and Travis County, although Texas oak and Texas ash seem to be more important in the Cut Plain. Quantitative data of analogous communities may be found in Van Auken et al. (1979, 1980, 1981). Scalybark oak is very important on the Cut Plain. Neither madrone, lacey oak nor pinyon occur on the Cut Plain, and the endemic Yucca rupicola of the Plateau is replaced on the Cut Plain by the endemic Yucca pallida.

Composition of slope communities west of the Frio River change dramatically. Woodlands are usually restricted to northern and eastern exposures and canyon bottoms, and taxa with a Mexican affinity become more important. Ashe juniper declines markedly in importance and at the Rio Grande in Val Verde County is almost absent. Shrubs such as blue sage (Salva ballotaefolia), sumac (Rhus spp.), leadtree (Leucaena retusa), cenizo (Leucophylum frutescens , Spanish dagger (Yucca treculeana), scrub oak, lasey oak, sotol (Dasylirion spp.), agarito (Berberis trifoliolata), Acacia spp. and other xeric adapted species are among the dominants. The western manifestation of the escarpment vegetation is best described by Bray (1905), Tharp (1944), Webster (1950), Flyr (1966), Smith and Butterwick (1975a, 1975b) and Johnston et al. (unpublished).

Special features of the escarpment zone include assorted karstic features such as sinkholes and grottoes, which are well known but little studied (Smith and Butterwick, 1975b; Williams, 1977b). These features are especially significant because they harbor peripheral and insular biota representative of Mexican/tropical or eastern temperate deciduous elements. Mesic microenvironments of these features and of steep, protected canyons in general harbor numerous plants whose main distribution lies in the forests of the Gulf Coastal Plain and include yaupon, eastern red cedar (Juniperus virginiana), Indian-cherry (Rhamnus carolineana), scalybark oak, Carolina supplejack (Berchemia scandens), inland seaoats (Chasmanthium latifolium, spicebush (Lindera benzoin), and dwarf palmetto. Narrow endemics include sycamore-leaf snowbell (Styrax plantanifolia) and Philadelphus ernestii. Some typical "Mexican" species at the eastern distributional extreme include madrone, Mexican tea (Ephedra antisyphlitica), and the fern Anemia mexicana.

Communities of the Relatively Undissected Uplands and Broad Valleys

Uplands of the Edwards Plateau are not today and historically never were an expansive, open, treeless grassland. However, exclusive of the Llano Uplift region, a grassland-woodland mosaic currently exists on relatively deep upland soils across extensive portions of the Plateau. Historically, grasslands were probably more extensive than today, having been reduced by encroachment of woody species, due in part to introduction of domestic livestock and elimination of fire (see Smeins 1980). Likewise, tall and mid grass have been replaced by short grasses on much of the eastern two-thirds of the Plateau. The Lampasas Cut Plain also historically supported grasslands, although a more mature landscape with fewer flat or gently rolling areas suggest that grasslands probably formed a patchy mosaic with woodlands. The Llano Uplift contained some grassland, although more favorable soil moisture relations in some areas indicate that oak-hickory woodland along with mesquite or mesquite-oak wood-land predominated.

Grassland Communities

Grasslands of the Balcones region are generally restricted to relatively flat divides, adjacent moderate slopes and broad, mature stream valleys. These areas have been heavily grazed by domestic livestock and subjected to various brush-control techniques. Hence, they are patchy and dynamic in time. Variation in species composition caused by soils and aspect is difficult to separate from that due to past disturbance (Dunlap, 1983; Fowler and Dunlap, 1986). Allred (1956) considered the Plateau region a southern extension of the Mixed Prairie. Thus, well-watered, moderately grazed uplands of the region resemble tall-grass communities, but increasing aridity to the west causes mid- and short-grass components to become increasingly important.

Little bluestem (Schizachyrium scoparium), Texas wintergrass (Stipa leucotricha), white tridens (Tridens muticus), Texas cupgrass (Eriochloa sericea), tall dropseed (Sporobolus asper), sideoats grama (Bouteloua curtipendula), seep muhly (Muhlenbergia reverchonii) and common curlymesquite (Hilaria belangeri) are among the dominants of moderately grazed areas (Smeins et al. 1976, Dunlap 1983). Heavily grazed grasslands and more xeric soils contain a larger proportion of short grasses such as curlymesquite, three-awn, Texas grama (Bouteloua rigidiseta), red grama (B. trifola), hairy grama (B. hirsuta), hairy tridens (Erioneuron pilosum) and Tridens muticus. Cedar sedge (Carex planostachys) is common in these grasslands.

Soil depth and texture is highly variable in most areas, and hence the grasslands may be extremely heterogeneous (Smeins et al., 1976). An example of the interaction of grazing and soils is found in Fowler and Dunlap (1986). Uplands of the Hays-Travis County area may support shortgrass communities, while slopes have more tall and mid grasses due to 1) a clayeyer, and hence more droughty soil on ridges, and 2) heavier grazing on ridges than adjacent slopes because of the behavior patterns of domestic livestock. Live oak, shin oak and woody species associated with these are components of the grasslands, forming clumps or mottes. These mottes, along with frequent short but steep scarps dominated by woody species give many areas a park-like physiognomy. On deep, mainly non- calcareous or moderately calcareous soil, Texas oak, post oak (Quercus stellata) and, especially on the east, blackjack oak (Q. marilandica) may be scattered or form woodlands alternating with grasslands in the uplands. Similar oak woodlands also occur along well-drained stream terraces.

Essentially all of the grasslands of the region are in some stage of secondary succession, and thus highly dynamic (see Beaty, 1973). Although these grasslands may not have been devoid of Ashe juniper, invasion or thickening of this species has been observed to cause "cedar breaks" to form in former grasslands (Buechner, 1944; also see Smeins, 1980). Mesquite is also a woody component of these grasslands which has increased in density in many areas, and live oak, shin oak and other woody species such as persimmon, agarita and sumac, may cover more area than in pre-European settlement times. Prickly pear (Opuntia spp.), noseburn (Tragia spp.), rabbit tobacco (Evax spp.) and zexmania (Zexmania hispida) are also common components.

Lampasas Cut Plain

Plant communities of the Lampasas Cut Plain are hardly distinct from those of the Balcones Canyonlands, but the general topography is flatter, there are fewer drainages and the character of the region as a whole is that of a grassland or open woodland (sensu Driscoll et al., 1984) savanna (Kuchler, l964), rather than a closed woodland or forest. Also, there are more northern elements and a larger extent of post oak-blackjack oak woodlands, especially in the east and where the Cut Plain contacts the western Cross-Timbers in the northwest. Southern elements such as Texas madrone, Lacey oak and Mexican pinyon are absent from woodlands, while scalybark oak and bur oak are more important.

Although usually considered most closely related to Mixed Prairie (Allred, 1956; Dodd, 1968; Risser et al., 1981), grasslands to the north and east of the Lampasas Cut Plain are considered extensions of the True Prairie (Dyksterhuis, 1946; Diamond and Smeins, 1985;). Thus, grasslands of the region in good condition contain tall, mid and short grasses such as little bluestem, Indiangrass, big bluestem (Andropogon gerardii), silver bluestem, Texas wintergrass, tall dropseed, sideoats grama and curlymesquite. Mesquite is also a woody component, and Ashe juniper forms "breaks," although not as extensively as in uplands of the Balcones Canyonlands. Common short grasses of more xeric soils or in heavily grazed areas are the same as those listed for the Balcones Canyonlands. Mesquite is commonly a problem for ranchers, especially in the west, and as in other regions of the Edwards Plateau, the landscape is patchy due to differential past grazing, brush clearing, and other land use practices in general.

Woodlands

In our treatment area, open mixed-oak woodlands occur on interfluvial divides, frequently over karstic features. Important species include post oak, live oak, cedar elm and Texas oak. Where sands occur, blackjack oak and Texas hickory (Carya texana) appear locally. These woodlands occur within a matrix of grasslands with affinities for the Blackland Prairie (True Prairie) to the east. Open live oak woodlands occur along broad stream valleys coastward from the steep Canyonlands. Widely spaced trees occur in a mixed to tall grassland context. Many refer to this community type as Oak Savannah (Kuchler 1964). Overgrazing has caused a general trend of replacement of open grasslands by shrubby oak species and tall or mid grasses by short grasses (Buechner 1944).

The Llano Uplift, or Central Texas Mineral Region, has been the focus of more workers than has the limestone portion of the Plateau. Detailed modern vegetation and floristic studies of the region include Whitehouse (1931, 1933), McMillen et al. (1968), Butterwick (1979), Walters (1980) and Walters and Wyatt (1982). Deep, sandy soils support open oak-hickory woodlands. Cedar elm, Texas hickory, live oak, blackjack oak, and post oak are common. Texas oak and Ashe juniper, nearly ubiquitous on the central and eastern parts of the limestone plateau, are conspicuously absent. Where heavy textured soils occur, primarily over shales, a mesquite woodland predominates. Disturbance on all types of deep soil favors mesquite, persimmon and whitebrush (Aloysia gratissima). Specialized vegetation of granitic massifs (e.g., Enchanted Rock) is well documented by detailed studies (Whitehouse, 1933; Walters and Wyatt, 1982). These areas are particularly important as locales to investigate successional processes.

Fault Zone East of the Escarpment and Blacklands

While most investigators recognize the Balcones fault zone south and east of the Plateau, few have described the distinctive vegetation which occurs there. A few investigators who describe this vegetation include Anderson, 1904; Tharp, 1926; Blair, 1965; Collins et al., 1975; Riskind, 1980; Gehlbach, 1984; Lynch, 1962, 1971. This fault zone of downthrown eroded Cretaceous materials, mostly of chalks, claystones, and marls, forms gently rolling terrain with shallow clayey soils (Mollisols). Frequently, Quaternary lag gravels cap these hills, more prominently on the south than on the north. The zone is dissected by numerous streams (Fig. 2), which shelter a riparian gallery forest. Bald cypress occurs as a component between the Nueces and Colorado, while bur oak and bastard oak occur from the Colorado to the Brazos. Otherwise, the gallery forest can also be characterized as an oak-elm-hackberry forest. Pecan, ash (Fraxinus pennsylvanica, texana, berlandieri (north to south)) and cottonwood are also important.

From the Medina River north and eastward, the vegetation of this zone is a tension or transition zone of woodlands, savannah, and prairie. Grasslands grade into Fort Worth Prairie communities through the Lampasas Cut Plain northward and Blackland Prairie to the east. Both of these grasslands are most closely related to the True Prairie of the North American Mid-continent (Dyksterhuis, 1946; Diamond, 1983). Woodlands grade into the Cross Timbers to the north and the Post Oak Savannah to the east. From the Medina River southward, but extending as far north as the Colorado River, brushy species of the Tamaulipan thorn scrub become more important, especially on well drained substrates. Species such as Texas persimmon, guajillo (Acacia berlanderi), Spanish dagger (Yucca treculeana, sacahuista (Nolina texana), sotol (Dasylirion texana), little-leaf sumac (Rhus microphylla), spiny hackberry, snakewood (Colubrina texensis), mountain laurel, Mimosa spp., lime prickly ash (Zanthophylem fagara) and blackbrush acacia become more common. Ashe juniper is common in this zone at least to the Nueces River on the south. On deeper soils mesquite, huisache and hackberry (Celtis reticulata) dominate with mid grass generally characteristic of Tamaulipan savannahs, such as Trichloris, Chloris, Bouteloua, Stipa, Sporoblous, Bothriochloa, Aristida, Hilaria and Erioneuron.

Northward from the Colorado, where chalk is exposed, vegetation is typical of the Balcones Escarpment (see Blair, 1965; Beaty and Gehlbach 1975) and the dissected uplands of the Lampasas Cut Plain. In the zone between Waco and Dallas, eastern red cedar may occur together with Ashe juniper, but it occurs as far south as the Blanco River.

To the east of the chalky, relatively steep Plateau margins is the Blackland Prairie. These grasslands once occurred over deep, clayey Vertisols. Few remnants of this once vast grassland remain. The climax dominants include little bluestem, big bluestem, Indiangrass, tall dropseed, and sideoats grama (Diamond and Smeins, 1985). Flat divides on the Plateau proper supported similar grassland. The few degraded grassy areas over native sod that remain in our region contain Texas grama, Texas wintergrass, buffalograss, curlymesquite, three-awn, muhly, and a variety of short grasses and weedy forbs.

CONCLUSION

The environment of the Balcones Fault Zone/Edwards Plateau region is highly variable, and supports forests, woodlands and grasslands. Superimposed on variation caused by these abiotic factors is variation caused by historical land-use patterns. Variation in grazing history and various brush-control techniques have created a particularly patchy landscape in the grasslands and woodlands Thus, the influence of environmental variables is often obscured, and "fence line" contrasts can be viewed throughout.

We have referred to the Balcones Canyonlands as the most distinctive biotic region of Texas. It has abundant endemic biota. Yet, despite its physiographic distinctness, the area stands out because it harbors an intermixture of biotic elements characteristic of adjacent regions. The mesic microsites and deep sandy soils of the Llano Uplift region contain eastern deciduous forest species, the southern margin harbors Tamaulipan thorn scrub elements, and the xeric portions contain elements from the Mexican Plateau and Chihuahuan Desert. Likewise, grasslands contain elements of the True Prairie, Great Plains (= Mixed) Prairie, Short-Grass Prairie, and Desert Plains Grassland. This mixing of floras, this aggregate biota which is unlike any other adjacent provincial unit (Blair, 1950), is a result of the biogeographic history of the Edwards Plateau, along with its size and high degree of climatic, edaphic and topographic variation. Indeed, the Plateau functions as a wide ecotonal refuge, a filter or melting pot wedged between the equally rich Austroriparian biota to the east and the Mexican biotas to the south and west. Its richness and distinctiveness cries out for investigation, discovery, and most importantly, responsible, enlightened stewardship.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the tireless typing and editorial skills of Bernie Rittenhouse, and to T. B. Samsel and Tom Diltz for facilitating reproduction of the landform figure. Recognition should also go to Suzanne Davis and Ann Morse in the Information Processing Center for their patience and skill in handling the revision process and in the production of the final printed copy.

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in Abbott, Patrick L. and Woodruff, C. M., Jr. eds., 1986,
The Balcones Escarpment, Central Texas:
Geological Society of America, p. 21-32