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.
REFERENCES
Adams, R. P., 1977, Chemosystematics - analysis of populational differentiation
and variability of ancestral and recent populations of Juniperus ashei. Annals
of Missouri Botanical Garden. 64:184-209.
Allred, B. W. and H. C. Mitchell, 1954, Major plant types of Arkansas,
Louisiana, Oklahoma and Texas and their relations to climate and soils. Texas
Journal of Science 7:7-19.
Allred, B. E., 1956, Mixed prairie in Texas. in Grasslands of the Great
Plains. Edited by J. E. Weaver and T. J. Fitzpatrick. Johsen Publishing
Co. Lincoln, Nebraska, pp. 209-254.
Amos, B. and C. R. Rowell, 1984, Phytogeographic analysis of the Edwards
Plateau, Texas based on woody taxa and endemic species. Proceedings of the
Symposium-Floristics and Vegetation of the Edwards Plateau. Southwestern
Association of Naturalists, Junction, Texas. In preparation.
Anderson, E., 1904, Plant societies of the Austin quadrangle. M.A. thesis.
University of Texas, Austin.
Baker, V., 1975, Flood hazards along the Balcones Escarpment in central Texas:
alternative approaches to their recognition mapping and management. Bureau of
Economic Geology. Geologic circular 75-5. University of Texas, Austin.
Beaty, H. E., 1973, Revegetational dynamics of an overgrazed grassland. M.A.
thesis, Baylor University, Waco, Texas.
______ and F. R. Gehlbach, 1975, Vegetational map of the central Texas
region. Institute of Environmental Studies, Baylor University, Waco, Texas.
Bentley, H. L., 1898, A report upon the grasses and forage plants of central
Texas. USDA Bulletin No. 10. G.P.O. Washington, D.C.
Blair, L. S., 1965, A structural analysis of the cedar-oak woodland on the
Austin chalk of Waco, Texas. M.S. thesis, Baylor University, Waco, Texas.
Blair, W. F., 1950, The biotic provinces of Texas. Texas Journal of Science
2:93-117.
Bomar, G. W., 1983, Texas Weather. University of Texas Press, Austin, Texas.
Bray, W. L., 1904, The timber of the Edwards Plateau of Texas; its relation to
climate, water supply and soil. USDA Division of Forestry Bulletin 49. 30 pp.
______, 1905, Vegetation of the sotol country in Texas. Bulletin of the
University of Texas, 60. Science Series No. 6, Austin, Texas.
______, 1906, Distribution and adaptation of the vegetation of Texas.
University of Texas Bulletin 82. Austin, Texas.
Bryant, Y. M., Jr. and H. J. Shaffer, 1977, The late Quaternary
paleoenvironment of Texas: A model for the archeologist. Bulletin of the Texas
Archeological Society 48:1-25.
Buechner, H. K., 1944, The range vegetation of Kerr County, Texas, in relation
to livestock and white-tailed deer. American Midland Naturalist 31:697-713.
Bush, J. K. and A. 0. Van Auken, 1984, Woody species composition of the upper
San Antonio River gallery forest. Texas Journal of Science 36:139-148.
Butterwick, M., 1979, A survey of the flora of Enchanted Rock and vicinity, Llano and Guillispie Counties, Texas. in
Enchanted Rock: A natural area survey, No. 14. LBJ School of Public Affairs,
University of Texas, Austin.
Collins, 0. B., F. E. Smeins and D. H. Riskind, 1975, Plant communities of the
Blackland Prairie of Texas. in M. K. Wali (ed.) Prairie: a Multiple
View. University of North Dakota Press, Grand Forks, North Dakota.
Correll, D. S. and M. C. Johnston, 1970, Manual of the Vascular Plants of
Texas. Texas Research Foundation, Renner, TX.
Cuyler, R. H., 1931, Vegetation as an indicator of geological formations.
Bulletin of the American Association of Petroleum Geologists, 15:67-78.
Diamond, D. D., 1983, Composition, diversity, and interspecific relationships
of grasslands within the true and upper coastal prairies of North America.
Ph.D. dissertation, Texas A&M University, College Station, TX.
_______, and F. E. Smeins, 1985, Composition, classification, and species
response patterns of remnant tallgrass prairies in Texas. American Midland
Naturalist 113(2):294-308.
Dice, L. R., 1943, The biotic provinces of North America. University of
Michigan Press, Ann Arbor, Michigan. 78 pp.
Driscoll, R. S., E. L. Merkel, D. L. Radloff, D. E. Snyder, and J. S. Higihara,
1984, An ecological land classification framework for the United States. USDA
Forest Service. Miscellaneous Publication 1439.
Dunlap, D. W., 1983, A quantitative descriptive study of the grassland
vegetation and soils of the eastern Edwards Plateau, Texas. M.A. thesis,
University of Texas, Austin. 185 pp.
Dyksterhuis, E. J., 1946, The vegetation of the Fort Worth prairie. Ecological
Monographs 16:2-29.
Fenneman, N. M., 1931, Physiography of western United States. McGraw-Hill, New
York, N.Y.
Flyr, David, 1966, The contemporary vegetation of the Amistad Reservoir area,
in: A preliminary study of the Paleoecology of the Amistad Reservoir
Area. D. A. Story, Y. M. Bryant, Jr. assemblers. Report to the National
Science Foundation.
Ford, A. L. and 0. W. Van Auken, 1982, The distribution of woody species in the
Guadalupe River floodplain forest in the Edwards Plateau of Texas. Southwestern
Naturalist 27:383-392.
Fowler, N. L., 1985, Density dependent population regulation in a Texas
grassland. Ecology 67:545-554.
______ and D. W. Dunlap, 1986, Grassland vegetation of the eastern Edwards
Plateau. American Midland Naturalist 115:146-155.
Gehlbach, F. R., 1981, Mountain islands and desert seas: A natural history of
the U.S.-Mexican borderlands. Texas A&M University Press, College Station,
TX.
______,1984, Woodlands and forests of the Balcones Scarp zone in Central Texas:
structure, succession, human modification and evaluation for landscape
planning. Proceedings of the Symposium--Floristics and Vegetation of the
Edwards Plateau. Southwestern Association of Naturalists, Junction, TX. In
preparation.
Godfrey, C. L., G. S. McKee and H. Oakes, 1973, General soils map of Texas.
Texas Agricultural Experiment Station, College Station, TX.
Gould, F. W., 1975, Texas Plants--a Checklist and Ecological Summary. Texas
Agricultural Experiment Station, Texas A&M University, College Station,
TX.
Hill, Robert T., 1892, Notes on the Texas-New Mexican Region. Bulletin of the
Geological Society of America 3:85-100.
Hunt, C. B., 1974, Natural regions of the United States and Canada. W. H.
Freeman & Co., San Francisco.
Johnson, E. H., 1931, The natural regions of Texas. University of Texas
Bulletin 3113. University of Texas, Austin.
Johnston, M. C., D. H. Riskind, M. Butterwick, J. Lamb, and S. Osborn (in
press). A botanical survey of the Lower Canyons of the Rio Grande. Chihuahuan
Desert Research Institute Special Publ., Alpine, Texas.
Kuchler, A. W., 1964, Potential natural vegetation of the conterminous United
States. American Geographical Society Special Publication No. 36.
Larkin, T. J. and G. W. Bomar, 1983, Climatic atlas of Texas. Texas Department
of Water Resources, Austin, TX.
Lonsdale, J. T., 1927, Igneous rocks of the Balcones Fault region of Texas.
University of Texas Bulletin 2744, Austin, TX.
Lundelius, E. L., Jr., 1967, Late Pleistocene and Holocene faunal history of
central Texas. in Pleistocene Extinctions: the search for a -
cause. P.S. Martin and H. E. Wright, Jr. Yale University Press, New Haven, Conn. pp.
287-319.
Lynch, D., 1962, Study of a grassland mosaic at Austin,Texas. Ecology
43:679-686.
______, 1971, Phenology, community composition and soil moisture in a relict at
Austin, Texas. Ecology 52:890-897.
Lyndon B. Johnson School of Public Affairs, 1978, Preserving Texas' Natural
Heritage. LBJ School Policy Research Project Report 31. Austin, Texas.
McMahan, C. A., R. G. Frye and K. L. Brown, 1984, The Vegetation Types of
Texas, including Cropland. Texas Parks and Wildlife Department, Austin, Texas.
McMillan, D., C. E. Jens, W. R. Adler, R. Y. Blystone, W. H. Gillespie, J. R.
Irwin, R. E. Janowsky, D. 0. Kille, T. R. McGlathery, R. R. Martin, R. W.
Morey, C. R. Mynard, and T. S. Patty, 1968, Factors influencing the narrow
restriction of Pilularia americana in Texas. Southwestern
Naturalist 13:117-127.
Palmer, E. J., 1920, Canyon flora of the Edwards Plateau of Texas. Journal of
Arnold Arboretum 1:233-239.
Riskind, D. H., 1980, A general introduction to the vegetation and dominant
plant communities of the McKinney homestead, Travis County, TX, in
Archaeological Investigations at the Thomas F. McKinney Homestead. Part I.
Bulletin of the Texas Archeological Society. 51:128-135.
Risser, P. G., C. E. Girney, H. D. Blocker, S. W. May, M. J. Patton and J. A.
Weins, 1981, The True Prairie Ecosystem. Hutchinson Ross Publishing Company,
Strousburg, PA.
Sellards, E. H., W. S. Adkins and F. B. Plummer, 1932, The geology of Texas,
Vol. 1. Stratigraphy. University of Texas Bulletin 3232, Austin, TX.
Smeins, F. E., T. W. Taylor, and L. B. Merrill, 1976, Vegetation of a 25-year
exclosure on the Edwards Plateau, Texas. Journal of Range Management
29(l):24-29.
______, 1980, Natural role of fire on the Edwards Plateau. in White, L. D., Prescribed range burning in the Edwards Plateau, Texas Agricultural Experiment Station Proceedings, College Station, TX. pp. 4-16.
Smith, J. and M. Butterwick, 1975a, A vegetational survey of the Devils
River-Dolan Creek area, in Devils River: A natural area survey, University of
Texas Division of Natural Resources and Environment,
University of Texas, Austin.pp. 36-57.
_______, 1975b, A vegetational survey of the Devil's Sinkhole-Hackberry Creek
Area. in Devil's Sinkhole Area - Headwaters of the Nueces River.
Division of Natural Resources and Environment, University of Texas, Austin. 21-46
Solcher, E. A., 1927, An analysis of the plant associations of Bexar County,
Texas. M.A. thesis, University of Texas, Austin.
Tharp, B. C., 1926, Structure of Texas vegetation east of the 98th meridian.
University of Texas Bulletin 2606, Austin, TX.
______,1939, The vegetation of Texas. Anson Jones Press, Houston, TX.
______,1944, The mesa region of Texas: an ecological study. Proceedings of the
Texas Academy of Science 27:81-91.
______,1952, Texas Range Grasses. University of Texas Press, Austin.
Thornthwaite, C. W., 1948, An approach toward a rational classification of
climate. Geographic Review. 38:55-94.
United States Fish and Wildlife Service, 1979, Unique Wildlife Ecosystems of
Texas. Albuquerque, NM. 164 pp.
Van Auken, 0. W., A. L. Ford and A. Stein, 1979, A comparison of some woody
upland and riparian plant communities of the southern Edwards Plateau.
Southwestern Naturalist 24:165-180.
______, A. L. Ford, and J. L. Allen, 1981, An ecological comparison of upland
deciduous and evergreen forests of Central Texas. American Journal of Botany
68:1249-1256.
______, A. L. Ford, A. Stein and A. G. Stein, 1980, Woody vegetation of upland
plant communities in the southern Edwards Plateau. Texas Journal of Science
32:23-35.
______ and J. Bush, 1985, Secondary succession on terraces of the San Antonio
River. Bulletin of Torrey Botanical Club 112:158-166.
Walters, T. W., 1980, The vascular flora and vegetation of granite outcrops in
the Central Mineral Region of Texas. M.S. thesis, Texas A&M University,
College Station, TX.
______ and R. Wyatt, 1982, The vascular flora of granite outcrops in the
Central Mineral Region of Texas. Bulletin of the Torrey Botanical Club
190:344-364.
Webster, G. L., 1950, Observations on the vegetation and summer flora of the
Stockton Plateau in northeastern Terrell County, Texas. Texas Journal of
Science 2:234-242.
Whitehouse, E., 1931, Ecology of Enchanted Rock vegetation. M.S. thesis.
University of Texas, Austin.
______, 1933, Plant succession on central Texas granite. Ecology 13:391-405.
Williams, J. E., 1977a, The vegetation of Wild Basin. Wild Basin Institute for
Environmental Studies. Wild Basin Publication 1. Wild Basin Wilderness, Inc.,
Austin, TX.
______, 1977b, Vegetation of Hamilton's Pool, Travis County, Texas. Unpublished report submitted to Natural Areas Survey, c/o Texas Conservation Foundation, Austin, TX.
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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
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