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Preliminary description of the vegetation of South Texas exclusive of coastal saline zones.

ABSTRACT. -- South Texas covers the 93,000 square kilometers south of the Edwards Plateau, and between the Rio Grande and San Antonio Rivers and the Gulf of Mexico. Ten vegetation associations of the region (exclusive of coastal wetlands and the lower Rio Grande Valley), comprising 29 plant communities, are described. These include two grassland (little bluestem-trichloris and seacoast bluestem-balsamscale), two woodland (hackberry-huisache and live oak-post oak), and six shrubland associations. The shrublands include two mesic associations (mesquite-granjeno and huisache-prickly pear) and four xeric associations (mesquite-prickly pear, guajillo-cenizo, blackbrush-twisted acacia, and creosote bush-prickly pear). Key words: vegetation; southern Texas; shrublands; plant communities.

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South Texas, as here defined, covers an area of 93,000 square kilometers, bordered on the north by the Edwards Plateau, on the east by the San Antonio River and the Gulf of Mexico, and on the west and south by the Rio Grande (Fig. 1). Numerous authors have included South Texas in their description of vegetation of larger areas, including: Bailey (1905); MacDougal (1908); Harshberger (1911); Clements (1920); Livingston and Shreve (1921); Tharp (1926, 1939); Dice (1943); Blair (1950); Allred and Mitchell (1955); Oosting (1956); Johnston (1963); Shelford (1963); Kuchler (1964); Gould (1969); and Daubenmire (1978). Clover (1937), Box and Chamrad (1966), Dodd and Holtz (1972), Drawe et al. (1978), Van Auken and Bush (1985), Vora (1990), and Vora and Messerly (1990) have described the vegetation of smaller areas of South Texas.

Variation in natural vegetation in South Texas is largely a function of four factors--rainfall, soil texture, soluum depth, and temperature. These four variables form the basic matrix of the vegetation complex, with such factors as salinity, drainage, topography, and exposure having a modifying effect on a local scale. Average annual rainfall in South Texas decreases from 86 centimeters in the northeast to 44 centimeters in the southwest, a distance of 270 kilometers. Most rainfall comes from moisture from the Gulf moving inland. The predominant winds in the region are from the southeast, and when tropical storms strike the area they also generally move from southeast to northwest, with the northeastern quadrant being the wet side. This results in vegetation of the northeast being transitional to the Eastern Decidous Forest, whereas vegetation in the southwest is transitional to the Chihuahuan Desert.

Soil textures in the region range from sands to montmorillonitic clays. The sands are found in five areas--large patches in the northwestern, southwestern, southern, and northeastern parts, and a strip in the north. In each case, the sands function as wet sites, supporting more mesic vegetation than the surrounding clays and loams. Clays predominate along much of the upper coastal plain and as a strip between sand and the Balcones Escarpment in the northeast, but also are found scattered throughout South Texas.

The Bordas Scarp is a fault line running from the southwestern part of South Texas northeastward (Fig. 1). This fault line of low caliche hills forms a rough "Y" across South Texas, stretching from the southwest to the northeast and dividing the region into three subregions. The northeastern subregion is an area of low rolling hills divided by numerous small creeks and rivers. The southeastern subregion is a relatively flat coastal plain with large areas of deep clays to the north and sands to the south. The western subregion is an area of gently sloping low hills with many of the drainageways saline. It is the driest of the three subregions, with the rocky hills along the Rio Grande the most xeric.

The southern extreme of the region is subtropical, with Cameron County having a 341-day growing season, and with many years in which there is no hard freeze. Bexar County in the north has a 265-day growing season. Subtropical species are abundant in southern South Texas, but decrease in abundance to the north. This is especially true for some shrubs, which are subdominants in the southern third of the region.

Nomenclature follows that of Gould (1975) for grasses and Correll and Johnston (1970) for other plants. The vegetation of those areas along the coast that derive their ecological characteristics from the saline coastal environment are not included here. For the purpose of this paper, the following units of classification will be used:

A. Biome. The highest hierarchical level. Based on structure or dominant abiotic factor. Example: Grassland.

B. Formation. Highest subdivision of a biome. Based on structure or dominant abiotic factor. Example: Prairie grassland.

C. Province. Highest subdivision of a formation. Based on structure or geographic location. Example: Midgrass prairie.

D. Association. The major subdivision of a formation. Based on species composition and designated by a binomial or trinomial based on codominat species. Example: Little bluestem-trichloris association.

E. Community. Basic subdivision of an association. Based on differences among relative importance of codominants or subdominants. Example: Little bluestem-trichloris/Texas wintergrass community.

F. Type. Subdivision of a community. Based on differences between subordinate species or relative importance of subdominant species.

GRASSLANDS OF SOUTH TEXAS

Two climax grassland associations occur in South Texas: little bluestem-trichloris and seacoast bluestem-balsamscale.

[FIGURE 1 OMITTED]

Little bluestem-trichloris Association

The major occurrences of this association are the Fayette Prairie on each side of the sand belt south of the Balcones Escarpment and the upper Coastal Prairie, beginning approximately in central Kleberg County and extending northward. The Fayette Prairie is a southwestern extension of the Midgrass Prairie Province. These grasslands occur on clay and clay loam soils in the higher rainfall region of South Texas. The topography of the Fayette Prairie is largely low rolling hills with numerous creeks and small rivers; that of the Coastal Prairie is relatively flat with numerous creeks and small rivers. A combination of widely flucuating soil moisture (the result of highly variable rainfall), the root-pruning action of montmorillonitic clays, and occasional fires resulted in this midgrass climax except along drainages and the top of some of the steeper hills. The effect of the montmorillonitic clay is greatest on the Coastal Prairie. Combined with fire, and without destruction of the grassland by overgrazing, this pruning action gives grasses a significant advantage over shrubs in competition for soil moisture. Upon disturbance of the climax by overgrazing or prolonged drought, these sites move toward mesic shrublands, generally the mesquite-granjeno association (Archer et al., 1988). Once established, the shrubland community is extremely stable.

The structure of this association is a midgrass prairie with widely scattered individual mesquite (Prosopis glandulosa) or oak (Quercus sp.) trees and clumps of prickly pear (Opuntia lindheimeri). There is a well developed understory of forbs and shortgrasses, with a strong early spring aspect. Patches of tallgrass prairie occur on the most favorable sites. The most common dominants within this association are little bluestem (Schizachyrium scoparium) and trichloris (Chloris pluriflora). Major subdominants include silver bluestem (Bothriochloa saccharoides), big bluestem (Andropogon gerardii), plains bristlegrass (Setaria leucopila), sideoats grama (Bouteloua curtipendula), Texas wintergrass (Stipa leucotricha), buffalograss (Buchloe dactyloides), hairy grama (Bouteloua hirsuta), knotroot bristlegrass (Setaria geniculata), indiangrass (Sorhastrum nutans), eastern gamagrass (Tripsacum dactyloides), and big sandbur (Cenchrus myosuroides).

There are four common communities within this association. The little bluestem-trichloris/Texas wintergrass community is the most common and is the typical community of the association. The little bluestem-big bluestem-trichloris community is found on the more favorable sites, most commonly at bottoms of slopes, near permanent water, on sandy loam sites, and on higher rainfall sites of the Coastal Prairie. Within this community, big bluestem occurs in patches, with its relative importance increasing as available soil moisture increases. On the most favorable sites, primarily along the eastern edge of South Texas, eastern gamagrass and indiangrass become important components. The transition to the marsh communities is marked by an increase in bushy bluestem (Andropogon glomeratus) and knotroot bristlegrass.

The silver bluestem-trichloris/buffalograss community is the most common grassland community on the drier sites within the Fayette and Coastal prairies. Little bluestem is a common species within the community, and often replaces buffalograss as soil moisture increases, thereby becoming transitional to the little bluestem-trichloris/Texas wintergrass community. Typical sites for this silver bluestem community are upper slopes and hardpans (Fig. 2). This is also a common early retrogression community resulting from drought or heavy grazing. As soil moisture becomes more limiting, hairy grama often becomes more common than buffalograss, especially on shallow sands, and sideoats grama may achieve subdominant status on upper slopes.

[FIGURE 2 OMITTED]

The little bluestem/knotroot bristlegrass-silver bluestem community is found on sites of poor drainage. As periods of flooding or intensity of grazing increase, little bluestem decreases and knotroot bristlegrass increases in importance. As length of flooding increases further, longtom (Paspalum lividum) becomes important and bushy bluestem replaces silver bluestem, eventually becoming the bushy bluestem-knotroot bristlegrass/longtom community. If fire is excluded, or overgrazing is intense, from either of these wet grassland communities, huisache (Acacia farnesiana) becomes increasingly important, eventually developing into the huisache-prickly pear association.

Seacoast bluestem-balsamscale Association

This association is the characteristic grassland on the medium-depth sands throughout South Texas. Seacoast bluestem (Schizachyrium scoparium var. littoralis) is a rhizomatous variety of little bluestem and, with the exception of the woodland communities, is the variety of the S. scoparium complex found on sandy soils in South Texas. These sandy prairies mostly occur in several large areas in the eastern (Refugio and Goliad counties), southern (Kenedy, Kleberg, Willacy, Brooks, Jim Hogg, and Starr counties), and western (Zapata, Dimmit, and LaSalle counties) part of the region. The topography of these sites is slightly rolling with few creeks or rivers. The increased availability of soil moisture on the sands allows grasses to develop into a midgrass prairie rather than the discontinuous canopy of short- and midgrasses common to adjacent loams and clays. Fire then can function to maintain these areas as prairie rather than the shrubland/grassland complex characteristic of adjacent nonsandy sites. Overgrazing destroys this prairie structure, decreasing the effectiveness of fire. As a result, many of these sites now support a mesquite-dominated shrubland community.

The structure of this association is a midgrass prairie with seacoast bluestem in patches and balsamscale in clumps. There is an extensive understory of forbs (which contribute to a distinct spring aspect) and shortgrasses, with the midgrass canopy becoming more open and bare ground increasing as rainfall decreases. There are scattered individual mesquite trees and small, scattered clumps of prickly pear, and, to the west, yucca (Yucca constricta). Seacoast bluestem and panamerican balsamscale (Elyonurus tripsacoides) are the common dominants (Fig. 3), with rowfeather threeawn (Aristida purpurascens), thin paspalum (Paspalum setaceum), brownseed paspalum (P. plicatulum), coast sandbur (Cenchrus incertus), big sandbur, tanglehead (Heteropogon contortus), big bluestem, snoutbean (Rhyncosia texana), tephrosia (Tephrosia lindheimeri), snakecotton (Froelichia floridana), mesquite, prickly pear, and yucca being the most common subdominants.

Topography, and hence soil moisture, is the factor most affecting community differentiation within this association. The seacoast bluestem-balsamscale/rowfeather threeawn community is the typical community of the association, and is found on the midslopes of the low rolling hills (Fig. 2). Seacoast bluestem decreases toward the tops of the hills where soil moisture levels are lower, and thin paspalum becomes more abundant, forming the balsamscale thin paspalum/rowfeather threeawn community. Drought and overgrazing have a similar effect of reducing the size and number of seacoast bluestem patches and increasing the abundance of thin paspalum, rowfeather threeawn, and coast sandbur. The grassland transitions from these sands to loams are dominated by either seacoast bluestem-brownseed paspalum/balasmscale or seacoast bluestem-tanglehead/rowfeather threeawn communities. This transition is the result of the increased water-holding capacity of the sandy loams, with the tanglehead community found on drier sites than the brownseed paspalum community. If soil moisture continues to increase, especially in the swells at the bottom of the slopes, or if fire is excluded, the seacoast bluestem-brownseed paspalum/balsamscale community develops into the mesquite/wolfberry-granjeno or mesquite-granjeno/hogplum community.

[FIGURE 3 OMITTED]

WOODLANDS OF SOUTH TEXAS

Two climax woodland associations occur within South Texas as defined in this paper.

Hackberry-huisache Association

This is the riparian association of South Texas. The width, structure, and composition of the association at any location is dependent upon the size of the stream or river and the soil texture along the banks. The association is best developed within the northeastern part of the region, where the river bottoms are broader and rainfall higher. Here the woodland may be one to two kilometers wide, the trees 20 meters tall, the canopy closed, and with as many as five well developed strata. In the drier southwestern portion, the woodland becomes narrow, often discontinuous, the trees fewer in species and smaller, and strata often reduced to two or three.

The most common dominants within the association are sugar hackberry (Celtis laevigata) and huisache. Major subdominants, rising to dominant status in various locations, include eastern cottonwood (Populus deltoides), post oak (Quercus stellata), live oak (Q. virginiana), cedar elm (Ulmus crassifolia), anaqua (Ehretia anacua), mesquite, pecan (Carya illinoensis), black hickory (C. texana), shagbark hickory (C. ovata), Texas persimmon (Diospyros texana), Texas ebony (Pithecellobium flexicaule), mustang grape (Vitis mustangensis), and muscadine (V. rotundifolia). Understory development is highly variable and dependent upon upper canopy thickness, soil texture, and frequency of flooding. Common understory species include Virginia wildrye (Elymus virginicus), trichloris, Texas wintergrass, knotroot bristlegrass, Texas bristlegrass (Setaria texana), brownseed paspalum, rustyseed paspalum (Paspalum langei), little bluestem, eastern gamagrass, old-man's beard (Clematis drummondii), mistflower (Eupatorium odoratum), and catbrier (Smilax bona-nox).

Community development within the association is complex because of the nature of discontinuous riparian environments. There are, however, three major groupings of communities within the association: northern and eastern, western and southern, and sandy. Along the northern and especially the eastern edges of South Texas, the association is most like the Eastern Deciduous Forest. Here moisture is more abundant and less variable, and the communities are dominated by combinations of sugar hackberry, cedar elm, eastern cottonwood, pecan, black hickory, and mustang grape, with huisache and mesquite mostly occurring along the edges. In the much drier and more variable southwestern area, the communities are dominated by sugar hackberry, huisache, anaqua, mesquite, Texas persimmon, Texas ebony, and muscadine, the precise combination depending upon the size of the drainage. Retama (Parkinsonia aculeata) and mesquite become increasingly important on the drier sites and after major disturbance of the climax community. Likewise, sugar hackberry and huisache become increasingly important in the northeast on the smaller streams and drainages or following disturbance of the climax woodland in that area. The oaks dominate on sandy riparian sites, post oak being most common in the northeast and live oak in the west, south, and along the coast.

Live oak-post oak Association

This association occurs throughout South Texas on sandy-surfaced soils where moisture is sufficient to support a woodland. The association is most extensive in the northeast where it forms extensive strips. Along the coast, it most often occurs as either groves (mottes) or in strips. The two dominant species are live oak and post oak. Subdominant species include mesquite, huisache, mustang grape, muscadine, and little bluestem.

The post oak-live oak/little bluestem community is the major community of the association in the northeastern part of South Texas, and is the primary community of the sand belt south of San Antonio and stretching through northern Wilson and Atascosa and southern Bexar counties, and into northern Frio County. This oak woodland is approximately 100 kilometers long and at places up to 15 kilometers wide. Post oak is the more important species on the uplands and live oak becomes dominant on the larger bottoms. Understory is poorly developed except where the oak canopy is broken, in which case a patch of midgrass prairie forms, dominated by little bluestem. Mesquite is an important subdominant only along the edges of the oak woodland or within the woodland where the canopy has been broken and the midgrass canopy reduced.

The live oak/mesquite-seacoast bluestem community is the representative community of this association along the coast. The oak occurs either in strips or in groves, the strips generally occurring along drainages and they are more common along the northeastern shoulder of the Bordas Scarp, from southern Live Oak County to southern Goliad and northern Refugio counties. The grove habitat is more common nearer the coast and through the sand country of Kenedy and Brooks counties. In both forms, the live oak forms a dense canopy with few other species (upper and lower strata) present. The trees are generally of uniform size within a grove or a given distance of a strip, but the sizes vary between groves or strips from small (less than five meters high and less than 10 centimeters dbh) to large (more than 15 meters high and more than one meter dbh). Areas dominated by midgrasses or mesquite or both occur as breaks and extensions into these oak strips and as patches within and between groves. These are similar to the seacoast bluestem balsamscale/rowfeather threeawn community and are generally dominated by seacoast bluestem, with or without mesquite. If the area dominated by the grasses is significantly less than that dominated by the oak, they should be considered a part of the oak community. The woodland is not as stable as it might first appear. Openings in the canopy occur occasionally and are then dominated by the grasses. If the factor causing the opening is temporary (for example, lightning or death of one or two dominant trees), then the grasses soon give way to mesquite and then young oak, and the opening closes. If the factor is prolonged (for example, drought or disease), the opening may enlarge. As the disturbance eventually ends, the opening begins to retract. The same ebb and flow takes place along the outer margins, with mesquite being more common there.

SHRUBLANDS OF SOUTH TEXAS

Perhaps no other type of vegetation is more closely associated with South Texas than the shrubland--the brush country. These shrublands are closely associated with the region yet they often are misunderstood, because attempts are made to explain them solely on the basis of knowledge of shrublands, even mesquite-dominated shrublands, from other, temperate, regions. There are, of course, many similarities. There are, however, many dissimilarities.

Too often, especially in North America, shrublands have been considered only as transitional types or types resulting from disturbance. This is especially the case in management applications. In many parts of the world, shrublands are as stable climax types as forests and grasslands, and hence should be afforded biome status. Such is the case in South Texas. Some shrublands there are the result of mismanagement--destruction of the grassland by overgrazing, exclusion of fire, herbivory by grazers instead of browsers. Others, however, are climax types. Failure to recognize this point has led to many problems with pasture management following brush control.

It is helpful to separate South Texas shrublands into two groups, mesic and xeric. The mesic shrublands are, in effect, dry woodlands, more closely approaching forests, and probably also grasslands, than deserts. Xeric shrublands occur on sites where available soil moisture becomes too limiting for grasses to form a continuous canopy, thereby affording protection from fire for those shrubs that can establish on these sites. This division into mesic and xeric shrublands is of fundamental importance in understanding the ecological dynamics of these shrublands and their responses to management techniques. Xeric shrublands respond in a manner similar to the response of most shrublands of the western United States. Mesic shrublands, however, respond in a manner similar to the response of deciduous woodlands of the eastern United States.

There are two mesic shrubland associations in South Texas--mesquite-granjeno and huisache-prickly pear. There are four xeric shrubland associations--guajillo-cenizo, blackbrush twisted acacia, creosotebush-prickly pear, and mesquite-prickly pear. The first three of these xeric shrublands occur on shallow soil sites and the fourth on saline sites.

Mesquite-granjeno Association

This is the major shrubland association of eastern South Texas and is common bottomland association in western South Texas. In northern South Texas, this association is dominant along drainages and lower slopes on nonsandy soils where the soil moisture levels are intermediate. If the sites become more mesic, the hackberry-huisache association dominates, with the mesquite-granjeno association dominating the transition area between the woodland of the bottoms and the grassland of the slopes or open prairies. If the area is sandy, the live oak-post oak association dominates with the mesquite-granjeno association dominating the transition to heavier-textured soils. In the southeastern part of South Texas, this association dominates the loamy textured sites where topography effectively excludes fire as a significant ecological factor. Most of coastal South Texas is not broad unbroken plains, but instead, broad plains broken by numerous creeks, small rivers, and saline or caliche pans. These function as natural firebreaks that effectively exclude prairie fires such as were common on the Great Plains. Dry lightning storms are rare in South Texas. If bolt lightning occurs, it is almost always accompained by rain. The idea of these broken prairies burning on a regular basis because of fires started by lightning is hard to accept. When such fires did start, it is extremely doubtful they would burn past the first of these natural barriers. Hence, for this area to burn on a regular basis (every 10 to 20 years, for example), each of these units would have to be hit individually, and because many of these units are less than five kilometers wide, that is rather doubtful. And 10 to 20 years is adequate time for these areas to become dominated by shrubs. In western South Texas, this association also dominates the drainages too dry to support the hackberry-huisache association.

The mesquite-granjeno association is the climax association on the above-mentioned sites. It also now dominates many sites that were climax grassland. As the grasslands of northern and eastern South Texas are destroyed by overgrazing, these shrublands become established. Once established, it is doubtful that they can return to grassland by fire alone, even if grazing is completely excluded. Once established, these shrublands are extremely stable, and much of eastern and northern South Texas has been converted from grassland to shrubland by this method.

The structure of this association is an open to dense shrubland, with three to four strata. Upper strata shrubs vary from two to 10 meters in height, and are primarily mesquite. Lower strata shrubs vary from one to four meters, and are more species diverse, although granjeno (Celtis pallida) is by far the most common. The herbaceous strata generally consists of an upper strata dominated by tall forbs and scattered clumps of midgrasses, and a lower strata dominated by forbs and shortgrasses. Although the grass and forb productivity is relatively low within this association, it is an important habitat type for white-tailed deer for protection and browse. When the shrubland is dense, there tend to be scattered small openings in the shrubland, forming a mosiac, with the openings dominated by mid- or shortgrasses.

Mesquite is always the dominant species within the association, and granjeno is the most common codominant. If not a dominant, granjeno always will be a subdominant within the association. Other common subdominants, which can rise to codominant status, are prickly pear, colima (Zanthoxylum fagara), hogplum (Colubrina texensis), whitebrush (Aloysia lyciodies), blackbrush (Acacia rigidula), wolfberry (Lycium berlandieri), bluewood (Condalia obovata), Texas persimmon, Texas ebony, and lotebush (Ziziphus obtusifolia). Important understory species include agarito (Berberis trifoliolata), guayacan (Porlieria angustifolia), tasajillo (Opuntia leptocaulis), silver bluestem, little bluestem, trichloris, plains bristlegrass, knotroot bristlegrass, Texas bristlegrass, pink pappusgrass (Pappophorum bicolor), vine-mesquite (Panicum obtusum), curly mesquite (Hilaria belangeri), buffalograss, hairy grama, Texas grama (Bouteloua rigidiseta), purple threeawn (Aristida purpurea), hooded windmillgrass (Chloris cucullata), mistflower, orange zexmenia (Zexmenia hispida), bundleflower (Desmanthus sp.), parietaria (Parietaria floridana), old-man's beard, sarcostemma (Sarcostemma cynanchoides), bindvine (Cynanchum barbigerum), and cudweed (Gnaphalium pensilvanicum).

[FIGURE 4 OMITTED]

There are five major communities within this association. The mesquite-granjeno/prickly pear community is the most common and widespread. The community is best developed (that is, greatest structural development and greatest number of subdominants) in the northeast, especially in Live Oak and Atascosa counties (Fig. 4). The mesquite-granjeno/hogplum community becomes important on sandy loam soils, especially drier sandy loams (that is, sand loam knolls or upper slopes). In this community, hogplum becomes the most important subdominant and most of the other subdominant species within the association are absent. Prickly pear and bluewood are the only two other common subdominant shrubs, and herbaceous productivity increases. If sand content continues to increase, the transition continues to the Seacoast bluestem-tanglehead/rowfeather threeawn community.

[FIGURE 5 OMITTED]

In the southern part of South Texas (from approximately Kleberg and Jim Wells counties southward), the mesquite-colima/granjeno community becomes the representative community on loams and clays (Fig. 5). This is the southern version of the mesquite-granjeno/prickly pear community, in which the subtropical species colima becomes an important component. Mesquite and colima form a dense overstory, with granjeno and bluewood forming a dense understory thicket. These communities can become so dense that it is difficult even to crawl through them, and herbaceous understory is often quite sparse.

The mesquite/granjeno-wolfberry community is most common on the smaller drainages within the sandy areas where the soil texture is sandy loam and along the edges of saline waterways near the coast. In this community, mesquite is the overstory dominant and granjeno and wolfberry dominate the lower shrub strata, with whitebrush and bluewood common associates. If fire moves through the sand sites often enough, the seacoast bluestem-brownseed paspalum/balsamscale community dominates instead.

[FIGURE 6 OMITTED]

The whitebrush-mesquite/granjeno community is found throughout South Texas, generally along drainageways of flat areas and along slope benches where clay content increases enough to significantly increase water-holding capacity. Typically, these communities become extremely thick whitebrush thickets with mesquite and granjeno rising above the whitebrush. The whitebrush often forms a solid canopy up to three meters high with almost no herbaceous understory development.

Huisache-prickly pear Association

This is the dominant association on areas of poor drainage or that are periodically flooded but where the flooding does not last long enough to destroy the understory components. Huisache is the dominant overstory species (Fig. 6), and it varies from scattered, savanna-like stands to dense woodlands, with the trees varying from three to 10 meters in height. Lower strata shrubs, generally prickly pear, occur in clumps within a moderate to dense stand of midgrasses and forbs. Important understory species include knotroot bristlegrass, vine-mesquite, longtom, brownseed paspalum, bushy bluestem, silver bluestem, little bluestem, trichloris, and old-man's beard. Fire is an important factor in the dynamics of this association, keeping the huisache canopy open enough to allow development of the dense midgrass understory. If fire is excluded, the huisache canopy becomes dense and the understory grasses decrease in importance, eventually decreasing to the point that insufficient fuel is present to carry a fire through the stand.

There are two major communities within this association. The huisache/prickly pear-silver bluestem community is the typical community on those areas that are periodically flooded, but where the water table is not near the surface. Retama is a common subdominant within this community, generally occuring along the drier edges. The midgrass subdominants vary from site to site, but silver bluestem is most common. The huisache/bushy bluestem-knotroot bristlegrass community is the typical community on those sites where the water table is near the surface. On the drier edges, prickly pear, silver bluestem, and little bluestem become increasingly important. Where the water line is approached, longtom becomes more important. Within both communities, old-man's beard can form almost exclusive colonies, sometimes as much as 15 meters wide. Both of these communities are important for both livestock and wildlife.

Mesquite-prickly pear Association

This association is found on saline or sodic sites in South Texas. These sites generally occur as drainage basins where most of the water evaporates rather than runs off, as drainages where the water is saline from leaching saline profiles, or as saline or sodic outcrops. On these sites, the overstory shrubs, primarily mesquite, vary from scattered individuals to a moderate stand, four to eight meters tall. The lower strata shrubs are moderate to dense, often in clumps, and generally one to three meters tall. The herbaceous understory consists of a discontinuous canopy of shortgrasses with scattered clumps of midgrasses and coarse bunchgrasses. Scattered areas of bare ground are common. The most common subdominants are prickly pear, lotebush, blackbrush, wolfberry, amargosa (Castela texana), alkali sacaton (Sporobolus airoides), and gulf cordgrass (Spartina spartinae). Major understory species include whorled dropseed (Sporobolus pyramidatus), seashore dropseed (S. virginicus), curly mesquite, red threeawn (Aristida longiseta), Texas grama, red grama (Bouteloua trifida), tobosa (Hilaria mutica), plains bristlegrass, pink pappusgrass, and trichloris.

There are three major communities within this association. The mesquite-prickly pear-lotebush community is found on the drier saline slopes. The mesquite are small and scattered, with the prickly pear and lotebush scattered to dense. There is generally a mostly continuous cover of shortgrasses, with scattered clumps of midgrasses. Near the coast, amargosa becomes important, forming almost solid stands on some sites. The Mesquite/prickly pear-sacaton community is the representative community on the moister saline sites away from the coast. Mesquite trees are small to medium in size and scattered, with retama often present. Prickly pear generally occurs as large scattered clumps on slight rises, and sacaton occurs as scattered clumps to dense stands in the swales. Where the sacaton clumps are scattered, shortgrasses are abundant. Nearer the coast, and on wetter sites, gulf cordgrass replaces sacaton and prickly pear becomes less abundant. This becomes the mesquite/cordgrass-prickly pear community. Along the coast, sea oxeye (Borrichia frutescens) becomes a strong subdominant.

Guajillo-cenizo Association

This is the association found on the tops of the caliche hills of the Bordas Scarp and other sites where caliche is at the surface. These shallow soils vary from almost nonexistent to 10 centimeters deep, and have low moisture holding capacities. The association consists of an open to dense shrubland, with the dominants generally two to three meters tall. The herbaceous understory is sparse, generally with more bare ground than grass cover, and consisting primarily of shortgrasses, low-growing semishrubs, and annual forbs. Scattered midgrasses occur, primarily in the deeper soil and litter on the immediate upslope side of the dominant shrubs. The primary dominants are guajillo (Acacia berlandieri) and cenizo (Leucophyllum frutescens). Common subdominants are blackbrush, Texas kidneywood (Eysenhardtia texana), Spanish dagger (Yucca treculeana), prickly pear, mountain laurel (Sophora secundiflora), guayacan, and blue sage (Salvia ballotaeflora). Major understory species include agarito, leatherstem (Jatropha dioica), curly mesquite, Texas grama, red grama, sideoats grama, slim tridens (Tridens muticus), hairy grama (Erioneuron pilsoum), red threeawn, purple threeawn, hooded windmillgrass, plains bristlegrass, Texas bristlegrass, silver bluestem, little bluestem, pink pappusgrass, and dogweed (Dyssodia tenuiloba).

Cenizo occurs only rarely north of central Live Oak County. Within this northeastern area, the association is represented by the guajillo/blackbrush-kidneywood community. South and west of central Live Oak County, the association is represented by the cenizo-guajillo/blackbrush community (Fig. 7). The two communities are similar except for the presence or absence of cenizo.

Blackbrush-twisted acacia Association

This association occurs on the upper slopes of caliche hills below the guajillo-cenizo association and on sites where caliche does not quite come to the surface. The soil depth on these sites generally varies from six to 40 centimeters. The association structure varies from dense stands almost entirely of blackbrush to open stands of shrubs one to two meters tall. There is almost no herbaceous understory development under the dense blackbrush stands to a fairly continuous cover of shortgrasses with scattered midgrasses in open stands. Fire is an important factor in maintaining the open nature of the lower elevations of this association. Where the grass cover is continuous, fire limits the spread of the shrubs. In the upper slope areas of the association, however, the grass cover is more discontinuous and fire is less important. Even on the lower slopes, soil moisture levels are not high enough to completely close the grass canopy and, therefore, the shrubs cannot be reduced below a certain level. On these shortgrass sites, discontinuities as narrow as harvester ant trails can stop fires if there is insufficient wind to carry the fire past. Overgrazing reduces the grass canopy and, therefore, favors the spread of shrubs up to the density that can be supported by the limited soil moisture.

[FIGURE 7 OMITTED]

The most common dominants within this association (Fig. 8) are blackbrush and twisted acacia (Acacia tortuosa). Common subdominants are prickly pear, Spanish dagger, guajillo, guayacan, mountain laurel, lotebush, allthorn (Koeberlinia spinosa), Texas kidneywood, Texas ebony, mesquite, and cenizo. Common understory species include agarito, leatherstem, hedgehog cactus (Ferocactus setispinus), curly mesquite, sideoats grama, Texas grama, pink pappusgrass, little bluestem, cane bluestem (Bothriochloa barbinodis), Arizona cottontop (Digitaria californica), trichloris, slim tridens, plains bristlegrass, Texas bristlegrass, hooded windmillgrass, red threeawn, purple threeawn, buffalograss, orange zexmenia, and dogweed.

[FIGURE 8 OMITTED]

There are three major communities within this association. The blackbrush-twisted acacia/prickly pear community is the typical community of the association and occurs on the mid-slope locations. The blackbrush/guajillo-prickly pear community forms the upper slope blackbrush thickets between the blackbrush-twisted acacia/prickly pear community and the guajillo-cenizo association. The structure of this community varies from a dense stand of almost pure blackbrush to a more open stand of blackbrush and guajillo, or blackbrush, guajillo, and cenizo. Prickly pear and guayacan are important subdominants within this community. The blackbrush-mesquite/twisted acacia community occurs on the lower slope locations dominated by the association, and is a common representative of the association on those relatively level sites where caliche or gypsum nears the surface. It is a more mesic community than the blackbrush-twisted acacia/prickly pear community, and is transitional to either the mesquite-granjeno association or the silver bluestem-trichloris/buffalograss community, depending upon the frequency of fire on the site. The mesquite are small, generally less than four meters high, and the blackbrush scattered to moderately dense. If the site is transitional to the shrubland, the subdominant shrubs become more dense than in the blackbrush-twisted acacia/prickly pear community. If the site is transitional to the grassland community, the subdominant shrubs become less dense and the midgrasses become more abundant.

Creosotebush-prickly pear Association

This is the most xeric association in South Texas. The hills of the Bordas Scarp are composed primarily of caliche. The hills along the Rio Grande north of Starr County, however, are gavelly, as are some outcrops further east. On these gravelly hill tops, the creosotebush-prickly pear association replaces the guajillo-cenizo association. The shrubland is open, with the dominants mostly one to two meters tall. The understory is discontinuous, mostly consisting of shortgrasses, semishrubs, and ephemerals, with scattered clumps of midgrasses in depressions. The dominants are cresotebush (Larrea divaricata) and prickly pear. The most common subdominants are lotebush, blackbrush, saltbrush (Atriplex canescens and A. matamorensis) allthorn, yucca, Spanish dagger, catclaw (Acacia greggii), guayacan, and cenizo.

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TERRY MCLENDON

Department of Range Science, Colorado State University, Fort Collins, Colorado 80523
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Author:McLendon, Terry
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Feb 1, 1991
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