M. KEITH OWENS, Research Scientist, Texas Agricultural Experiment Station, Uvalde.
Abstract: Some 21 million acres of south Texas are dominated by brush. The changes in the vegetation are usually attributed to increased grazing pressure by domestic livestock, the suppression of natural fires to protect man-made improvements, and possibly from a shifts in climatic factors. Once shrub communities are established on a site, it is extremely unlikely that the site will convert to a grassland without sustained cultural inputs. Simply adding back the factors missing in savanna maintenance (increased fire frequency and decreased grazing pressure) are unlikely to cause a shift in the current vegetation. The long-lived shrubs and trees on south Texas rangelands have reached a “steady-state” where managers must take advantage of all the available tools to manage shrublands for domestic livestock and wildlife.
South Texas rangelands are aptly named the Brush Country. Almost 21 million acres of south Texas is now dominated by a mixture of mesquite, Acacias, hackberry, and other shrubs. An additional 25 million acres in northern Mexico are also dominated by shrubs. The prevalence of shrubs in this huge area is basically because they are the best adapted plant form for the soils, precipitation patterns, and land management practices. However, that is not to say that the particular species of shrub and that the spatial pattern of shrubs cannot be managed, but we must remember that shrubs always have been, and always will be, present in our plant communities. In order to understand the ecology of the region, we must understand the environmental constraints of the area, what vegetation may have existed here in the past, how it changed to the vegetation we have today, what are the prevalent plant species, and where will they be found on the landscape.
Environmental constraints
In south Texas, the mixed brush country is bordered by the Edwards Plateau (Hill Country) on the northern edge, by the Coastal Plains on the eastern edge, and by the Rio Grande on the southern and western edge (Hatch et al. 1990). Annual rainfall in the area varies from 30 inches on the east side to less than 18 inches on the west side. The most constant factor concerning annual rainfall is its extreme variability. Annual rainfall in the Uvalde area has ranged from a low of less than 10 inches during the drought of the 1950s to a high of over 42 inches in 1979. The variability is most evident when we consider the pattern of precipitation within a year. During 1979, an extremely wet year, the month of October was completely dry, while over 17.5 inches of rain fell in November. This month-to-month variability reinforces the view that “south Texas is a land of continual drought interrupted by periodic flooding“.
The other most dependable feature of the mixed brush environment is the heat. Although the Chamber of Commerce may advertise an average July air temperature of 85oF, daily high temperatures are often over 100oF for most of the summer. These high summer air temperatures contribute to extremely high soil temperatures. We have measured soil temperatures at a 1-inch depth as high as 150oF in the interspaces between mesquite trees. At the same time, soil temperatures under the trees reached a maximum of about 83oF. Since many seeds perish at temperatures above 130oF, the interspaces between trees can be an inhospitable site for plant establishment. Winter temperatures, on the other hand, are quite mild with only a few days below 32oF. The average frost-free period ranges from 260 to 340 days over the region and in some years, a killing frost does not occur. This is especially important in the establishment and growth of neo-tropical plant species such as leadtree (Leucaena spp.).
The type of soils in south Texas depends largely upon landscape position. The general topography can be best described as flat to gently rolling with elevation ranging from sea-level to 1000 feet. Relatively deep, fertile soils are found along the many river and creek bottoms. Silty-loams and clay-loams are the dominant soils found in the broad flat areas and low stony hills are dominated by a shallow calcareous soil with a hard pan at 1.5 to 3 feet.
Historic vegetation
The vegetation of south Texas has been shaped by numerous factors during the last 200 years. The changes in the vegetation are usually attributed to increased grazing pressure by domestic livestock and the suppression of natural fires to protect man-made improvements. More recently, the change in carbon dioxide concentration in the atmosphere has been implicated in the widespread change from grasslands to shrublands.
It is difficult to say with certainty what the vegetation looked like prior to settlement by Europeans. We depend on the accounts of early explorers and travelers to depict how trees, shrubs and grasses were distributed. A traveler in 1580 reported “an open country, with plains and few dense woods…short pasture…with large, and dense brush tickets” (Hanselka and Kilgore 1987). Nearly 3 centuries later, Michler reported in 1849:
” As a general thing… Concerning land both on the Frio and Leona, from the rivers back, that it may be divided into 4 parallel strips. – the first, next to the river, consisting of heavy timber, and a heavy, black soil; the second, a mezquite (sic) flat, of small width, and the soil of a lighter nature and very fertile; the third, a range of low hills covered with loose stones and thick chaparral; the fourth, a wide open prairie… Each of these strips is distinct, and parallel to the general course of the river.” (Quoted from Weniger 1984).
From these passages, and others who wrote similar descriptions, we can conclude that the shrubs that are present today were probably present before Europeans moved into the area. The distribution of the shrubs has changed considerably though. It would be difficult, if not impossible, to find a “wide open prairie” in south Texas now.
By the early 1900’s, the Brush Country had changed from a grassland savanna to shrublands. An early resident on the Rio Grande Plains later wrote about his childhood and described the Rio Grande Plains as follows:
“For this is hard country, brush country, mean country, heart-break country. Ugly in summer, drought-stricken, dusty, glaring, but in winter it is hideous. In winter the bare trees, granhena and mesquite and huisache that the rest of the seasons have certain grace, these are like a dead orchard…. The brush is like twisted debris from a hurricane, it is like a bad dream. The nap is up, the whole country seems to have been rubbed the wrong way. The brush lies all around like a dry jungle creeping.” Allen (1977, quoted in Scifres and Hamilton 1993).
What may have changed the shape of the vegetation in recent history? Certainly continuous heavy grazing was a factor. Domestic livestock were brought into the region with the missions and early settlers. In the 1760s the San Antonio missions alone were grazing 5,000 cattle and 17,000 sheep and goats in the area around San Antonio. By 1768, an additional 16,000 cattle were pastured around the Goliad mission. Since the country to the northwest of San Antonio was inhabited by first the Apache and then the Comanche Indians, it is doubtful that much grazing took place in those areas (Weniger 1984). Most of these animals probably grazed in the south Texas plains. By 1854, early travelers were recording the effects of domestic livestock when they wrote “prairies were close-cropped, and we passed many carcasses of cattle that had miserably perished by … starvation”. The number of domestic livestock on Texas rangelands at that time (about 1850) was estimated at 240,000 head of cattle, and an untold number of sheep. This number continued to increase until the heyday of the western cowboy during the 1880s.
Another factor influencing the establishment and spread of shrubs has been the suppression of fires. Although some authors think natural fires were not a significant ecological factor (e.g. Weniger 1984), many others will argue that point (Scifres 1980, Scifres and Hamilton 1993, Wright and Bailey 1982). Fires started from 2 main sources: lightning and humans. Lightning-caused fires were most likely started during hot, dry summers, or perhaps during any period of stress such as a drought. As Scifres and Hamilton (1993) noted, this is not the same time as when humans would start fires for management. Cool-season fires, when plants are dormant, are generally easier to control and less threatening to man-made improvements. This alteration of the seasonality of fires can have great ramifications on the plant community, as described in another chapter of this symposium. At the same time as fire seasonality was changing, fire frequency was also changing. The amount of fine fuel available for fires was reduced due to grazing and drought which lead to infrequent fires. Small woody plants had a chance to become established, and after establishment, resprouting shrubs are difficult to control with fire alone.
A third factor which would promote shrub and tree establishment would be changes in the local climate. Increasing levels of atmospheric CO2 have been recorded since the advent of the Industrial Age. As CO2 has increased by 30% (from 280 to 350 ppm) in the last century, shrubs have increased on semiarid rangelands. A possible explanation for this is the different photosynthetic pathways of C3 shrubs and C4 grasses. Basically, increasing atmospheric CO2 favors the C3 photosynthetic pathway which results in the shrubs becoming more efficient competitors with the C4 grasses. For a detailed explanation of this hypothesis see Polley (1997). Changes in the local climate can have a more direct effect with a much shorter time-frame. Archer (1994) lists a series of climatic factors which would promote the change of savanna grasslands to shrublands:
1. Increasing rainfall – Increasing rainfall would promote the establishment and survival of shrubs. If fire frequency is reduced, or fires are eliminated altogether, then small shrubs would be more likely to establish and grow. This scenario is certainly evident in the “episodic establishment” of seedlings so widely reported for semiarid and arid rangelands.
2. Periodic drought – Droughts are common in south Texas and the most susceptible plant form are the grasses. Most shrubs are tolerant of drought (e.g. paloverde which can drop all of its’ leaves and photosynthesize in the stems) and will survive short-term droughts. As the grasses die, gaps are formed in the vegetation mosaic which can then be colonized by shrubs.
3. Shifts in the seasonality of precipitation – Increased precipitation during cool winter months favors the C3 shrubs while increased summer moisture would favor the C4 grasses.
4. Increased temperature – Increased air temperatures increase soil temperatures, which can limit seed and seedling survival in interspaces between trees.
Although each of these 3 factors (i.e., grazing, fire, and climate) can, and probably did, affect the rangelands of south Texas, the changes we see today are most likely a result of the interaction between all 3 factors. For instance, Archer (1994) points out the following scenario: when rainfall increases during a particularly wet year, shrub seedling emergence and survival also increases due to the favorable growing conditions. Mature grass plants already present on the site would also respond and increase their growth resulting in more herbaceous biomass. This increased biomass would serve as fine fuel and the likelihood of a widespread fire would increase. The fire would the kill the small shrubs and trees before they developed an extensive crown for resprouting. If the fine fuel was decreased – through grazing – then the widespread fires would not occur and the chance of the shrubs successfully establishing would increase.
Invasion process
There are certain conditions which must be met for shrubs to successfully invade grasslands. There must be an adequate supply of viable seed, that seed must arrive in a “safe site” for germination, and the seedling must survive for at least 1 growing season. Some of the shrub/tree seeds from south Texas are extremely long-lived while others are very ephemeral. Mesquite seed can potentially survive in the soil for decades but guajillo seed has a short life in the seed bank (Owens et al. 1995).
A “safe site” is simply an area which has an increased probability of seed survival and seedling emergence. The importance of surviving the first growing season is evident because research has shown that if shrub seedlings of some species (e.g. mesquite) can reach that age, then they have a high probability of surviving to maturity (Brown and Archer 1989).
The process of shrublands invading into grasslands has been studied at the La Copita Research Area, near Alice, TX (Archer 1995). Through a long series of research experiments, Archer and colleagues have documented the following steps towards establishing a mixed brush community:
1. A mesquite tree becomes established in the grassland. Mesquite seeds are dispersed mainly by animals. The pod surrounding the seed is high in sugar and is very palatable. After the pod is eaten and digested, the hard seeds remain in the gut of the animal. Stomach or rumen acids scarify the hard seed coat and then the seed is excreted with feces. Prior to the widespread introduction of domestic livestock, seed dispersal of mesquite seeds was dependent on native wildlife. Wildlife densities were low enough that mesquite seeds were not spread across the entire landscape, but only in areas frequented by wildlife. Wildlife, including white-tailed deer, coyotes, and raccoons, can ingest and distribute mesquite seeds, but not quite as efficiently as domestic livestock.
2. The mesquite tree matures and provides habitat for other shrubs. As the mesquite tree grows, it provides perches for birds within the grassland. Seed-eating birds forage within the wooded riparian areas and disperse seeds beneath the mesquite tree. The tree canopy modifies the environment by sequestering nutrients, decreasing temperatures, and altering the hydrologic cycle. These changes facilitate the establishment of other shrubs.
3. Interspecific competition increases. In some instances, the density of desirable grass plants decreases under mesquite canopies due to competition (Archer 1995), but in other situations, the mesquite canopy provides a grazing and environmental refugia for these grasses (pers. obs.). There is typically very little or no establishment of new mesquite seedlings under the canopy of the mature plants.
4. The mature mesquite tree dies. Eventually the mesquite tree dies and the undercanopy shrubs become the dominant plants. The mixed brush community is now firmly established on the site.
Major plant species
The many shrub species which grow in south Texas can be characterized into several classes, or functional types, based on their preferred landscape location and their “value” as forage or cover species. Shrubs can be classified as occurring predominantly on shallow ridges, on clay-loam flats, or in bottomlands. Many shrubs provide a valuable forage base for livestock and wildlife, while other shrubs are neutral, and still others are poisonous or have anti-quality compounds. Some shrubs that have little value for forage provide critical cover habitat for widlife. For instance, knifeleaf condalia (Condalia spathulata) does not provide any forage but provides cover for northern bobwhites (Colinus virginianus). The following discussion will start by separating plants into their typical landscape locations and then discuss their nutritional value within that landscape.
Shallow ridge sites. These sites are widespread throughout south Texas and are characterized by very shallow soils, dense brush cover and very little herbaceous understory. The most common shrubs on these sites are guajillo (Acacia berlandieri), blackbrush acacia (A. rigidula), ceniza (Leucophyllum frutescens), and shrubby bluesage (Salvia ballotiflora). These 4 shrubs typically comprise more than 80 percent of the total shrub cover. Another important shrub, while not plentiful is highly preferred by white-tailed deer (Odocoileus virginianus), is Texas kidneywood (Eysenhardtia texana). Guajillo and blackbrush acacia are considered desirable browse plants, shrubby bluesage is grazed occasionally, and ceniza is grazed infrequently. In one study on the Lyles Ranch near Uvalde, we found that shrubby bluesage comprised up to 20% of deer diets during the winter (Hervey 1989) but acted as a barrier to foraging for both Spanish goats and white-tailed deer during the summer (Etzenhouser et al. 1997). A consistent observation from both studies is that ceniza acts as a barrier to foraging and is infrequently grazed. Guajillo and blackbrush acacia may comprise up to 90% of the total deer and goat diet for certain periods of the year (Hervey 1989, Etzenhouser 1997). Both of these species have been considered desirable shrubs because they both have crude protein concentrations of >20% (Varner and Blankenship 1985). More recent research has shown that the majority of this nitrogen is actually tied up in secondary compounds such as tyramine and phenethylamine which are toxic in large doses and may impair reproductive function in small doses (Forbes et al. 1996). The actual amount of shrub which must be ingested before poisoning the animal is not known.
Clay-loam flats. The most dominant shrub or tree on these sites is mesquite (Prosopis glandulosa). Other common shrubs include spiny hackberry (Celtis pallida), whitebrush (Aloysia gratissima), guajillo, twisted acacia (A. schaffneri), brazil (Condalia hookeri), and javelina bush (C. ericoides). There are many more shrub species in these sites, but they are usually present in only small amounts. Whitebrush functions as a barrier to foraging, with little or no forage value. In low-lying areas, whitebrush may actually form impenetrable thickets which are used by feral hog populations. Mesquite leaves are waxy and are not desired forage. Only the mesquite pod is highly preferred forage, with all of the problems discussed previously. Spiny hackberry, brazil, and javelina bush are all palatable shrubs which are used by white-tailed deer.
Bottomlands. Mature mesquite, sugar hackberry (Celtis laevigata), live oak (Quercus virginiana) and sometimes eastern cottonwood (Populus deltoides) grow along the riverbottoms and in areas with extra moisture. Although some of these plants provide forage (e.g. acorns from oaks), their scarcity precludes them from being major components of animal diets.
Conclusions
South Texas rangelands have evolved over time from grassland savannas to mixed brush thickets. Original grasslands were probably maintained by a low grazing intensity and natural wildfires. Shifts in local climatic patterns, coupled with the changes in fire and grazing frequency, resulted in a different plant community. Native shrubs, which were limited in their distribution, gained a toehold on grasslands and expanded their range and density rapidly.
Once shrub communities are established on a site, it is extremely unlikely that the site will convert to a grassland without sustained cultural inputs. Simply adding back the factors missing in savanna maintenance (increased fire frequency and decreased grazing pressure) are unlikely to cause a shift in the current vegetation. The long-lived shrubs and trees on south Texas rangelands have reached a “steady-state” where managers must take advantage of all the available tools to manage shrublands for domestic livestock and wildlife.
Literature Cited
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Brown, J. R. and S. Archer. 1989. Woody plant invasion of grasslands: establishment of honey mesquite (Prosopis glandulosa var. glandulosa) on sites differing in herbaceous biomass and grazing history. Oecologia 80: 19-26.
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Comments: Dale Rollins, Professor and Extension Wildlife SpecialistUpdated: Mar. 18, 1997