Physiographic features

This site is located in the 81C, Eastern Edwards Plateau Major Land Resource Area (MLRA). It is classified as an upland site. It includes smooth areas of gently sloping topography ranging to moderately sloping areas in a benched landscape. In this “stair-step” position, the Shallow site occurs as narrow contour bands in complex with another site type, such as the Adobe (081CY355TX) site. Slopes are generally 1 to 5 percent, but range up to 8 percent in the benched situation. Elevation ranges from 600 to 2,300 feet.

Table 2. Representative physiographic features

Table 3. Representative physiographic features (actual ranges)

Climatic features

The climate is humid subtropical and is characterized by hot summers and relatively mild winters. The average first frost should occur around November 15 and the last freeze of the season should occur around March 19.

The average relative humidity in mid-afternoon is about 50 percent. Humidity is higher at night, and the average at dawn is about 80 percent. The sun shines 70 percent of the time possible during the summer and 50 percent in winter. The prevailing wind direction is southeast.

Drought is calculated as 75% below average rainfall. It should be noted that timing of rainfall may be more significant than average rainfall.

Approximately two-thirds of annual rainfall occurs during the April to September period. Rainfall during this period generally falls during thunderstorms, and fairly large amount of rain may fall in a short time. Hurricanes provide another source of extremely high rains in a short time. A review of the rainfall records suggest that rainfall is below “normal” at least 60 percent of the time. Therefore, the erratic nature of the rainfall should be considered when developing any land management plans.

The impact of droughts in the Edwards Plateau cannot be under-estimated. Not only are droughts devastating to the land but also to those that manage the land. Droughts occur roughly every 20 years but not always. A severe drought in 2012 coupled with extreme heat resulted in a die off of juniper over millions of acres as well as other native plants.

Table 4. Representative climatic features

Climate stations used

• (1) MEDINA 1NE [USC00415742], Medina, TX

• (2) SAN ANTONIO/SEAWORLD [USC00418169], San Antonio, TX

• (3) KERRVILLE 3 NNE [USC00414782], Kerrville, TX

• (4) BLANCO [USC00410832], Blanco, TX

• (5) CANYON DAM [USC00411429], Canyon Lake, TX

• (6) BURNET MUNI AP [USW00003999], Burnet, TX

• (7) AUSTIN GREAT HILLS [USC00410433], Austin, TX

• (8) GEORGETOWN LAKE [USC00413507], Georgetown, TX

• (9) PRADE RCH [USC00417232], Leakey, TX

Influencing water features

This being an upland site, it is not influenced by water from a wetland or stream. These upland site may shed some water via runoff during heavy rain events. The presence of good ground cover and deep rooted grasses can help facilitate infiltration and reduce sediment loss.

Wetland description

N/A

Soil features

In a representative profile for the Shallow ecological site, the soils are shallow, calcareous clays. The soil material is underlain by hard limestone at a depth of 8 to 20 inches. Limestone fragments are typically less than 15 percent by volume but may range up to 35 percent. These soils take in water readily when they have good cover of mid to tall grasses and plant residue. Surface crusting is common when plant cover and soil conditions deteriorate. Fractures in the limestone bedrock generally contain fine soil particles and store moisture. Plant roots penetrate these cracks and crevices, and thus have access to more moisture and plant nutrients than is apparent in the soil. The plants on this site are relatively unpalatable because of low phosphorus content which occurs because of the limited depth and high lime content.

It should be noted that this site may contain small inclusions of other soils/sites, such as Clay Loam or Low Stony Hill, that were not delineated separately because of mapping intensity. Field investigations and verification should always preclude any inventory efforts.

The following representative soils associated with the Shallow ecological site are Doss and Purves. These are the representative map units associated with the Shallow ecological site:

Doss silty clay, 1 to 5 percent slopes
Doss-Brackett association, undulating
Purves association, undulating

Table 5. Representative soil features

Animal community

This site is used for the production of domestic livestock and to provide habitat for native or exotic wildlife species. Cow-calf operations are the primary livestock enterprise although stocker cattle are also grazed. Sheep and goats were formerly raised in large numbers and are still present in reduced numbers. Sustainable stocking rates have declined drastically over the past 100 years because of the deterioration of the reference plant community. An assessment of vegetation useful to the management species is needed to determine correct stocking rates.

A large diversity of wildlife utilizes this site for at least a portion of their habitat needs. Historically, large migrating herds of bison, resident herds of pronghorn, white-tailed deer, and other species were prominent. Prairie chickens (Tympanuchus spp.) were even reported for some areas within the region. With the demise of these species and the changes in the plant community, the variety of wildlife has changed.

With the eradication of the screwworm fly (Cochilomyia hominivorax), the increase in woody vegetation, and insufficient natural predation, white-tailed deer numbers have increased drastically and are often in excess of carrying capacity. Where deer numbers are excessive, overbrowsing and overuse of preferred forbs and browse has caused the deterioration of the plant community. Progressive management of deer populations is needed to keep populations in balance and provide an economically important ranching enterprise. Achieving a balance between woodland and more open plant communities on this site is an important key to deer management. Because of the overlapping diet preferences, competition among deer, sheep, and goats can be an important consideration in livestock and wildlife management. Smaller mammals included many kinds of rodents, jackrabbit, cottontail rabbit, raccoon, skunk, opossum, and armadillo. Mammalian predators include coyote, red fox, gray fox, bobcat, and mountain lion. Many species of snakes and lizards are native to the site.

Many species of birds are found on this site including game birds, songbirds, and birds of prey. The major economically important game birds include the Rio Grande turkey, bobwhite quail, and mourning dove. Turkey prefer plant communities with substantial amounts of shrubs and trees interspersed with grassland. Quail prefer plant communities with a combination of low shrubs, bunchgrass, bare ground, and low successional forbs. The different species of songbirds vary in their habitat preferences. In general, a habitat that provides a large variety of grasses, forbs, shrubs, vines, and trees benefit the most species. This complex ecosystem of grassland, savannah, shrubland, and woodland will support a good variety and abundance of songbirds. Birds of prey are important to keep the numbers of rodents, rabbits, and snakes in balance; therefore, the different plant communities of the site will sustain different species of raptors. These raptors can prey on domestic animals as well.

Various kinds of exotic wildlife have been introduced on the site including aoudad sheep, blackbuck antelope, and axis, sika, fallow, and red deer. Their numbers should be managed in the same manner as livestock and white-tailed deer to prevent damage to the plant community. Feral hogs are present and can cause damage when their numbers are not managed.

Plant Preference by Animal Kind:
This rating system provides general guidance as to animal forage preference for plant species. It also indicates possible competition and diet overlap between kinds of herbivores. Grazing preference changes from time to time, especially between seasons, and between animal kinds and classes. An animal’s preference or avoidance of certain plants is learned over time through grazing experience and maternal learning (http://extension.usu.edu/behave/Grazing). Preference does not necessarily reflect the ecological status of the plant within the plant community. For wildlife, plant preferences for food are rated. Refer to detailed habitat guides for a more complete description of a species habitat needs.

Preferred (P) – Percentage of plant in animal diet is greater than it occurs on the land.
Desirable (D) – Percentage of plant in animal diet is similar to the percentage composition on the land.
Undesirable (U) – Percentage of plant in animal diet is less than it occurs on the land.
Not Consumed (N) – Plant would not be eaten under normal conditions; only consumed when other forages not available.
Used, but Degree of Utilization Unknown (X) – Percentage of plant in animal diet is unknown.
Toxic (T) – Rare occurrence in diet and, if consumed in any tangible amounts, can result in death or severe illness in animal.

Hydrological functions

The existing plant community with representative plant species, current soil conditions (soil health), current management, and climate determine the dynamics of the water cycle. The hydrology of this site is complex and still not fully understood. Plant and litter cover are important factors which protect the site from erosion; however, total production and particularly the types of plant species present have a greater impact on hydrologic dynamics (infiltration capacity, runoff, and soil losses). For example, cover values could be similar among the three plant states (transition diagram), but hydrology (infiltration capacity, water holding capacity, and runoff) will be different. The common denominator is not cover. Furthermore, the most important factors are the types of plants and their relative abundance; in other words, biomass is first and cover is second.

Another important factor is the structure and morphology of the root system associated with the plant species. The items associated with high hydrologic function are organic matter content, non-compacted soil surface (lower bulk density), intact soil structure, high porosity, high aggregate stability and the presence of soil biotic factors, such as earthworms, fungi, blue-green algae, and mosses (when moist).

State 1

With reference to the transitional pathway diagram, the Tall and Midgrass Prairie (reference community) is associated with the maximum hydrologic function. The high degree of hydrologic function in State 1 is because of the dominance of rhizomatous tall and midgrasses. As explained above, when properly managed, these species provide adequate cover. However, one of the keys to high hydrologic function is the structure or morphology of the root system. During high rainfall periods, water will percolate beyond the immediate surface root zone via fractures in the limestone. As this water moves downward, it contributes to the recharge of aquifers if the underlying soils and geology are appropriate.

When conditions are representative of a high composition of tall and midgrass species, little runoff occurs. For example: 50-year average = 35 inches rainfall, Johnson City; 1 inches runoff, 0.26 ton per acres soil loss. The exception is high-intensity thunderstorms. High-intensity storms will produce some runoff; however soil loss will still be less than 1 ton per acre. Rangeland hydrology research and models, such as the Rangeland Hydrology and Erosion Model, reveals that precipitation, runoff, and erosion increases as the storm return period increases with 2-year, 5-year, 10-year, 25-year, or 50-year storms.

Return period analysis based on 50 years of climate:

Storm Return Period Precipitation (in) Runoff (in) Erosion t/ac
Average 50 years 34.1 1.1 0.3
5 years 39.8 1.9 0.5
10 years 43.1 2.5 0.7
25 years 49.7 3.6 0.9
50 years 51.8 7.3 0.9

Based on 50 years of climate data, there is a 96 percent chance there will be a runoff and delivered sediment for these conditions. (Rangeland Hydrology and Erosion Model Predictions—model calibrated from field data).

Return Period Analysis
To help interpret the table, note that a 5-year value will be exceeded, on the average, about once every 5 years, or twice every 10 years. There is a one in five, or 20 percent, chance that a value equal to or greater than the 5-year value will occur in a given year. There is a (100 - 20), or 80 percent, chance that the precipitation, runoff, erosion, or sediment yield will be less than the 5-year value. In the results shown in the table, there is a 20 percent chance that the annual erosion will exceed about 0.5 ton per acre. At best, any predicted runoff or erosion value, by any model, will be within plus or minus 50 percent of the true value. Erosion rates are highly variable.

State 2

Improper grazing management starts the chain reaction causing loss or reduction of the rhizomatous tall and midgrasses. Prolonged improper grazing management, no brush management, no prescribed fire, and the introduction of invasive species results in impaired hydrologic function. During the transition phase from State 1 to State 2: infiltration decreases, runoff increases, and soil loss may begin because of shifts in grass composition from tall and midgrasses species to short species (some bunch and sod formers), higher percentages of bare ground, rainfall splash, soil compaction and capping, loss of organic matter, and deterioration in structure. Hydrologic conditions worsen with continued improper management.

Return period analysis based on 50 years of climate:

Storm Return Period Precipitation (in) Runoff (in) Erosion t/ac
Average 50 years 34.1 3.2 1.7
5 years 39.8 4.3 2.8
10 years 43.1 5.6 3.2
25 years 49.7 7.9 3.5
50 years 51.8 11.6 3.6

Based on 50 years of climate data, there is a 100 percent chance there will be runoff, erosion, and delivered sediment for these conditions. (Rangeland Hydrology and Erosion Model Predictions—model calibrated from field data).

With a combination of increasing bare ground, shorter cover, less leaf area, and high-intensity storms; this site can contribute to an increased frequency and severity of flooding within a watershed.

State 3

This state is representative of the Juniper Shrubland Community dominated by woody plants. Understory species become increasingly sparse because of shading and competition from woody plants. As Ashe juniper becomes mature (10 to 20 feet tall) and density increases, bare ground increases. Research has shown that hydrologic function in the interspace area between shrubs deteriorates. Vegetative ground cover decreases as woody canopy increases.

Return period analysis based on 50 years of climate:

Storm Return Period Precipitation (in) Runoff (in) Erosion t/ac
Average 50 years 34.1 3.8 3.7
5 years 39.8 5.5 5.4
10 years 43.1 6.8 6.7
25 years 49.7 8.3 8.3
50 years 51.8 13.1 8.5

Based on 50 years of climate data, there is a 100 percent chance there will be runoff, erosion, and delivered sediment for these conditions. (Rangeland Hydrology and Erosion Model Predictions—model calibrated from field data).

As the site becomes dominated by woody species, especially oaks and juniper, the water cycle is further altered. Interception of rainfall by tree canopies is increased which reduces the amount of rainfall reaching the surface. Interception can be as great as 10 to 46 percent. Stem flow is increased by the funneling effect of the canopy resulting in increased soil moisture at the base of the tree.

Juniper litter can be an advantage and a disadvantage Even though litter is water repellent; the litter can store water and conserve moisture in the upper soil horizon. In some cases though, the water repellency can result in an increased runoff in the interspace areas. Excess litter also holds moisture like a sponge and prevents it from moving downward into the soil profile. This impact is worsened when rainfall comes in small amounts. Juniper litter has a decreased decomposition rate and enters the carbon soil pool more slowly.

When evergreen species such as Texas live oak and juniper dominate, their transpiration reduces the chance for deep percolation, especially during a drought situation. Juniper and oak roots are coarser and can reach deeper into soils with stones and crevices than grasses with their finer roots. This gives them access to water for survival at the expense of the grasses and a competitive advantage.

It should be noted that not all sites are recharge sites. Recharge is highly dependent upon porous geology. Some research reveals that the reductions in evapotranspiration from tree and shrub removal are offset by transpiration from the herbaceous plants that replace the shrubs and by evaporation from newly exposed soil. In these cases, it becomes a management decision as to which plants are best suited to use the available soil moisture for the desired use.

Because of the very high shrink-swell clay soil and the formation of surface cracks in dry periods, rainfall infiltration can still occur even when ground cover is poor. With a combination of a sparse ground cover and intensive rainfall, this site can contribute to an increased frequency and severity of flooding within a watershed. Soil erosion is accelerated, quality of surface runoff is poor, and sedimentation increased.

Brush management combined with good grazing management can help restore the natural hydrology of the site. However a threshold may have been crossed, if the soil surface conditions have deteriorated because of excessive overland flow or runoff, loss of organic matter, and excessive erosion. The return to State 1 hydrology depends on the length of time and severity of soil loss in State 3. Therefore, restoration to State 1 hydrology may not be possible or realistic.

If a mature woodland canopy develops, a buildup of leaf litter occurs which increases the organic matter of the soil, builds structure, improves infiltration, and retards erosion. Some values of a properly functioning water cycle are restored on this site when a woodland plant community persists. Unfortunately, not all factors will ever return to the original condition.

Recreational uses

This site has the appeal of the wide open spaces. The abundant tall and mid grasses and scattered oaks produce beautiful fall color variations. The area is also used for hunting, birding and other eco-tourism related enterprises.

Wood products

Honey mesquite and oaks can be used for firewood and the specialty wood industry.

Inventory data references

The data contained in this document is derived from analysis of inventories, clipping studies, and ecological interpretation from field evaluations.

Other references

Anderson, J.R., C.A. Taylor, Jr., C.J. Owens, J.R. Jackson, D.K. Steele, and R. Brantley. 2013. Using experience and supplementation to increase juniper consumption by three different breeds of sheep. Rangeland Ecol. Management. 66:204-208. March.

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Bestelmeyer, B.T., J.R. Brown, K.M. Havsted, R. Alexander, G. Chavez, and J.E. Hedrick. 2003. Development and use of state-and-transition models for rangelands. Journal of Range Management. 56(2): 114-126.

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http://extension.usu.edu/behave/ (Accessed 6/6/2013)

Contributors

Carl Englerth
Dr. Kenneth Spaeth, RMS, NRCS, Fort Worth, Texas
Joe Franklin
Mark Moseley
Edits by Travis Waiser, MLRA Leader, NRCS, Kerrville, TX

Approval

Bryan Christensen, 9/19/2023

Acknowledgments

Reviewers and Contributors:
Joe Franklin, RMS, NRCS, San Angelo, Texas
Ryan McClintock, Biologist, San Angelo, Texas
Bryan Hummel, Natural Resources Technician, Joint Base San Antonio–Camp Bullis, Texas
Mark Moseley, ESS, NRCS, Boerne, Texas
Ann Graham, Editor, NRCS, Temple, Texas
Jessica Jobe, Project Leader, NRCS, Kerrville, Texas
Travis Waiser, Soil Scientist, NRCS, Kerrville, Texas

Site Development and Testing Plan:

Future work, as described in a Project Plan, to validate the information in this Provisional Ecological Site Description is needed. This will include field activities to collect low, medium and high intensity sampling, soil correlations, and analysis of that data. Annual field reviews should be done by soil scientists and vegetation specialists. A final field review, peer review, quality control, and quality assurance reviews of the ESD will be needed to produce the final document. Annual reviews of the Project Plan are to be conducted by the Ecological Site Technical Team.

QC/QA completed by:
Bryan Christensen, SRESS, NRCS, Temple, TX
Erin Hourihan, ESDQS, NRCS, Temple, TX