Ecological dynamics

The Reference Plant Community consists of mid and shortgrasses with few tallgrasses. Some perennial forbs are present with small numbers of annual forbs and a few scattered woody shrubs. Productivity is moderate with most of the production coming from sideoats grama (Bouteloua curtipendula) and blue grama (Bouteloua gracilis). Lesser amounts of buffalograss (Bouteloua dactyloides), hairy grama (Bouteloua hirsuta), sand dropseed (Sporobolus cryptandrus), and perennial three-awn (Aristida wrightii) are found on the site. Vine mesquite (Panicum obtusum) and western wheatgrass (Pascopyrum smithii) are found growing in depressional areas. Small pockets of sand bluestem (Andropogon hallii) and Indiangrass (Sorghastrum nutans) may be found scattered throughout the site. Little bluestem (Schizachyrium scoparium) will occur in small amounts where the soil becomes shallower. The more commonly found forbs are dotted gayfeather (Liatris punctata), scarlet globemallow (Sphaeralcea coccinea), Engelmann’s daisy (Engelmannia peristenia), baby white aster (Chaetopappa ericoides), halfshrub sundrop (Calyophus serrulatus), trailing ratany (Krameria lanceolata) and annual forbs. The primary woody species found are yucca (Yucca glauca) and broom snakeweed (Gutierrezia sarothrae), with an occasional catclaw mimosa (Mimosa aculeaticarpa var. biuncifera) and plains pricklypear (Opuntia polycantha); however, trees are seldom found on this site.

The site occurs on slightly to moderately sloping areas on upland plains where some small amount of geologic erosion may have occurred and the soils are somewhat “thinner” than those of the associated Deep Hardland ecological site that occurs on the more level terrain. Higher calcium carbonate content throughout the soil profile accounts for the amount of sideoats grama growing on this site. This differs from the closely associated Deep Hardland site that is dominated by blue grama. The forb component is more apparent in years of above average rainfall. Pronghorn favor this site because of the variety of forbs present. Cryptogamic crusts are more common on this site than on nearby Deep Hardland sites. Production on this site is quite close to that of the Deep Hardland sites. The two main indicator plants on the Limy Upland ecological site are sideoats grama and yucca. Yucca has a tendency to increase on limy upland sites that have had regular spring and early summer deferment for many years with good yucca seed production. Yucca blooms are very palatable to deer, pronghorn and cattle.

Fire played a role in the ecology of this site as well as all other high plains sites. The general role of fire was to sustain natural grassland and suppress shrubby species. Fire helped to keep a balance between the grasses, forbs and shrubs. However, in the shortgrass region, fire was probably secondary to climate in promoting the historic vegetative state. A drier climate (<20 inches annual precipitation) creates a situation where the subsoil is dry more often than it is wet. Plant roots grow in response to moisture and this dryer climate favors shortgrasses with fibrous root systems or short rhizomatous grasses. Yucca is a major increaser on this site and natural fire no doubt kept yucca suppressed significantly. Annual forbs are stimulated by fire and diversity is generally increased. Heavy grazing after a fire can have a negative effect if conditions are dry and remain so for an extended period.

Periodic grazing and trampling by migrating herds of bison and elk as well as resident herds of pronghorn antelope occurred during drought periods. Bison moved about in large herds over the region somewhat regulated by water sources and fire frequency. However, long rest periods followed once the large herds of bison moved out of the area, allowing the resilient grassland to re-establish and maintain its reference community structure.

Variations in climatic factors, especially the amount and timing of precipitation, greatly influence the productivity of ecological sites and are largely responsible for the fluctuations in the amount of vegetative growth from one season to the next. It is not unusual for fluctuations of greater than 50 percent to occur from one year to another. These types of climatic variation are part of the overall environment in which the reference plant community developed. However, it needs to be pointed out that long-term drought (4 to 6 years of rainfall 50 percent below the mean) can act in concert with other forces to affect changes in plant communities. For instance, extended drought weakens plants and makes them more susceptible to the effects of overgrazing. Drought conditions coupled with fire can be damaging and need long periods of time to fully recover. Extremely dry summers followed by wet winters can favor cool-season annual grasses at the expense of perennial warm-season species. A well-adapted, healthy community could better withstand such rigors of drought. However, even they experience damage that would result in some departure from the former stable state. Usually, the departure would be temporary.

When domestic livestock were brought to the plains in the 1870’s, it was largely an open range situation. By 1890, however, most of the area had been fenced and livestock were confined to theses areas continually.

The major forces influencing the transition to the Shrub/Shortgrass community is continued over-grazing by livestock and the decrease in the frequency and intensity of fire. As livestock and wildlife numbers increase and grazing use exceeds a plants ability to sustain defoliation, the more palatable and generally more productive species decline in stature, productivity and density. Early day settlers often had little information upon which to base stocking rates. In many cases, more animals were grazed than the grassland resources could safely support. The tendency of this site is to become a shortgrass dominant site if long-term grazing abuse occurs. This will lead to a decline in the vigor of sideoats grama and other desirable mid and tallgrass species. Blue grama and buffalograss will increase because they are better able to withstand grazing pressure. With constant grazing pressure, the blue grama will eventually become sod bound and lose its bunch grass appearance.

Yucca will increase on the site if the grass cover is weakened and the yucca makes seed for several years. With the weakened grass cover, broom snakeweed will often gain a major foothold on the site. On some of the western portions of MLRA 77C, cholla cactus (Cylindropuntia imbricata) has increased on some deep hardland and limy upland sites. The decrease in density and stature of the mid and tallgrasses, an increase in shortgrasses, and an increase of yucca and other woody vegetation brings about a new plant community, the Midgrass/Shortgrass/Shrub Community (1.2).

In the Midgrass/Shortgrass/Shrub Community (1.2), the transition back to the reference community is possible with proper grazing management and chemical brush and pest management. Prescribed burning could be used if the conditions allow. The production of vegetation has shifted from mostly herbaceous vegetation to increasing amounts of woody shrubs. Herbaceous vegetation is still the largest production in this phase. Nutrient cycling, the water cycle, watershed protection and biological functions have changed little.

If heavy grazing continues with no form of brush and pest management, a threshold will be crossed to a Shortgrass/Shrub Community (2.1). In this state, typical vegetation will be low vigor, blue grama with increasing amounts of low quality shortgrasses. Bare areas will increase with annuals filling the voids. Perennial three-awn will invade this site when the more desirable grasses are weakened and/or removed. Yucca, and occasionally broom snakeweed, will increase dramatically. Nutrient cycling, the water cycle, watershed protection and biological functions have been severely reduced. The plant community is so degraded that it cannot reverse retrogression without extensive energy and management inputs. Restoration of the Shortgrass/Shrub Community (2.1) will require prescribed grazing with rest periods during the growing season, re-seeding bare areas with adapted native grass species, chemical and/or mechanical brush management, and some form of pest management. With the reduced amounts of grass fuel, prescribed burning is usually not an option in this phase.

When long-term, abusive grazing occurs, this site will regress to the Shrub/ Shortgrass/Annuals Community (3.1). In this degraded state, yucca and broom snakeweed will dominate the site (>50 percent). Typical herbaceous vegetation will be perennial three-awn, low quality shortgrasses, and low vigor, sod bound blue grama. The large, connected bare areas will have numerous annual species present. The loss of herbaceous cover and increased bare ground encourages accelerated erosion. Nutrient cycling, the water cycle, watershed protection and biological functions are not functioning well in this phase. Restoration of the Shortgrass/Annuals Community (3.1) to reference conditions will require major energy, economic and management inputs. Conservation practices required include prescribed grazing with several consecutive (3-4 years) rest periods during the growing season, re-seeding bare areas with adapted native grass species, and chemical brush and pest management. Prescribed burning is not an option in this phase. Full recovery and maintenance of the reference community requires continued proper grazing management as well as occasional brush and pest management.

NOTE: Rangeland Health Reference Worksheets have been posted for this site on the Texas NRCS website (www.tx.nrcs.usda.gov) in Section II of the eFOTG under (F) Ecological Site Descriptions.

STATE AND TRANSITIONAL PATHWAYS: (DIAGRAM)
The following diagram suggests some pathways that the vegetation on this site might take in response to various treatment or natural stimuli over time. There may be other states not shown on the diagram. Those shown are some of the most commonly seen. This information is intended to illustrate the changes in vegetative states that can occur in a given set of circumstances, and may not happen this way in all cases. Local professional guidance should be sought when making plans to manipulate plant communities for specific purposes.

As a site changes in the structure and makeup of the plant community, the changes may be due to management or due to natural occurrences or both. At some point in time thresholds are crossed. Once changes have progressed to a certain point, the balance of the community has been altered to the extent that a return to the former state is generally not possible. Some form of energy must be applied in order to make the community respond in that direction. These changes in plant communities occur on all ecological sites with some sites being more resistant to change than others. Also, some sites seem to be more resilient being able to heal or restore more easily than other sites. Usually, changes in management practices alone, such as different grazing methods, will not result in restoration of former vegetative states. An example of an energy input that might be necessary to effect change might be the implementation of chemical brush management and complete growing season rest to reduce domination of woody shrubs and promote more perennial grasses and forbs. This might have to be done more than once and could take several years. Such a vegetative shift could not be accomplished by regulation of grazing alone. The amount of energy required to effect a change would depend on the present vegetative state and the desired state.