Ecological dynamics

The soils, topographic location, climate, periodic droughts and fire influenced the stabilization of the reference plant community on this site as was the case on most all High Plains ecological sites. Grazing and/or browsing by local and nomadic wildlife influenced the plant community as well. The resulting plant community was a Midgrass/Tallgrass Dominant Community (1.1). Midgrasses tend to dominate over most of the site with sideoats grama (Bouteloua curtipendula) being the overall dominant species, lesser amounts of vine mesquite (Panicum obtusum), Arizona cottontop (Digitaria californica), plains bristlegrass (Setaria leucopila), hooded windmillgrass (Chloris cucullata), sand dropseed (Sporobolus cryptandrus), tumble windmillgrass (Chloris verticillata), silver bluestem (Bothriochloa laguroides), hairy grama (Bouteloua hirsuta), fall witchgrass (Digitaria cognata), gummy lovegrass (Eragrostis curtipedicellata), black grama (Bouteloua eriopoda), and sand muhly (Muhlenbergia arenicola). The dominant tallgrass is little bluestem (Schizachyrium scoparium) with lesser amounts of Indiangrass (Sorghastrum nutans) and sand bluestem (Andropogon hallii). In areas where tighter soils occur, the shortgrasses include blue grama (Bouteloua gracilis) and buffalograss (Bouteloua dactyloides). Some cool-season grasses occur in small amounts such as western wheatgrass (Pascopyrum smithii), Canada wildrye (Elymus canadensis) and needle & thread (Hesperostipa comata). Typically associated forbs included dotted gayfeather (Liatris punctata), prairie clover (Dalea purpurea), catclaw sensitivebriar (Schrankia uncinata), golden dalea (Dalea aurea), gaura (Gaura spp.), rushpea (Hoffmanseggia glauca), Engelmanndaisy (Engelmannia peristenia), lyreleaf greeneyes (Berlandiera lyrata), sagewort (Artemisia ludoviciana), scarlet globemallow (Sphaeralcea coccinea), Fendler’s penstemon (Penstemon fendleri), wild alfalfa (Psoralidium tenuiflorum) and numerous annual forbs. Woody species include sand sagebrush (Artemesia filifolia), yucca (Yucca glauca), and catclaw mimosa (Mimosa biuncifera). Nutrient cycling, the water cycle, watershed protection and biological functions are functioning at their peak.

Natural fire likely played an important role in the function of most plains sites, especially the tall grass communities. Tall grasses such as sand bluestem and little bluestem were dependent upon fire to stimulate them and remove old growth that would accumulate on the soil surface. Fire also kept shrubs from getting too thick. Fire helped to keep a balance between the grasses, forbs and shrubs. Wildlife habitat was improved by opening up canopies and stimulating forb growth. The deep rooted species that grow on the site are not easily damaged by fire. Yucca and shrubs will usually re-sprout, but are suppressed for a time allowing grasses to dominate. If periodic fire does not occur, then the yucca and woody plants will slowly increase and with grazing pressure can begin to dominate the site. Since fire is not always available to be applied, then practices such as brush management may necessary from time to time to help keep the community in balance.

Periodic overgrazing and trampling by migrating herds of bison and resident herds of pronghorn antelope probably occurred during drought periods. However, long rest periods followed once the large herds of bison and antelope moved out of the area, allowing the resilient grassland to re-establish itself and maintain its reference community structure.


The major forces influencing the transition from the reference 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

Under good management this is one of the most productive sites in MLRA 77B and will give good animal performance. Little bluestem and sideoats grama are fairly resistant to grazing pressure but will decline if continuous heavy grazing persist. The tallgrasses are fairly sensitive to overgrazing and will begin to decrease more quickly if continuous heavy grazing occurs for long periods. If excessive grazing pressure continues, ecological retrogression occurs. The tendency of this site is to become a shortgrass dominant; sideoats grama and little bluestem will give way to blue grama. There will be an increase in perennial and annual forbs, with increasing amounts of yucca and sand sagebrush. The decrease in density and stature of the midgrasses and tallgrasses and an increase in shortgrasses and the density of the yucca and woody vegetation brings about a new plant community, the Shortgrass/Midgrass/Shrub Community (1.2).

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

If long-term heavy grazing continues, a threshold will be crossed to a Shrub/Shortgrass Community (2.1). In this degraded state, typical vegetation will be low vigor blue grama and bare areas will open up with annuals filling the voids. Perennial threeawn will invade this site when the more desirable grasses are weakened and/or removed. Yucca and sand sagebrush will increase dramatically. Occasionally, broom snakeweed (Gutierrezia sarothrae) will increase to the point of domination. The loss of herbaceous cover and increased bare ground encourages accelerated erosion. Nutrient cycling, the water cycle, watershed protection and biological functions have been severely reduced. The plant community is so degraded that it cannot reverse the transition without extensive energy and management inputs. Restoration of phase (2.1) will require prescribed grazing with rest periods during the growing season, re-seeding bare areas with adapted native grass species, and 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.

Recovery can occur fairly rapid if the competitive plants are controlled and proper grazing management is applied. Full recovery and maintenance of the reference plant 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 (Narrative):
The following diagram suggests some pathways that the vegetation on this site might take. There may be other states not shown on the diagram. This information is intended to show what might happen in a given set of circumstances; it does not mean that this would happen the same way in every instance. Local professional guidance should always be sought before pursuing a treatment scenario.

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. This means that once changes have progressed to some certain point, the balance of the community has been altered to the extent that a return to the former state is not possible, that is, not possible unless some form of energy is applied to make it happen. These changes take place on all ecological sites, but some sites support communities that are more resistant to change than other sites. Also, some sites are more resilient, that is, they tend to be able to heal or restore themselves more easily. Usually, changes in management practices alone, such as grazing techniques, will not be sufficient to restore former plant communities. An example of energy input might be the implementation of chemical or mechanical brush management to decrease the amount of woody shrubs and increase the amount of grasses and forbs. This shift in community balance could not be brought about with grazing alone. The amount of energy required to bring about a change in plant community balance may vary a great deal depending on the present state and upon the desired result.

STATE AND TRANSITIONAL PATHWAYS DIAGRAM: