Ecological dynamics

The information contained in the State and Transition Diagram (STD) and the Ecological Site Description was developed using archeological and historical data, professional experience, and scientific studies. The information presented is representative of a very complex set of plant communities. Not all scenarios or plants are included. Key indicator plants, animals and ecological processes are described to inform land management decisions.

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. The plant community as found by European settlers in the early 1800’s developed under the prevailing climate over time along with the soils in their topographic location. Grazing and/or browsing by local and nomadic wildlife influenced the plant community as well. The resulting plant community was the Tallgrass/Midgrass Community (1.1), which consisted of tall and midgrasses, associated forbs and scattered woody shrubs. The dominant tallgrasses were sand bluestem (Andropogon hallii) and little bluestem (Schizachrium scoparium). The midgrasses that were present include sideoats grama (Bouteloua curtipendula), sand lovegrass (Eragrostis trichodes), dropseeds (Sporobolus spp.), sand paspalum (Paspalum strumarium), fall witchgrass (Digitaria cognata), hairy grama (Bouteloua hirsuta), blue grama (Bouteloua gracilis), silver bluestem (Bothriochloa laguroides), plains bristlegrass (Setaria leucopila), red lovegrass (Eragrostis secundiflora), gummy lovegrass (Eragrostis curtipedicellata), sandbur (Cenchrus insertus), hooded windmillgrass (Chloris cucullata) and perennial three-awns (Aristida spp.). Some cool-season grasses occur in small amounts such as Canada wildrye (Elymus canadensis) and needle & thread (Heterostipa comata). Typically associated forbs included western ragweed (Ambrosia psilostachya), dotted gayfeather (Liatris punctata), annual wild buckwheat (Polgenum convolvulus), queens delight (Stillingia sylvatica), prairie clover (Dalea purpurea), catclaw sensitivebriar (Mimosa aculeaticarpa var. biuncifera), golden dalea (Dalea aurea), camphorweed (Heterotheca pilosa), sand lily (Mentzelia decapetala), gaura (Gaura spp.), prairie spiderwort (Tradescantia occidentalis), erect dayflower (Commelina erecta) and numerous annual forbs. Woody species include sand sagebrush (Artemesia filifolia), skunkbush sumac (Rhus aromatica), sand plum (Prunus angustifolia), yucca (Yucca glauca) and southwestern rabbitbrush (Chrysothamnus pulchellus). Nutrient cycling, the water cycle, water-shed 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. Tallgrasses 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. Shrubs usually resprout, but are suppressed for a time allowing grasses to dominate. If periodic fire does not occur, then the 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 climax community structure. The major forces influencing transition from the historic climax community is continued overgrazing 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.

The productive capacity of this site is moderately high if good management is applied. The tallgrasses are fairly sensitive to overgrazing and will begin to decrease if continuous heavy grazing is applied. They respond to rest very well and if grazed moderately and rested periodically toward the mid to late growing season, they will generally persist. If excessive grazing continues ecological retrogression occurs. As retrogression proceeds, the tallgrasses give way to an increase in midgrasses such as sideoats grama, dropseed species and perennial threeawns. The better quality forbs are replaced with less palatable species such as gaura, western ragweed, annual wild buckwheat and camphorweed and there will be an increase in annual forbs. Sand sagebrush and skunkbush can increase to >20% of the total plant community, on some sites yucca plants may start to increase. The decrease in density and stature of the tall grass vegetation and increase in density of the woody vegetation brings about a new plant community, a Midgrass/Shrub Community (2.1).

In the Midgrass/Shrub Community (2.1), the transition back to near reference is reversible 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 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 Dominant/Annuals Community phase (3.1). In this degraded state, typical vegetation will be annuals, western ragweed, perennial threeawn, red lovegrass, gummy lovegrass, sandbur and a significant canopy of sand sagebrush, yucca and possibly skunkbush. 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 retrogression without extensive energy and management inputs. Restoration of phase (3.1) 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.

The sandy site is not particularly resistant to heavy grazing but exhibits amazing resilience. If even a small seed source of the tallgrasses remains and some old root crowns are still viable, then recovery is possible with minimal re-seeding. Recovery can occur fairly rapid if the competitive plants are controlled and proper grazing management is applied. 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. 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.

As it relates to this site, the plant community balance is more fragile than some of the short grass sites are. This balance is necessary for the community to function properly. Tall grasses are not as resistant to grazing as short and mid grass species. The soil is more fragile since it is sandy and can result in plant and soil disturbance from hoof action. If cover is very poor, wind erosion will occur.