Ecological dynamics

The reference plant community 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 Draw site in MLRA 77B may host two slightly different plant communities depending on soil moisture. Where long-term soil moisture has been low, plant community (1.1) will be dominated by midgrasses with lesser amounts of shortgrass species. Few tallgrasses, forbs and woody plants will be found. In instances where soil moisture conditions are more favorable, plant community (1.2) will still be dominated by midgrass species with a higher percentage of tallgrasses present and lesser amounts of shortgrass species. Forbs and woody plants will be more prevalent along areas that tend to pond water for longer periods.

The dominant midgrass species are western wheatgrass (Pascopyrum smithii), vine mesquite (Panicum obtusum), and sideoats grama (Bouteloua curtipendula). Lesser amounts of blue grama (Bouteloua gracilis) and buffalograss (Bouteloua dactyloides) will be present along with smaller compliments of switchgrass (Panicum virgatum), sand bluestem (Andropogon hallii) and Indiangrass (Sorghastrum nutans).Where slightly saline influences exist, alkali sacaton (Sporobolus airoides) may be a significant component. The significant presence of western wheatgrass makes this site preferred in the cool season as well as in the summer. At times there may be small holes of water present in the drainage channels. These provide a good source of water for wildlife and occasionally some plants such as curly dock (Rumex crispus) and smartweed (Polygonum spp.) may be found growing in wetter years. Productivity on this site is fairly high due to a deep soils and extra runoff from adjacent upland sites. Since the site occupies a location lower on the landscape, animals prefer to take shelter from wind during the colder part of the year.

Commonly found forbs are Baldwin ironweed (Vernonia baldwinii), Illinois bundleflower (Desmanthus illinoensis), gaura (Guara spp.), sagewort (Artemisia spp.), curlycup gumweed (Grindelia squarrosa), heath aster (Aster ericoides), Engelmann’s daisy (Engelmannia peristenia), Maximilian sunflower (Helianthus maximilianus), scurfpea (Psoralidium tenuiflorum), western ragweed (Ambrosia psilostachya), and numerous annuals. Few scattered elm (Ulmus spp.), hackberry (Celtis spp.), and cottonwood (Populus deltoides) occur but these are not as prevalent as on loamy or wet bottomland sites. A few shrubs such as willow baccharis (Baccharis salicina), wild plum (Prunus spp.) and occasional yucca (Yucca glauca) and/or sand sagebrush (Artemisia filifolia) will be present. Nutrient cycling, the water cycle, watershed protection and biological functions are functioning at their peak.

Grazing by large herbivores played a major role in shaping the site vegetatively. It is well documented that large herds of bison often grazed the site and domestic livestock prefer it as well. As bison migrated with the seasons, these sites received heavy grazing pressure from time to time but had long recovery periods. There is considerable evidence of haying of these sites by early day settlers. The increased productivity was recognized and the quality of the forage was good.

Natural fire also played a major role in grassland ecology. The general role of fire seems to have been to perpetuate grasslands and keep any encroaching woody vegetation at bay. Woody plants were scattered along the channels where they could often escape fires, but there is little doubt that fire kept the number of woody plants controlled. Fires may have occurred as often as every 5 to 7 years on the average and this site usually had an above average fuel load compared to other plains sites.

The major forces influencing the transition away from the reference plant 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.

Western wheatgrass acts as a strong increaser with initial grazing pressure. If abusive grazing is practiced for many years, western wheatgrass and other midgrasses will give way to increasing amounts of buffalograss and blue grama. These short grasses are better adapted to grazing pressure. The more desirable forbs decrease rapidly with abuse and western ragweed increases along with a host of annual forbs. In some cases, cool-season annual grasses such as Japanese brome (Bromus spp.) and little barley (Hordeum pusillum) will become excessive and compete strongly with perennials. Woody plants such as yucca and/or sand sagebrush often start to increase and become a higher percentage of the total composition. The decrease in density and stature of the mid and tallgrass species along with an increase in shortgrasses brings about a new plant community, the Shortgrass/Midgrass Plant Community phase (2.1).

In the Shortgrass/Midgrass Plant Community (2.1) phase, the transition back to near reference conditions is possible with proper grazing management and chemical brush 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 the Degraded Shortgrass/ Annuals/Shrubs Plant Community phase (3.1). In this degraded state, typical vegetation will be low vigor blue grama and buffalograss. Bare areas will open up with annuals filling the voids. Perennial threeawn will invade when the more desirable grasses are weakened and/or removed. Yucca and/or sand sagebrush will increase dramatically. Often pricklypear (Opuntia spp.) will gain a foothold on the site. 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 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. Where active gully erosion has occurred, mechanical shaping and re-vegetation will be needed. With the reduced amounts of grass fuel, prescribed burning is usually not an option in this phase.

If good plant cover is not maintained on this site, erosion from water can become a problem. Gullies may appear and the channels, which are usually grass covered, become deeper and are sometimes devoid of cover.

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:
As a site changes in the structure and makeup of the plant community, the changes may be due to management, 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 tipped to the extent that a return to the former state is not possible unless some form of energy is applied. 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. 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.

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.