Ecological dynamics

The soils, topographic location, climate, periodic droughts and fire influenced the stabilization of the reference plant community for this site. this plant community as found by early European settlers was a Shortgrass/Midgrass Community (1.1) with few tallgrasses. There was a good variety of perennial forbs with a few scattered woody shrubs. The calcareous soils influenced the species composition of the site. Productivity is moderate with most of the production coming from sideoats grama (Bouteloua curtipendula) and blue grama (Bouteloua gracilis). Other commonly found grasses were buffalograss (Bouteloua dactyloides), sand dropseed (Sporobolus compositus) and perennial threeawn (Aristida purpurea). Vine mesquite (Panicum obtusum) and western wheatgrass (Pascopyrum smithii) could be found in runoff concentration areas. The more free lime there is in the soil, the higher the sideoats grama and little bluestem (Schizachyrium scoparium) composition. Major forbs included dotted gayfeather (Liatris punctata), slimleaf scurfpea (Psoralea tenuiflora), Engelmann daisy (Engelmannia peristenia), lyreleaf greeneyes (Berlandiera lyrata), baby white aster (Chaetopappa ericoides), catclaw sensitivebriar (Mimosa microphylla), scarlet globemallow (Sphaeralcea coccinea), and plains zinnia (Zinnia grandiflora). The major woody shrub is plains yucca (Yucca glauca) and there were often small amounts of catclaw mimosa (Mimosa aculeaticarpa var. biuncifera), plains pricklypear (Opuntia polycantha), broom snakeweed (Gutierrezia sarothrae), and cholla (Cylindropuntia imbricata) present. Yucca acts as a strong increaser on this site and if the site is constantly rested in the spring when the yucca blooms make seed, the increase is hastened.

Natural fire played a major role in the ecology of this site as with most plains sites. The major effect of periodic fire was to suppress woody shrubs and promote a grassland community. With the absence of fire it is much easier for woody shrubs to proliferate and become dominant. Natural fires probably occurred every 10 to 15 years and perhaps more often throughout the plains region. This was often enough to hold most woody species in check and encourage grass dominance. Fire also promoted diversity of forb growth for a couple of years following the burn, which drew the attention of wildlife species such as pronghorn and mule deer. The main obstacle to the use of fire in the present day is the liability situation and the unpredictability of precipitation. As precipitation falls below a mean of 18 inches, measuring the positive effects of fire becomes more difficult. Fire can be a valuable tool for managing vegetation when used by trained experts in a proper manner.

Large herbivores, mainly bison, roamed the prairie, grazed heavily, and then moved on allowing for long recovery periods. Healthy, productive grassland ecosystems were maintained under this type of natural grazing regime. The grazing of domestic livestock, mainly cattle, began in the 1870’s. Early day ranchers saw a land of endless grass and over estimated the ability of the land to support large numbers of stock. As the land was fenced in the 1880’s, stock began to be more confined leading to continuous grazing pressure on much of the range. Since that time, continuous grazing has been common and stocking rates have been for the most part excessive.

Hydrologically speaking, the site produces a considerable amount of runoff which finds its way to the numerous small and medium sized drainages. If plant cover is a healthy grassland community, the runoff is slowed and the water quality of runoff is acceptable. Poor cover with significant bare soil promotes runoff and siltation.

This becomes a vicious cycle with infiltration being limited by poor cover and poor moisture relationships limiting the growth of healthy plants. Deep rooted perennial grasses and forbs aid in an efficient water cycle.

This is not a particularly diverse site as far as habitat for wildlife is concerned. However, grassland birds, pronghorn and mule deer inhabit the site. Woody cover is not sufficient to provide cover for whitetail deer and turkey. Scaled quail are often found on the site as they need no more brush than a few yucca and cholla plants to provide their cover needs. In reference conditions, this site could support the native plains wildlife, while other sites with more woody vegetation supported different species. In planning with producers to meet wildlife habitat needs, the potential of each ecological site needs to be realized and included in the overall plan.

Although recent climatic warming trends and increased atmospheric carbon dioxide may be enhancing vegetative change, the major forces influencing the transition from the historic climax plant community are continuous heavy grazing 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.

If abusive grazing continues for long periods of time, ecologic retrogression occurs. As retrogression proceeds, this site will move towards the Shortgrass/Midgrass/Shrub Community (1.2) with an increase in woody and annual plants. The low vigor midgrasses will decrease and the shortgrass species such as blue grama, perennial threeawn and annuals will increase to the point of dominating the site. Shrubs such as yucca and possibly broom snakeweed will become a significant part of the plant community. Plant composition will be less diverse than the reference community. In this phase, ecological processes have changed somewhat, but the pathway back toward the reference community can be initiated through prescribed grazing, primarily deferment during the growing season for three to four years. Chemical pest and brush management will be necessary to reduce the shrub and annuals population. This can be accomplished by aerial application or individual plant treatment (IPT). In the Shortgrass/Midgrass/Shrub Community (1.2) the fine fuel load is still adequate for prescribed burning if climatic conditions allow.

If heavy continuous grazing continues, along with periodic droughts, retrogression will move towards a Shortgrass/Shrub Community (2.1). This phase will see deterioration of the perennial grass community, low vigor shortgrasses will dominate with minimal presence of midgrasses. Broom snakeweed and cactus may increase dramatically. Yucca can become the dominant plant on this site with no fire or brush management. Bare ground may exceed 40 percent with annuals filling the voids. The Shortgrass/Shrub Community (2.1) will have low production potential and the ecological processes are not functioning very well. Poor hydrological conditions prevail. Productivity and diversity of the plant community has generally suffered.

The plant community is so degraded that it cannot reverse retrogression without extensive energy and management inputs; a major threshold has been crossed. Restoration of the Shortgrass/Shrub Community (2.1) to the Shortgrass/Midgrass State requires chemical brush and pest management, re-seeding and prescribed grazing. Due to the lack of fine fuel and poor continuity, prescribed burning is not an option.

If grazing abuse continues for many years, this site will become a Shrub/Shortgrass Community (2.2). Yucca will become the dominant plant with a dramatic increase in broom snakeweed and cactus. Low vigor shortgrasses and perennial threeawn will occupy the interspaces. Bare ground can exceed 50 percent on some sites with annuals filling the voids. No presence of midgrasses remaining. Restoration of the Shrub/Shortgrass Community (2.2) requires extensive chemical brush and pest management, re-seeding and prescribed grazing for many years to restore this phase to near reference.

When given good management this site, as well as other sites, respond favorably and some measure of recovery can be expected fairly rapidly.

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:
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. Changes in the structure and composition of the plant community may be due to management, to natural occurrences or both. At some point in time thresholds are crossed as indicated by the dashed lined box on the State and Transition Diagram. This suggest that once changes have progressed to a certain point, the plant 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 brush management to decrease the amount of woody/cacti 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)