Ecological dynamics

The reference plant community consists of a mixture of midgrasses with lesser amounts of shortgrass and tallgrass species, along with a respectable amount of forbs and scattered woody plants. It could be classified as a true mixed-grass prairie. The productivity is fairly high due to a deep soil and a good soil, plant, air, water relationship. The main grass species are sideoats grama (Bouteloua curtipendula), blue grama (Bouteloua gracilis), vine mesquite (Panicum obtusum), and lesser amounts of little bluestem (Schizachyrium scoparium). There are also moderate amounts of Arizona cottontop (Digitaria californica), plains bristlegrass (Setaria vulpiseta), sand dropseed (Sporobolus cryptandrus), hooded windmillgrass (Chloris cucullata), and fall witchgrass (Digitaria cognata) present. In certain locations small amounts of sand bluestem (Andropogon hallii) and Indiangrass (Sorghastrum nutans) are present.

Forbs are present in moderate amounts in the reference community but are moisture dependent. Forbs will range from 8 to 12 % of total vegetative composition by weight. Major perennial forb species include catclaw sensitivebriar (Mimosa nuttallii), Engelmanndaisy (Engelmannia peristenia), prairie acacia (Acacia angustissima), green eyes (Berlandiera spp.), Rushpea (Hoffmannseggia spp.), scarlet gaura (Gaura spp.), Western ragweed (Ambrosia psilostachya), scarlet globemallow (Sphaeralcea coccinea), halfshrub sundrop (Calylophus berlandieri), and dotted gayfeather (Liatris punctata). The main woody species are sand sagebrush (Artemisia filifolia), catclaw (Mimosa spp.), yucca (Yucca spp.), occasional condalia (Condalia spp.), and mesquite (Prosopis glandulosa). Mesquite is usually much more prevalent present day than historically. If this site is subjected to abusive grazing practices, the vegetative community will lose the tallgrasses and much of the midgrasses fairly rapidly. Shortgrass species will increase along with annual forbs. Mesquite is the major woody increaser although catclaw and yucca can also act as increasers.

Grazing by large herbivores played a major role in shaping the site vegetatively. 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. Continuous heavy grazing with domestic livestock has occurred on many sandy loam sites and deterioration of the original plant community has been the result.

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. Fires may have occurred as often as every 7 to 12 years on the average and this site usually had an above average fuel load compared to some other plains sites. There is documentation from the mid 1800's that mention mesquite being present in the western rolling plains but it seems to indicate that most mesquite occurred along water courses and in scattered locations throughout an otherwise total grassland ecosystem. Fire likely kept most woody species suppressed and favored the mid and tallgrasses. Forb diversity was also promoted by periodic fires, which was beneficial to the wildlife population.

Grazing pressure began to be severe in the 1880’s and the diversity and productivity of the site has generally declined except where excellent management has been practiced for long periods. The taller warm-season grasses such as Indiangrass and sand bluestem have disappeared in most instances. If abusive grazing is practiced for many years, midgrasses will give way to increasing buffalograss (Bouteloua dactyloides) and blue grama, along with hooded windmillgrass, and sand dropseed. These shortgrasses can adapt better to grazing pressure. The more desirable forbs decrease rapidly with abuse and western ragweed increases with a host of annual forbs. In some cases, annual grasses such as Japanese brome (Bromus japonicus) and little barley (Hordeum pusillum) begin to be competitive. Perennial three-awn (Aristida purpurea) invades with long-term abuse and can dominate areas within a site. If good plant cover is not maintained on this site, erosion from water can become a problem. Small gullies may appear and bare ground increases. Infiltration is decreased and runoff increases.

In reference condition good grass cover and a variety of species made this site desirable for deer, turkey, and many ground nesting birds. Small mammals and predators such as coyotes and bobcats find the site attractive as well. In historic climax, the site did not have sufficient woody cover to sustain whitetail deer but this has changed over the past 150 years and many of these sites have an abundance of woody cover. It is difficult to find large acreages of this site in near reference condition.

Poor cover and decreased plant diversity brought about by poor grazing management disrupt the natural processes such as the water cycle and nutrient cycle. The site has the capacity to store a good deal of water in the soil profile but poor vegetative cover inhibits this process. If little water enters the soil, then the more productive grass species tend to do poorly. Opportunistic plants such as weedy forbs and shortgrasses decrease the long term stability of the site. Deeper rooted grasses and forbs are more efficient at nutrient cycling and aid in reducing surface runoff.

Plant Communities 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 or due to natural occurrences or both. Changes may occur slowly or fairly rapidly, depending on the type of events that effect change. At some point in time thresholds are crossed, which means that once changes in vegetative makeup have progressed to a certain point, the balance of the community has been altered. When this point is reached, a return to the former community state is generally not possible – unless some significant energy inputs are provided to induce a response in that direction. These changes in plant communities occur on all ecological sites with some being more resistant to changes than other sites. Some sites seem to be more resilient and are more easily restored to former vegetative states than are other sites. Usually, changes in grazing management alone, such as improvement in grazing techniques, will not be sufficient to induce the desired change in plant communities. An example of energy input that might be needed to induce change might be the implementation of chemical brush management and complete growing season rest in order to reduce the domination of woody shrubs and promote the dominance of perennial grasses and forbs. This action might have to be done more than once and might take some time. Such a vegetative shift would not be possible with grazing management alone. The amount of energy input needed to effect change depends on present vegetation and the desired result.

The following diagram suggests some of the more predictable pathways the vegetation on this site may take when subjected to poor grazing management. This diagram generally depicts some of the same situations shown in the photographs of the plant communities.