Ecological dynamics

The reference plant community is limited most by the nature of the soils occurring on the site. The high shrink/swell clays plus a relatively dry climate limit the species to those that can tolerate those harsh conditions. In general, the reference plant community can be best described as a Tobosa – Alkali sacaton (Pleuraphis mutica - Sporobolus airoides) community with a limited amount of other grass species, a limited forb population and a somewhat scattered shrub component. Old photographs of this site taken in the early 1900’s show that the mesquite and other shrubs tended toward open stands of moderate sized plants and there were some fairly large areas with no brush species present. Other grass species that occur in smaller amounts on the Clay Flat ecological site are: blue grama (Bouteloua gracilis), buffalograss (Bouteloua dactyloides), white tridens (Tridens albescens), vine mesquite (Panicum obtusum), whorled dropseed (Sporobolus pyramidatus), sand dropseed (Sporobolus cryptandrus), and occasionally a small amount of Texas wintergrass (Nassella leucotricha). The shrubs common to the site are mesquite (Prosopis glandulosa), lotebush (Ziziphus obtusifolia), wolfberry (Symphoricarpos orbiculatus), cholla (Cylindropuntia imbricata), tasajillo (Cylindropuntia leptocaulis), prickly pear (Opuntia spp.), ephedra (Ephedra spp.), and occasionally four-wing saltbush (Atriplex canescens) and shadscale saltbush (Atriplex confertifolia). In reference condition, it is thought that shrubs tended to be more scattered with mesquite more likely to be confined to water courses or on small areas of more loamy soils that sometimes occur within the broader clay flat areas. Present day examination shows that mesquite has increased more than any other woody species on the site. Mesquite is generally smaller in size than the plants growing on associated sites. They tend to be four to six feet in height and very much multi-stemmed.

Tobosa has always been the dominant grass species on this site but with grazing pressure it has become more so. Alkali sacaton may be somewhat diminished in many places due to it being preferred over the tobosa during most of the year. Blue grama and buffalograss are usually grazed very heavily. Vine mesquite is also a highly preferred species on the site. No doubt these more palatable species have declined over time due to selective grazing. Tobosa accounts for the majority of the production on the site. Tobosa is uniquely adapted to heavy clay soils because of its tough, scaly, deep root system. There is always some old growth present, thereby protecting itself from over utilization. The accumulation of litter is important on the site because of the droughty nature of the soil and the need to conserve moisture and limit evaporation. Tobosa and alkali sacaton seem to be able to withstand moderate to heavy grazing pressure with minimal ill effects. The warm-season grasses found on the site are C-4 type metabolism plants and the cool-season grasses are C-3 plants. All the shrubs are C-3 with the possible exception of Atriplex. Forbs are a combination of C-3 and C-4 plants. The resistance of the site to change is high. Resilience is also high provided the site has not been denuded of cover. The productivity of the site is rather high when well managed. If abused for long periods of time, this site, like most others will certainly manifest major changes in species composition and productivity or both. Lack of fire and heavy grazing are the major factors effecting change. Long-term droughts can act in combination with poor management to compound the negative effect on plant communities.

Fire played a very important role in the ecology of the clay flat site. Tobosa responds very well to fire and needs fire to restore vigor and eliminate old litter buildup that can occur over several years. When burned, tobosa increases greatly in palatability to large herbivores. Opening up the thick cover of tobosa allowed an increase of forbs for a year or two and allowed seed to be made. Alkali sacaton also responds well to fire and will usually increase somewhat under a fire ecology. Vine mesquite is also very fire responsive. Animals were drawn to the site after a burn which probably allowed some of the more preferred sites to rest naturally. Fire contributed to a greater diversity of plants and was important to the maintenance of a vigorous grassland community on this site. Prescribed fire is a recognized tool in managing the plant community on this site and is being used with some success. It has been found through research and experience that the optimum frequency with which to burn upland tobosa communities is about 7 to 8 years, depending somewhat on the growing seasons between fires. Fire is probably going to be necessary in managing this site for optimum livestock grazing or for wildlife use if those are the objectives.

Hydrologically speaking, the site has little runoff and therefore does not have the potential to affect watersheds as much as sites with more slope. However, if good protective cover is not maintained on the site, some sediment can find its way downstream. Silting in of small ponds within the site has always been a problem. If the cover is ever lost on this site, the clayey surface forms a thick hard crust that sheds water and increases evaporation.

Revegetation of this site is very difficult to accomplish because of soil characteristics. The brushy species, namely mesquite, are very hard to control with herbicides on this site. Mechanical control such as grubbing or root plowing destroys a lot of the perennial grass cover and more often than not, annuals such as annual broomweed (Amphiachyris dracunculoides) prevail for two or three years, even with reseeding. Eventually, the plants most adapted to the site will return and again become dominant.

The animal community that uses the site for habitat includes white-tailed deer, bob-white quail and small mammals. Most of these sites have sufficient woody shrubs to provide cover for deer, and quail utilize the area as nesting cover and for loafing areas where the grass cover is not too thick. Forb diversity and palatable browse is not sufficient to provide a reliable food source over the long term. Some of the associated sites supply better habitat elements than does the clay flat.

State & Transition Diagram Narrative:
The following diagram suggests some pathways that the vegetation on this site might take in response to various management treatment or natural stimuli over time. There may be other states that could result that are not shown in the diagram. Those shown are some of the states most commonly observed. Local guidance should be sought when making plans to manipulate plant communities for specific purposes.

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 occurring. At some point in time thresholds are crossed. This means that once changes in vegetation have progressed to some certain point, the balance of the community has been altered to the extent that a return to the former vegetative state is generally not possible, that is, not possible without some form of energy being applied in order to make the plant community respond in that direction. These changes in plant communities occur on all ecological sites with some sites being more resistant to change than others. Also, some sites seem to be more resilient, being more easily restored. Usually, changes in management practices alone, such as improved grazing methods, will not result in major changes to the community. An example of energy input that might be required to effect change might be the implementation of chemical brush management and complete growing season rest to reduce domination of woody shrubs and shift the community to one of more perennial grasses and forbs. This action might have to be done more than once and might take several years. Such a vegetative shift could surely not be accomplished by a change in grazing management alone. The amount of energy required to effect change would depend on the present vegetative state and the desired state.