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.

Two water associated characteristics affect vegetative composition in playas: frequency and duration of inundation. In addition to these factors, the nature of the drainage area is also very important. If the drainage is from rangeland, then there are minimal influences from things such as fertilizers and pesticides that are associated with production agriculture. For many of the playas, even some of those in native vegetation, much of the runoff are from cropland drainage areas. This can affect the nature of the vegetation to some degree. Siltation is increased if the drainage area is from cropland and incidence of contaminants is higher.

The drainage to storage ratio of playas has much to do with whether they pond little water or a lot of water. A very large playa with a small drainage area may never pond much water and will probably not support a predominance of hydrophytic vegetation. By the same token, a small playa with a large drainage area may support hydrophytic vegetation much of the time.

To discuss the nature of playa vegetation, it is necessary to recognize that the water regime varies greatly among the many playas in the region. As previously stated, the drainage to storage ratio governs the amount of runoff. With longer periods of inundation, grass species tend to disappear because they cannot stand being covered by water for very long. Most of the grass dominated playas are largely western wheatgrass with some small amounts of buffalograss (Bouteloua dactyloides) and vine mesquite (Panicum obtusum) present. There will be some forbs present such as frogfruit (Phyla nodiflora) and coreopsis (Coreopsis tinctoria) especially in the wetter spots. As the period of inundation increases, western wheatgrass (Pascoyrum smithii) and other grass species will disappear and species such as spike rushes (Eleocharis spp.) and smartweed (Polygonum spp.) will become dominant. In even wetter environments, species such as arrowhead (Sagittaria spp.) and rushes (Juncus spp.) prevail. Species composition is also affected by the timing of the moisture. As a person observes playa after playa on a given day, it becomes obvious that they are all a bit different and it is also difficult to explain the variations. Since rainfall patterns vary so much from year to year and also within the particular growing season, playa vegetation will also vary from year to year and within a given year. It should be noted that over the long haul (> 50 years), playas will probably be dry more than wet. However, in the period when they are wet, they contribute significantly to the biodiversity of the southern Great Plains.

It should be noted that probably 80 percent of the playas on the high plains have been affected by cultivation on their drainage area and by such structures as roads, culverts, terraces and modifications such as pits or ponds constructed in the playa basin itself. Because of these effects, it is hard to picture what the playas looked like in pristine times.

As to the part played by grazing on the overall ecology of playas, grazing plays a lesser role than does water regime. The grazing potential of playas is seasonal and cattle prefer to graze the lakes in summer when the forage is lush and green. The productive capacity of many playas is quite high and they can provide a good deal of grazing. It is certainly possible to graze so heavily as to adversely affect vegetation, especially grass vegetation. The general effect of abusive grazing over several years seems to be to decrease the number of perennials and increase the number of annual species. Along with this comes a decrease in overall plant diversity. Most of the hydrophytic perennials exhibit a surprising ability for their root stocks to go into dormancy when the playas become dry. These roots can live for many months and even years of dry conditions and when wet conditions again prevail, the plants quickly sprout and grow rapidly. It is very difficult to state a particular transitional pathway for playa vegetation based on grazing management. The best general statement would be that perennial grasses and forbs are the most susceptible plants to abusive grazing practices, and that strong perennial grasslikes are resistant to grazing damage. Overgrazing of playas is very detrimental to the host of wildlife that relies on the playas for habitat. One of the biggest detriments is the reduction of valuable wildlife cover. It is probable that the value for native wildlife exceeds the value for grazing. Dozens of species depend on playa vegetation for a home and for a food supply. In pre-settlement times, the buffalo most likely depended on water in the playas in order to graze out on the Llano Estacado. In dryer years with no standing water, the grazing was limited to areas along the major streams. At this point it is worth mentioning that production agriculture which includes the occasional tilling of the playas tends to promote more annual forb growth and contributes to the spread of noxious species such as wooly leaf bursage (bur ragweed).

Playas are very important in their affect on overall hydrological conditions. They receive runoff and in that runoff comes certain natural contaminants that are processed and recycled in the playas. Infiltration through the heavy clay soils is very slow but does occur. There is definitely some recharge to the underground aquifer through the playa system. It is difficult to speculate what effect hundreds of playas full or water during the summer months might have on rainfall locally. The evaporation is high and water going into the atmosphere may have some effect on rainfall. Playas act as natural filters and improve the quality of ground water. Because of their scattered presence over the high plains, they play an important part in trapping excess runoff from heavy rainfall events, thus limiting the distance runoff water travels down slope, thereby helping to control erosion.

Natural fire played a part in the ecology of this site as with all plains sites. The main effect of fire on the playa site was to stimulate growth and reduce old decadent plant cover from previous years. Most hydrophytic perennial species respond well to fire. In the playas, the root systems of these plants are protected by either wet soil or by being in dormancy. Fire helps in the reduction of old growth and stimulates new shoots and helps in breaking down accumulated plant material, thus aiding the nutrient cycle. Fire may have also helped in favoring the perennials over the annual species.

All species of plains wildlife benefit from the playa system. From the predators to small mammals to grassland birds to waterfowl, the playa lakes provide a diversity of habitat that the shortgrass plains upland sites cannot. They are an extremely important part of the shortgrass plains ecosystem.

State and Transitional Pathways 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 effect.

As previously stated, it is difficult to put forth a predictable state and transitional pathway that applies to playa lakes. Water regime is the biggest influencing factor. Grazing abuse can also affect playa communities but the predictability of that effect is somewhat questionable. Modification of playas by changes in drainage patterns, and structural modifications are the main man induced effects, and are more drastic as to the impact on vegetation than grazing by livestock.

Plant Communities based on frequency and duration of inundation:

In general, the more shallow the playa and the less frequently inundated, the more likely grasses such as western wheatgrass will dominate. The greater the depth and the more frequent the inundation, the more likely hydrophytic forbs and grasslikes will dominate. Many of the species now present in many playas that are associated with tillage and / or altered hydrology were probably not present in the natural grassland state of the high plains. In observing playas located within large areas of native rangeland, it was noted that well over half of these playas had a dominance of eleocharis macrostachya (creeping spikerush). The dryer moisture regimes had a predominance of western wheatgrass. A much fewer number had a predominance of smartweed and other hydrophytic forbs.

In cropland areas, the situation tends to show more annual forbs and pronounced increase in wooly leaf bursage. The playas on the south plains (from approximately Lubbock County south) are somewhat different in their ecology due to surrounding soils being sandy and, in some cases, increased salinity.