Ecological dynamics

The Reference Plant Community for this site depends on water regime. 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, as would be the case in the reference plant community, then there are minimal influences from factors 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. The percentage of exotic plant species and annuals may be much higher. Siltation is increased if the drainage area is from cropland and incidence of contaminants is likely to be higher.

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. This site is in a constant state of change. The unpredictable climate of the Southern High Plains dictates this condition. This drying and flooding allows vastly different plant communities to exist on the same playa, increasing the diversity of the site. Studies of vascular plants in the Southern Great Plains indicate that at least 346 species occur in playas. 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 (50-60 percent) playas in the Midgrass/Forb Community - Dryer State (1.1) are largely western wheatgrass (Pascopyrum smithii) with lesser amounts of buffalograss (Bouteloua dactyloides) vine mesquite (Panicum obtusum), knotgrass (Paspalum distichum) and barnyardgrass (Echinochloa muricata). Total production of forbs may be equal to the grass species in wetter years. Typical forbs present include frogfruit (Phyla incisa), coreopsis (Coreopsis lanceolata), plains ironweed (Vernonia marginata), and lambsquarter (Chenopodium album). Dryer phase playas generally do not support woody vegetation.

Grazing plays a lesser role than does the water regime. The grazing potential of playas is seasonal and cattle prefer to graze the lakes in the spring and 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 on the dryer phase playas. The general effect of abusive grazing over several years seems to decrease the number of perennials and increase the number of annual species. Along with this comes a decrease in overall plant diversity. If long-term abusive grazing does occur on the dryer phase playas and the frequency of flooding increases, this site tends to move towards the Grass-likes/Forbs/Annuals Community (1.2). 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 then 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 strong perennial grass-likes species 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.

As the period of inundation increases with moderate frequency, western wheatgrass and other grass species will disappear as this site moves towards the Grass-likes/Emergent Hydrophytic Plant Community (2.1). Creeping spikerush (Eleocharis macrostachya) and sedges (Abildgaardia spp.) will dominate. Dryer phase forbs will be replaced by smartweed (Polygonum pensylvanicum), California loosestrife (Lythrum californicum), saltmarsh aster (Symphyotrichum divaricatum), and curly dock (Rumex crispus).

On playas that have frequent inundation with prolonged periods of flooding, the plant community will move towards the Grass-likes/Hydrophytic Plant Community (3.1). Rushes (Juncus spp.), knotgrass (Paspalum distichum) and smartweed dominate with lesser amounts of arrowhead (Sagittaria lancifolia) and pondweed (Potamogeton spp.) present. Species composition is also affected by the timing of the moisture. Early growing season (May) moisture tends to favor smartweed. This site typically has few shrubs and these generally occur around the periphery of the wetter playa basins. The primary woody shrub is willow (Salix spp.). 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. If playas are inundated through the growing season and then dry up in the fall they may be bare during the following winter and early spring and can be subject to wind erosion until plants emerge in the summer. It should be noted that over the long haul (>50 years), playas will probably be dry more than wet. However, in the periods when they are wet, they contribute significantly to the biodiversity of the southern Great Plains.

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. 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 without any standing water, the grazing was limited to areas along the major streams.

Playas are very important in their affect on overall hydrological conditions. They receive runoff and in that runoff come 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.

It should be noted that probably 80 percent of the playas on the High Plains have been affected by cultivation in 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. At this point it is worth mentioning that production agriculture which includes the occasional tilling of the playas and adjacent areas tends to promote more annual forb growth and contributes to the spread of noxious species such as woollyleaf bursage (Ambrosia grayi), Texas blueweed (Helianthus cilaris) and smallhead sneezeweed (Helenium microcephalum).

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.

Plant Communities and Transitional Pathways (diagram):
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. In general, the more shallow the playa and 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 grass-likes 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.