Ecological dynamics

The soils, topographic location, climate, periodic droughts, declining water tables, and fire influenced the development of the Reference Plant Community for this site. The Midgrass/Shortgrass Community (1.1) is a combination of mid and shortgrasses with a few salt tolerant forbs and shrubs. Tallgrasses occur in isolated areas where the environment was more hospitable. Grasses make up approximately 80% of the total vegetation while forbs and shrubs make up approximately 20%. The dominant grass species are alkali sacaton (Sporobolus airoides) with lesser amounts of inland saltgrass (Distichlis spicata), creeping muhly (Muhlenbergia repens), whorled dropseed (Sporobolus pyramidatus), alkali muhly (Muhlenbergia asperifolia), and meadow dropseed (Sporobolus asper). The principal forbs are salt tolerant annuals. Principal shrubs include four-wing saltbush (Atriplex canescens) and willow baccharis (Baccharis salicina). No trees are present on the site.

While natural fire and periodic grazing by large herbivores influenced vegetative composition to some degree, the principal environmental factors driving the reference vegetation are salinity and the presence of a high water table. This site was not highly preferred by herbivores due to the high salt content in the vegetation. Grazing did occur periodically, especially when animals needed salt in their diet, and could obtain salt from grazing the vegetation. Natural fire did help to promote a grassland aspect and kept shrubs controlled. In pre-settlement times, this site occupied a relatively small portion of the landscape along draws and around ancient lacustrine salt lakes where salt water was near the surface. The presence of salty playas was noted by early day explorers. This site could be found most often along the ancient drainages that run from northwest to southeast across the southern high plains and around the associated salty playas. The soils were formed in old alluvium and lacustrine sediments. The site occurs over salt domes where salt water is near the surface. This ecological site in reference conditions has some variability in vegetation due to the degree of salinity and depth to water. In spots where the soils were highly saline, no vegetation would be present and the percent bare ground on this site could be as high as 40 to 50 percent.

In more recent times the spread of salty areas has greatly increased over the southern high plains. Salty “seeps” have outcropped along the old draws and in many cases even in areas that originally were neither saturated nor saline. There are some differing opinions as to the cause of the increase of wet saline areas. One possible cause for the increase may be the removal of much of the fresh water that occurred in the upper Ogallala formation allowing salt water below to make its way to the surface. Many of the acres now referred to as “wet saline” acres were cropped for many years and have been abandoned. Many were placed in the CRP program in the mid 1980’s. After cropping ceased, the water table under these areas rose and salt could be seen precipitating out of the soil surface; a byproduct of evaporation. These areas grew in size every year for at least 2 or 3 decades. As vegetation began to appear on land abandoned from cropping, salt cedar (Tamarix ramosissima), an exotic woody species, began to appear and has increased greatly over much of these areas. Forbs such as kochia (Bassia prostrata), Russian thistle (Salsola kali), and other salt tolerant introduced annual forbs have appeared as the first plants to occupy the site along with a few perennial grasses. It can be argued that these relatively recent saline areas have been caused by the influence of man’s activities, including depletion of the Ogallala and a change in farming practices.

Because of the rather recent appearance of this unique situation, it has been necessary to re-map soils on many of these areas. Soils that occurred sixty years ago have changed in chemistry and hydrology to the point that the old soil series have had to be re-named. Some series are hydric in nature with saturation to the surface common for at least part of the year. Many of these soils were previously categorized as upland sandy loams and clay loams.

The attempt to describe the reference community for a wet saline site must be done by looking at areas where it is reasonably certain that the vegetative community is not some recent development. Many of the sites determined to be wet saline today certainly do not resemble what was present on natural wet saline areas in pre-settlement days and probably never will. These recently formed wet saline areas, however, need to be addressed as many of them now support at least some native vegetation. It must be noted that the vegetative states described and present on these sites today may not be stable and may change over time.

In cases where domestic livestock are continually forced to overgraze the unpalatable vegetation on this site along with no fire, ecological retrogression can occur. As retrogression proceeds, this site will move towards a Shrub/Shortgrass/Annuals Community (2.1). Alkali sacaton and dropseed species will be replaced with inland saltgrass, Muhlenbergia species and annuals. Willow baccharis will increase along with an invasion of salt cedar. In this phase, ecological processes have changed somewhat, but the pathway back toward the reference community can be initiated through prescribed grazing, prescribed burning and brush management.

Continued overgrazing, along with periodic droughts, water table decline, and increased salinity, causes the site to move towards a Shrub Dominant Community (3.1). Salt cedar increases on the site to the point of domination with very little understory plants present. The percent bare ground may be even less than reference with salt cedar plants filling the voids. This phase of retrogression has low forage production potential and the ecological processes are not functioning very well. Poor hydrological conditions prevail. A major threshold has been crossed when the plant community is so degraded that it cannot reverse retrogression without extensive energy and management inputs. Restoration of the Shrub Dominant Community (3.1) requires extensive brush management, re-seeding, and prescribed grazing. Soil amendments and cultivation may be necessary under severe conditions.

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)

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 and/or natural occurrences. At some point thresholds are crossed as indicated by the lined box on the State and Transition Diagram. This suggests 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 unless some form of energy is applied. These changes take place on all ecological sites. Some sites support communities that are more resistant to change than others. Also, some sites are more resilient and can 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 the desired result.