Ecological dynamics

This is an upland grassland site occurring on nearly level to moderately sloping topography. The Reference Plant Community is best characterized as warm-season mid-grasses with somewhat lesser amounts of short-grass species and scattered short shrubs. Tallgrasses occur but are somewhat infrequent, and are found in micro environments that receive extra runoff. Perennial and annual forbs make up from 5 to 10 percent of total annual production, depending upon precipitation. Shrubs account for approximately 5 to 8 percent of total annual production, with some variation in species often occurring from one locality to another. Production levels are moderate to moderately high, and accessibility for grazing and browsing animals is good. Bunchgrasses are most prevalent with lesser amounts of sod-forming species. Inherent fertility is moderate and the site is favored by domestic livestock. Diversity is high in the reference community, but will decrease with long-term, heavy grazing pressure. This site is fairly extensive within the MLRA and may encompass both large and small acreages depending on location.

The reference grassland community consists predominantly of sideoats grama (Bouteloua curtipendula), blue grama (Bouteloua gracilis), and black grama (Bouteloua eriopoda), with smaller amounts of vine mesquite (Panicum obtusum), wolfstail (Lycurus phleoides), galleta (Pleuraphis jamesii), fall witchgrass (Digitaria cognata), sand dropseed (Sporobolus cryptandrus), Hall's panicum (Panicum hallii), plains bristlegrass (Setaria leucopila), perennial threeawn (Aristida spp.), hairy grama (Bouteloua hirsuta), gummy lovegrass (Eragrostis curtipendicellata), and sand muhly (Muhlenbergia arenicola). A few cool-season species such as needle and thread (Hesperostipa comata), Canada wildrye (Elymus canadensis), and bottlebrush squirreltail (Sitanion hystrix) will be found, and are most prevalent in years when winter moisture is above normal. Small amounts of little bluestem (Schizachyrium scoparium) occur in inclusions of shallow sandy loams or in areas where slightly sandier textures occur. Sand bluestem (Andropogon hallii) and Indiangrass (Sorghastrum nutans) will be found in micro environments within the site where moisture is more favorable. Tallgrasses generally do not make up more than 5 to 10 percent of the total plant community. Some of the more prevalent forb species are scarlet globemallow (Sphaeralcea coccinea), rushpea (Hoffmannseggia jamesii), catclaw sensitivebriar (Mimosa roemeriana), dotted gayfeather (Liatris punctata), sand lily (Mentzelia nuda), western ragweed (Ambrosia psilostachya), trailing ratany (Krameria sp.), lyreleaf greeneyes (Berlandiera larata), plains zinnia (Zinnia grandiflora), wild alfalfa (Psoralidium tenuiflorum), scarlet gaura (Gaura coccinea), and croton (Croton spp.). Forb production tends to be very moisture-dependent and can vary considerably from year to year. The suffrutescent half-shrub broom snakeweed (Gutierrezia sarothrae) is found in small amounts and can sometimes increase with abusive grazing. Woody short shrub species most commonly found are sand sagebrush (Artemisia filifolia), plains yucca (Yucca glauca), Mormon tea (Ephedra antisyphilitica), and winterfat (Krascheninnikovia lanata). Mesquite (Prosopis glandulosa), cholla (Cylindropuntia imbricata), and pricklypear (Opuntia macrorhiza) are found in small amounts. Shrubs tend to be scattered, but yucca, mesquite, and sand sagebrush can increase with abusive grazing practices and no natural fire.

General observations would suggest that blue grama, black grama, and lesser amounts of other short-grass species are now dominating many of the sandy loam sites in this MLRA. In many cases, there are only small amounts of sideoats grama present, and that species was almost certainly present in greater amounts in the reference community, especially on the sandy loams that have a coarse-loamy classification. The tighter sandy loams (fine-loamy classification) tend to produce somewhat more blue grama and less sideoats grama. Continuous moderate grazing will usually result in an increase in blue and black grama over time, and a decrease in sideoats grama. Prolonged abusive grazing practices will nearly always result in a significant increase in perennial threeawn, sand dropseed, hairy grama, sand muhly, gummy lovegrass, and annual forbs. Sand sagebrush and yucca are both increasers on this site. Yucca can sometimes increase to a moderate plant population (greater than 1000 crowns per acre), but the sand sagebrush does not usually form a dense canopy. If a seed source is available, mesquite can also invade the site. The Sandy Loam ecological site appears to be somewhat less resistant to grazing-induced changes in plant community than the associated Clay Loam or Loamy sites. The plant community can shift to a short-grass dominated site with very few mid-grasses if heavy grazing pressure is applied over several years. The shorter grass species such as blue grama, buffalograss (Bouteloua dactyloides), and even black grama are more resistant to grazing pressure. After many years of continuous heavy grazing, sideoats grama will still be found in the community, but will retreat to the protection of clumps of yucca, sand sagebrush, or other short shrubs. With good grazing management practices, selective brush management, and growing season rest, the balance between mid- and short-grasses can often be restored, but it will usually take a few years to do so. The more dominant the short-grass species become, the more difficult it is to restore the community balance. Some above-average precipitation in conjunction with prescribed grazing speeds this process a great deal. Careful grazing management techniques are essential to maintain the diversity and productivity of the reference community.

All the sites in this MLRA were historically grazed/browsed by bison, elk, pronghorn, and mule deer (along with numerous small herbivores such as prairie dogs, rabbits, ground squirrels, etc.) in pre-settlement times. The habits of the larger herbivores were semi-migratory and after grazing an area, they moved on to other localities where grazing resources were more attractive. Grazed areas received rest naturally and generally the recovery periods following grazing were adequate for vegetation to regain vigor and replenish reserves. The same locations might not have been grazed again for several months, and perhaps even years, depending on rainfall patterns and animals' movements. When grazing occurred, it was likely quite intense, and in some cases, the physical animal impact may have been significant, but the recovery period allowed for plant and soil resources to heal prior to being impacted again. This MLRA has always had fewer water resources than vegetative regions to the east, and this may have had some influence on the magnitude of the large herd effect of bison on grazing resources. The huge seasonal migrations of bison required dependable water sources that could supply sufficient water for large numbers of animals and be reached within a day or two. The drier western parts of the plains in which this MLRA resides do not have nearly as many water sources as do the Texas Rolling Plains located to the east of the Llano Estacado. However, the grasslands in this MLRA did develop under an ecology that included native grazing animals as an integral part of the processes. With settlement of the area, the development of the ranching industry, and especially the advent of barbed wire, the confinement of domestic livestock on smaller areas has had great effect on plant communities. Free-ranging animals that moved at will depending on locally available forage provided a graze/rest cycle that allowed the natural recovery of native plants, whereas the system of more continuous grazing employed post-settlement, usually does not meet the overall needs of native vegetation unless careful management practices are implemented. Also, there is considerable evidence that overstocking of the range occurred in the late 1800’s and early 1900’s, and, unfortunately, it still does today in some places. Much of the rangeland abuse that occurred was due to lack of knowledge of the capability of the resources.

Natural fire has also had an impact on these grasslands historically. It is accepted that natural fires occurred on the plains fairly frequently, at least every 8 to 10 years, with some areas burning even more frequently. The sites most likely to be burned were those where grass fuel was most abundant. The sandy loam site has historically produced moderate amounts of fine fuel, and its physiographic features would suggest that fire may have occurred with some regularity. The type of adjacent sites and the general topographic and vegetative attributes of those sites also had a major effect on the frequency of fire. In this MLRA, there are sites that exhibit large amounts of bare soil and rocky terrain with sparse plant cover, as well as sites that have physical barriers such as gullies, streambeds, etc. that could have limited fire movement and continuity. It is possible to say that where fire occurred with regularity, it had a major impact on maintaining grasslands and retarded the encroachment of woody shrubs and cacti in many areas. Fire also influenced the grazing habits of herbivores, as they were attracted to the fresh and succulent vegetation that often resulted after a natural fire. Community diversity, especially an increase in forb species most likely resulted from a natural fire. Prescribed fire can be utilized on certain sites under proper conditions as a management tool. It can, in some instances be used to retard the proliferation of certain woody species, and can be used on coarse grasses such as alkali sacaton (Sporobolus airoides), giant sacaton (Sporobolus wrightii), or tobosagrass (Pleuraphis mutica) to increase palatability. It should be noted that fire followed by a series of below average rainfall years can definitely have an adverse effect on some perennial grass species. Extreme care and attention to environmental factors should always be exercised when using fire as a tool, and becomes even more important in arid and semiarid regions.

When the ecological principles affecting plant communities are understood and when proper rangeland management practices such as prescribed grazing and selective control of invasive species are implemented, the native range can often approach the diversity and productive capacity of the reference plant community. Even abused ranges can recovery with time and good management.

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.

STATE AND TRANSITIONAL PATHWAYS: (DIAGRAM)

Narrative:
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.