Ecological dynamics

The assumed Historic Climax Plant Community (HCPC) – (1.1) of the Clayey Ecological site of MLRA 70B consists of short- and mid-grasses, with a limited amount of forbs and a limited shrub component. The dominant grass species in the HCPC is galleta (Pleuraphis jamesii), with lesser amounts of blue grama (Bouteloua gracilis) and buffalograss (Bouteloua dactyloides). In some areas, alkali sacaton (Sporobolus airoides) can be prominent. Species found in small percentages include sand dropseed (Sporobolus cryptandrus), sand muhly (Muhlenbergia arenicola), silver bluestem (Bothriochloa laguroides), tumble windmillgrass (Chloris verticillata), Hall's panicum (Panicum hallii), white tridens (Tridens albescens), vine mesquite (Panicum obtusum), and western wheatgrass (Pascopyrum smithii). Forbs found in historic climax include rushpea (Hoffmannseggia densiflora), scarlet globemallow (Sphaeralcea coccinea), lyreleaf greeneyes (Berlandiera larata), upright prairie coneflower (Ratibida columnifera), annual broomweed (Amphiachyris dracunculoides), scarlet gaura (Gaura coccinea), wooly plantain (Plantago purshii), curlycup gumweed (Grindelia squarrosa), annual greenthread (Thelesperma filifolium), annual sunflower (Helianthus annuus), and a few other annual species. Forbs are estimated to be no more than 5 to 8 percent in the HCPC. The more common shrubs are mesquite (Prosopis glandulosa) and lotebush (Ziziphus obtusifolia), which are both sparse in the HCPC. Cholla cactus (Cylindropuntia imbricata) is the most prevalent cactus species and is generally scattered over the site in the HCPC, but can act as a strong increaser, becoming the dominant shrubby plant on this site, and increasing to greater than 350 plants per acre in some cases. Pricklypear (Opuntia phaecantha) is also found frequently. These cactus species seem well adapted to the clayey soil environment. Mormon tea (Ephedra spp.) and fourwing saltbush (Atriplex canescens) occur infrequently. The shrubby mesquite seldom reaches more than 6 feet in height. The only tree species present is an occasional hackberry (Celtis laevigata var. reticulata). Galleta will always make up the majority of the total annual production for the site, but as long-term grazing pressure increases, the blue grama and buffalograss nearly always decrease somewhat. Alkali sacaton may be more prevalent on portions of the site that receive more natural runoff. The productive capacity of the site in terms of biomass is quite high for this moisture regime.

Soil factors are very influential in the development of the HCPC. The clayey texture and the blocky structure of the subsoil limit the diversity and the nature of plant species present. These soils can store a great deal of soil moisture but this moisture is released to plants rather sparingly. Subsoils are very hard when dry and the blocky structure is somewhat restrictive to plant roots. When dry, deep cracks appear. When wet, these soils are plastic and sticky. Livestock traffic can cause severe compaction during wet periods, especially in areas where vegetative cover is poor. The plant species that occupy this site are drought tolerant and resistant to damage from grazing. They are able to send roots down into the hard blocky subsoil, extract tightly held soil moisture, and are able to endure the high shrink – swell characteristics of the soil. Species composition is not as diverse as on some of the adjacent sites. This site is generally very stable and the plant community is resistant to change caused by livestock grazing pressure and natural influences.

Climate and grazing pressure have major effects on plant community dynamics on this site. While this site is productive in the amount of plant material grown annually, it is not the most preferred site for livestock grazing. The galleta, which usually dominates the site, is not a highly palatable grass, and is somewhat coarse compared to blue grama and buffalograss. Digestibility of galleta is lower due to its coarse nature and animals naturally limit their consumption.

This is a major reason that the total biomass present on this site is usually more than other short grass sites. Livestock (cattle) will often heavily graze the more palatable blue grama and buffalograss occurring on the site and only lightly graze the galleta. This type of spot grazing favors the tobosa and probably results in an even more dominant stand over time. Galleta as well as alkali sacaton are more palatable in the early spring when the new growth is emerging, and cattle do well on it. However, by mid-summer, the leaves have matured and are becoming coarse and less palatable.

On this particular site, natural fire was almost certainly a major ecological influence on the plant community. Periodic burning renders galleta and alkali sacaton much more palatable for herbivores than in unburned areas. The post-burn new growth is relished by domestic livestock, as it most likely was by bison in pre-settlement days. Fires functioned as a means of moving concentrations of grazing animals to areas of more nutritious grazing resources. In addition, the fuel load produced by galleta and alkali sacaton on the clayey site is much greater and exhibits better fuel continuity than on other short grass sites. Fires would have been more intense, therefore causing greater damage to woody shrubs and cacti.

It is estimated that fires occurred on the average of every 8 to 10 years in pristine times which would have also had a marked effect on keeping shrubs suppressed. Cactus in particular is quite susceptible to damage by fire, especially if fuel loads are in the 2000 pounds per acre range. Even if cacti are not completely killed by the burning process, insect damage post-burn nearly always occurs. It would reasonable to assume that periodic fire helped to maintain this site as a grassland community with only a few shrubs. In years of favorable soil moisture, prescribed burning is a useful tool in managing this site.

The Shortgrass Community (HCPC) (1.1) was relatively stable and resilient within the climate, soil, and fire regime. With heavy grazing pressure, long-term drought, , lack of fire, and invasion of cholla, this site can transition into the Shrub/Shortgrass Community phase (2.1). Long-term drought in conjunction with overgrazing is a principal cause of rangeland deterioration on this site and virtually all sites in this MLRA.

Another possible transitional pathway that occurs on this site when heavy continuous grazing, lack of fire, and brush invasion result in a significant increase in both mesquite and cholla, with shrub canopies approaching 15 to 25 percent. This community phase is the Shortgrass/Shrub Community (2.2).

Cholla cactus is a major increaser on this site. There was a small amount of cholla present in the HCPC, but in many places, this species has become a significant problem. Cholla can present a barrier to grazing if present in dense populations, and common ranching practices such as simply riding horseback through the heavily infested pastures can be treacherous. The increase of cholla does not seem to be altogether due to abusive grazing practices since it has increased significantly on properly grazed range as well as heavily grazed sites. Some historians speculate that cholla was spread by sheep grazing in the late 1800’s. Supposedly, the tunas would catch in the wool and be transported about. There were considerable sheep present in the area 150 years ago but few if any sheep are being grazed in the region today. Cholla seems to increase in drought cycles as does pricklypear. Mesquite and lotebush can increase on the site with long-term abusive grazing. The major increases of woody shrubs and cacti seem to follow episodic patterns with wet years following long-term drought being the prime times of seedling germination and establishment. Mesquite plants seem to remain rather small in stature on clayey sites, possibly due to less favorable plant, soil, water, and air relationships. This site is resistant to any major changes in plant community due to moderate grazing pressure. Vegetative shifts to different plant communities are usually confined to significant increases in cholla and/or mesquite.

Major deterioration in the grass component of the plant community can happen with severe abuse, but is somewhat rare. In the event of severe abuse, cattle are forced to consume the more unpalatable species, and it is possible to reduce cover to the point where bare soil increases and surface erosion accelerates, with gullies forming. If this occurs, large populations of annual weedy species can be expected to invade and production of perennial species is greatly lowered. This extreme scenario would constitute an irreversible threshold, and would not be common on this site with reasonable livestock 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.