Ecological dynamics

The soils of the Clay Slopes Ecological Site are generally deep and well drained. Runoff is rapid. Permeability is slow and available water capacity is high. The rooting zone is deep, but the clay layers might restrict rooting depth. Quality of forage plants is generally high. The site responds to good grazing management and is moderately resilient.

The reference plant community is the Mixed-grass Prairie Community (1.1) . Pre-settlement disturbances included grazing or browsing by endemic pronghorn antelope, deer and migratory bison, severe droughts and frequent fires. Wildfires are thought to have occurred at 4 to 12 year intervals in this region (Frost 1998). The frequent fires, weather patterns and relatively frequent droughts generally favored grasses over woody plants, but there was also a wide variety of forbs. Big and little bluestem were the dominant grasses throughout the MLRA, contributing as much as 40 percent of the annual production. Due to higher precipitation and cooler climate in the northern and eastern portions of the area, big bluestem was generally more abundant in that portion of the area. Other characteristic grasses in the reference community included sideoats grama, Arizona cottontop, vine-mesquite, tall dropseed, Texas wintergrass and meadow dropseed. Buffalograss and curly-mesquite were the most common shortgrasses. Occasionally, wet winters and springs caused an abundance of Texas wintergrass and cool-season annuals. See the Plant Composition and Annual Production Table below for estimated composition of the species.

The Mixed-grass Prairie Community (1.1) was relatively stable and resilient within the climate, grazing and fire regimes until European settlement in the mid 1800’s brought animal husbandry, windmills and fencing. The demise of the Native American Indians, the onset of the livestock industry and reduction of periodic fires changed the ecological dynamics of the vegetation on the Clay Slopes site. Although recent climatic warming trends and increases in atmospheric carbon dioxide may be enhancing vegetation change, the major forces influencing transition from the historic climax plant community to a woodland state are heavy continuous grazing by livestock and the decrease in frequency and intensity of fire (Brown and Smith 2000).

The intense grazing by cattle, sheep and goats in the late 1800’s began a transition in the physiognomy towards a Midgrass Prairie Community (1.2) encroaching woody species. As livestock increased and grazing use exceeded the plants ability to sustain defoliation, the more palatable and generally more productive species gave way to less palatable or more grazing resistant species. There was a decrease in the more palatable grasses and forbs and a reduction in intensity and frequency of fires. The frequency and intensity of fires was reduced because of less fire fuel and fire suppression. Fencing and water developments also helped bring about overgrazing by concentrating livestock.

When abusive grazing continues, the more palatable tall grasses give way initially to secondary species such as sideoats grama, tall dropseed, Arizona cottontop, vine-mesquite, and Texas wintergrass. The better quality forbs decrease while less palatable species, such as orange zexmenia, globemallow, eryngo, primrose, gaura and annuals increase. The shift in plant cover and decline in soil properties favors woody plant encroachment. The woody and herbaceous increasers are generally endemic species released from competition and fire suppression. Woody species such as mesquite, agarito, prickly pear and catclaw acacia increase in density and stature. In this phase Midgrasses, especially little bluestem and other mixed-grasses, still dominate annual herbage production, but the encroaching woody species increase in composition and production compared to the Mixed-grass Prairie Community (1.1).

If the Midgrass Prairie Community (1.2) is continually overgrazed, and fire is excluded, it transitions into a plant community dominated by woody plants. This process is amplified by droughts that occur at approximately 20-year intervals in the region. More grazing resistant grasses such as curly-mesquite, buffalograss, meadow dropseed and Texas wintergrass and less palatable forbs begin replacing the mixed-grasses. As the midgrass species cover declines, litter, mulch and soil organic matter decline and bare ground, erosion and other desertification processes increase. The microclimate in the grassland areas becomes more arid.

The Midgrass Prairie Community (1.2) transitions into a Shortgrass/Mixed-brush Community (2.1) when the woody plants exceed 10 to 15 percent canopy and/or the plants reach fire resistant age (about two years) and size (about four feet in height). This threshold also marks the beginning of a new state, the Woodland State (2.0), in which woody species dominant the site. Prescribed grazing or rest from grazing will generally not restore the grassland community without additional brush management

Mesquite generally dominates the Shortgrass/Mixed-brush Community (2.1), although oaks and juniper may often be present. The grass component is a mixture of low palatability mixed-grasses, shortgrasses and low palatability forbs. Meadow dropseed and threeawns increase with continued livestock overgrazing. Texas wintergrass, annual broomweed and annual brome also increase, especially during wet cycles. Exposed soil in open spaces crusts readily, subjecting the site to water erosion.

During this stage, the process of retrogression can be reversed with relatively inexpensive brush control practices and prescribed grazing management. If these practices are not applied and overgrazing continues, the woody canopy will continue to increase in dominance and ground cover and a woody-plant dominated community, the Mixed–brush/Shortgrass/Annuals Community (2.2) evolves. Once the brush canopy exceeds 30 to 40 percent, annual production for the understory is generally made up of unpalatable shrubs, cool-season grasses and annuals. Brushy species such as mesquite, prickly pear, tasajillo and agarito dominate production. Shortgrasses, cool-season grasses and annuals persist, but their production is limited.

Until maximum canopy cover by woody species is reached, water erosion can be a problem where soil is exposed. The exposed soil crusts readily, creating opportunity for further soil erosion. Considerable litter and soil movement occurs on exposed soil during heavy rainfall events. Interception losses increase with canopy cover and reduce the amount of soil moisture available for herbaceous production. Once canopy cover reaches potential, however, the hydrologic processes, energy flow and nutrient cycling stabilize.

Major high cost and high energy accelerated management practices are required to restore the Mixed-Brush/Shortgrass/Annuals Community (2.2) back to the Grassland State. Generally, mechanical or herbicidal brush management practices such as aerial spraying, dozing and/or individual plant treatments (IPT) along with other conservation practices such as range planting, prescribed grazing and prescribed burning are necessary for the ecological site to return to a grassland state.

The Clay Slopes site is suited primarily for range, but many acres have been put under cultivation. The soils on the lesser slopes are arable, especially with terracing. Many acres previously cultivated for crops have been returned to native or introduced grass species and are managed as pasture or range.

State and Transition Model:
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.