Ecological dynamics

The information contained in the State and Transition Diagram (STD) and the Ecological Site Description was developed using archeological and historical data, professional experience, and scientific studies. The information presented is representative of a very complex set of plant communities. Not all scenarios or plants are included. Key indicator plants, animals and ecological processes are described to inform land management decisions.

The reference plant community of the Clay Flat 23-30” PZ Ecological Site is assumed to be a Midgrass Prairie Community (1.1). The vegetation developed under the influence of the prevailing climate, frequent fires and periodic herbivory by bison, pronghorn antelope, deer and insects. Before European settlement in the 1800s frequent wildfires kept woody and forb species at low levels. Historians postulate that fires, set either by American Indians or lightning, re-occurred at four to six-year intervals on the eastern part of the Texas Rolling Plains (Frost 1998). The prevailing climate and frequent, intense fires likely were more influential in shaping the plant community into an open Midgrass Prairie Community (1.1) than intermittent heavy grazing by bison and pronghorns and periodic droughts.

The site is quite droughty and the shrink-swell characteristics of the soil produce a gilgai micro-relief with micro-highs and micro-depressions adding diversity to the plant community. Deep cracks, which form when the soils are dry, limit the plant community to those species that can tolerate frequent root pruning. Sideoats grama, vine mesquite, tobosa blue grama and buffalograss, dominated the Midgrass Prairie Community (1.1) making up approximately 45 to 55 percent of the total herbage production. Tobosa and curly mesquite were common on the western and southern portion of the MLRA where annual precipitation averages less than 25 inches annually. Sideoats grama, vine mesquite, and buffalograss were the primary dominants in the northern portion. The frequent fires favored grasses, especially tobosa, over woody plants and forbs.

The Midgrass Prairie Community (1.1) was relatively stable and resilient within the climate, soil and fire regime until settlement. Settlement by Europeans in mid to late 1800s brought elimination of the bison herd, removal of the American Indian and a large increase of livestock. The development of the windmill and barbed wire fencing in the 1880s brought about overstocking of cattle, sheep and goats throughout the region (Archer 1994 and Smeins, et al. 1997).

Long-term overgrazing of the Clay Flat site causes a reduction of the more palatable grasses and forbs and an increase in less palatable grasses and forbs. When retrogression is cattle induced, vine mesquite, sideoats grama and other palatable grasses and preferred forbs decrease in the plant community. Tobosa and curlymesquite are the primary increasers on the western and southern portions of the MLRA, while buffalograss, silver bluestem and white tridens are the primary increasers in northern and eastern portions. Total herbage production declines as shrubs, shortgrasses and forbs replace the midgrasses. There is a concomitant decline in vegetative ground cover, mulch and soil organic matter. The shift in plant cover and decline in soil properties favors woody plant encroachment. This, along with the reduction in intensity and frequency of fires, allows invasion of species from adjacent sites or the increase of more grazing resistant endemic species.

Under continuous overgrazing by livestock the Midgrass Prairie Community transitions into a Mixed-Grass Prairie Community (1.2) with invading woody species. Tobosa is persistent where it occurs, but shortgrasses generally begin to dominate primary production in this phase. The encroaching woody species also contribute an increasing percentage of the annual production. With proper grazing and periodic prescribed burning, the Mixed-Grass Prairie Community can be maintained or the retrogression reversed. If however, it is continually overgrazed and fire is excluded as a disturbance, the process of succession proceeds toward woody plant dominants and replacement of the more preferred grasses with less preferred species. As grass cover declines, litter and soil organic matter decline and bare ground, erosion and other desertification processes increase. Increasing woody dominants are primarily mesquite and pricklypear. When the woody plant community is well established and exceeds 15 percent canopy, grazing management strategies, such as periodic rests from grazing, generally will not restore the community to grassland. When this threshold occurs, the site has transitioned into a new state, the Shrubland State (2) and a new plant community, the Shortgrass/Mixed-Brush Community (2.1).

In the absence of fires and the occurrence of periodic droughts, the mesquite, lotebush and pricklypear continue to increase in size and density regardless of grazing management. Initially, the grassland component is a mixture of curly mesquite, buffalograss, white tridens, tobosa and low quality forbs. With continued livestock abuse, these species are largely replaced by low quality species such as three-awns, red grama, whorled dropseed and annuals. Tobosa is persistent where it occurs. Cool season species, such as Texas wintergrass and winter annuals, particularly annual brome and annual broomweed increase. Once woody plant cover exceeds 35 to 40 percent, forage production becomes limited, being generally made up of unpalatable shrubs, tobosa, shortgrasses, forbs and annuals. This community is recognized as a Shrub/Shortgrass/Annuals Community (2.2). Production varies greatly from season to season, depending on timing and amount of precipitation. Wet winters and springs bring on cool-season annuals such as annual bromegrass, annual broomweed and mustards. Extended wet spells induce increases in Texas wintergrass.

During the retrogression process, desertification, including erosion, continues in the interspaces until maximum ground cover by woody species is approached. The microclimate becomes drier as interception losses increase with canopy cover. Once canopy cover reaches potential, however, the hydrologic processes, energy flow and nutrient cycling stabilize under the shrubland environment (Thurow 1991). In this state, the site is poor range for livestock and low quality deer habitat providing only cover and low quality browse and forbs.

Major expense and energy are required to restore Shrub/Shortgrass/Annuals Community (2.2) to a Grassland State (1.1 or 1.2). Generally mechanical or herbicidal treatments and range planting followed by grazing deferment, prescribed grazing and prescribed burning is required for the site to approach the historic climax community. Care must be exercised in using mechanical treatments that require re-seeding because annuals such as annual broomweed can reduce seeding success and persist for several years. The restoration process may take several years before the site reaches potential grassland production.

The Clay Flat site was sometimes cultivated for food, fiber and hay, creating a Cultivated State (3.1). Although some grain and winter cereal crops are planted today, most of the fields in the site are used as permanent pasture or game food plots. Adapted annuals, adapted perennials or native perennials should be used. Some areas originally planted to crops have been abandoned and let “go back” to native range without cultural or management inputs, creating a unique plant community expressing itself in the process of secondary ecological succession, depending upon the degree of soil degradation. Such areas generally re-establish naturally with species from adjacent areas. If the woody invaders are not controlled periodically with brush management, such as Individual Plant Treatment (IPT), woody species will eventually dominate the site through secondary succession.

State and Transition Diagram:
A State and Transition Diagram for the Clay Flat (R078CY095TX) site is depicted below. Thorough descriptions of each state, transition, and pathway follow the model. Experts base this model on available experimental research, field observations, professional consensus, and interpretations. It is likely to change as knowledge increases.
Plant communities will differ across the MLRA because of the natural variability in weather, soils, and aspect. The Reference Plant Community is not necessarily the management goal; other vegetative states may be desired plant communities as long as the Range Health assessments are in the moderate and above category.

The biological processes on this site are complex. Therefore, representative values are presented in a land management context. The species lists are representative and are not botanical descriptions of all species occurring, or potentially occurring, on this site. They are not intended to cover every situation or the full range of conditions, species, and responses for the site.

Composition by dry weight and percent canopy cover are provided to describing the functional groups. Most observers find it easier to visualize or estimate percent canopy for woody species (trees and shrubs).

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.