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 Clay Bottomland Ecological Site occupies narrow bands of well drained soils on nearly level flood plains of the rivers and streams in MLRA 80A. The site may be inundated once or twice a year. Small depressions, from old channels, occur throughout. The soils are fertile, but the fine soil texture often restricts plant growth. Formerly called, and sometimes still referred to as, Heavy Bottomland, the reference plant community is tallgrass/midgrass. This plant community found on the site by settlers in the early 1800’s was a fire induced tallgrass, midgrass, shortgrass bottomland with a few scattered shrubs and trees along the drainage ways. This site evolved under frequent fire and periodic heavy grazing by bison, pronghorn antelope and deer. It is postulated that fires occurred as often as four to six-year intervals in this region (Frost 1998) prior to European settlement. The frequent fires favored grasses over woody plants and forbs. The fires were likely more influential in shaping the reference plant community into an open tallgrass plant community than the grazing by bison and pronghorn antelope and periodic droughts. There is a natural flux in vegetation composition within the reference plant community. This continuing flux is directly related to changes in the amount of water present on site. Amounts and species of tallgrasses, midgrasses, shortgrasses, and forbs are always in continuous change (dynamic equilibrium). This site is not only driven by local climate, but can also be affected by off-site precipitation accumulations that eventually drain into the area. Site plant community composition will generally be approximately 78% grasses and grasslike, 10% Forbs and 12% woodies. Individual tallgrass species composition will be approximately: switchgrass (15-35%), big bluestem (10-20%), Indiangrass (10-20%), and little bluestem (5-15%). Other grasses, ranging from a trace to 15%, are eastern gamagrass, prairie cordgrass, Florida paspalum, Canada wildrye, switchcane, broadleaf uniola, western wheatgrass, sideoats grama, sedges, Scribner's panicum, purpletop, fringeleaf paspalum, tall dropseed, meadow dropseed, longspike tridens, knotroot bristlegrass, beaded panicum, white tridens and Texas wintergrass. Smartweed and sedges occupied wetter depressions. Some of the common forbs were compassplant, tall gayfeather, Heath's aster, goldenrod, sagewort, leadplant, Illinois bundleflower and prairie scurfpea.

The tallgrass plant community was relatively stable and resilient within the climate, soil, and fire regime until European settlement in the 1800s. The mid 1800’s brought elimination of the bison herds, removal of the American Indian, and a large increase in domestic livestock. The development of the windmill and barbed wire fencing during the 1880’s promoted overgrazing throughout the region. Overstocking by domestic livestock induced a reduction of cattle-palatable grasses and forbs. Total herbage production declined as the grazing-resistant midgrasses and forbs began replacing the tallgrasses and climax forbs. There was a corresponding decline in vegetative ground cover, mulch, and soil organic matter. The shift in composition of the plant communities and decline in soil properties favored woody plant encroachment. This, along with the reduction in intensity and frequency of fires, allowed the invasion of species from adjacent sites and the increase of more grazing resistant species. Under this fire reduction scenario, the reference transitioned into a tallgrass/shrubland community. In this plant community, grasses still dominate, but the encroaching woody species begin to contribute increasing amounts to total annual vegetative production. During the dynamics of this stage, the transition to a shrubland can be reversed by mechanical and/or chemical brush control methods and prescribed grazing management that provides fine fuel loadings necessary for prescribed burning at the natural four- to six-year intervals. Prescribed burning generally does not kill mesquite once the plants reach greater than 2 years of age, but fire can suppress mesquite of any age if the fire can cause top kill. Easter redcedar can usually be controlled by fire, provided a good fuel load is present, until a height of approximately 6 feet, then, control becomes more difficult and more input extensive.

If fire is excluded, and the tallgrass community continues to be overgrazed, the transition toward woody plant dominance continues. In certain areas within MLRA 80A, salt cedar may also become invasive on this site. Grazing resistant grasses increase as cattle-palatable climax forbs and tallgrasses continue to decline. Grass cover, litter and soil organic matter decline as bare ground, erosion and other desertification processes increase. The microclimate in the grassland areas becomes more arid. When the woody plant component reaches 25 to 35 percent canopy, grazing management strategies, such as rest from grazing, generally may, or may not, reverse the transition. A combination of proper grazing and prescribed burning should be successful in, at least, maintaining this grass dominant community’s status quo. However with continued heavy livestock grazing and no brush management the tallgrass/midgrass community will continue its transition into a woody overstory of hardwood trees where woody plants dominate. American elm, oak, sumac, ash, walnut, pecan, Indigobush, bumelia, western soapberry, coralberry, hackberry and roughleaf dogwood are common trees.

Natural fertility, presence of shade, proximity to water, and nutritious forage make this site a preferred grazing area. The wet nature of the site protects it from grazing during wet conditions, but during dry conditions it is often the first site to be overused. If overgrazing continues and brush control practices are not applied, the woody canopy will increase in size and density until a dense woody plant dominant community develops. Dominance occurs at about 50 percent woody plant canopy cover. At this threshold, the grassland component will not produce enough fine fuel for fires to effectively suppress the woody plants. At this point, the site completes the transition into a new plant community type, the hardwood overstory/mixed-brush understory community. This state is dominated by hardwoods and mixed-brush to the exclusion of most climax herbaceous species except buffalograss and annuals in the woody plant interspaces. Once canopy cover exceeds 35 to 50 percent woody plants, forage production is very limited except in wet periods when annuals provide extra forage. Shortgrasses and cool-season grasses and forbs are present but sparse due to shading and competition from the woody plants. Easter redcedar, mesquite and understory brush continue to increase in size and density regardless of grazing management. Large areas of bare ground may appear between woody plants where small depressions occur. Desertification, including erosion, continues in the interspaces until maximum ground cover by woody species is approached. Once shrub cover reaches potential, the hydrologic processes, energy flow and nutrient cycling stabilize under the woody vegetation environment.

Major expense and energy are required to restore the hardwood and mixed-brush community back to a grassland plant community state. Restoration of site in this stage is very difficult to accomplish because of soil characteristics. An integrated approach is required. Mechanical or herbicide treatments such as dozing, individual plant treatments (IPT), herbicide spraying and range planting followed by grazing deferment, prescribed grazing and prescribed burning, are essential for the site to return to near the postulated historic climax community. The brushy species, namely redcedar and mesquite, are hard to control with herbicides on this site. Re-invasion occurs due to the residual seed bank. Mechanical control such as grubbing or root plowing can destroy the perennial grass cover and more often than not, annuals or broom snakeweed prevails for two or three years, even with reseeding. Radical land clearing with bulldozers may ultimately be necessary. The restoration process may take several years of repeated treatments. Therefore, maintaining the site in at least the hardwood 25 to 30% stage, or better, through proper stocking and brush management, including the use of prescribed burning, is highly recommended.

State and Transition Diagram:
A State and Transition Diagram for the Clay Bottomland (R080AY045OK) 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.