Central Mixed Grass Prairie

This mixedgrass prairie system ranges from South Dakota into the Rolling Plains and the western Edwards Plateau of Texas. It is bordered by the shortgrass prairie on its western edge and the tallgrass prairie to the east. The loessal regions in west-central Kansas and central Nebraska, the Red Hills region of south-central Kansas and northern Oklahoma are all located within this system. Because of its proximity to other ecoregions, this system contains elements from both shortgrass and tallgrass prairies, which combine to form the mixedgrass prairie ecological system throughout its range. The distribution, species richness and productivity of plant species within the mixedgrass ecological system is controlled primarily by environmental conditions, in particular soil moisture and topography. Grazing and fire are important dynamic processes in this system. The relative dominance of the various grass and forb species within different associations in the system also can strongly depend on the degree of natural or human disturbance. This system can contain grass species such as Bouteloua curtipendula, Schizachyrium scoparium, Andropogon gerardii, Hesperostipa comata, and Bouteloua gracilis, although the majority of the associations within the region are dominated by Pascopyrum smithii or Schizachyrium scoparium. Numerous forb and sedge species (Carex spp.) can also occur within the mixedgrass system in the Western Great Plains. Although forbs do not always significantly contribute to the canopy, they can be very important. Some dominant forb species include Ambrosia psilostachya (grazing increases dominance), Psoralidium tenuiflorum, Echinacea angustifolia, Helianthus species, and Ratibida columnifera. Oak species such as Quercus macrocarpa can occur also in areas protected from fire due to topographic position (usually moister north-facing slopes). This can cause an almost oak savanna situation in certain areas, although fire suppression may allow for a more closed canopy and expansion of bur oak beyond those sheltered areas. In those situations, further information will be needed to determine if those larger areas with a more closed canopy of bur oak should be considered part of Western Great Plains Dry Bur Oak Forest and Woodland (CES303.667). Likewise, within the mixedgrass system, small seeps may occur, especially during the wettest years. Although these are not considered a separate system, the suppression of fire within the region has enabled the invasion of native woody species such as Juniperus virginiana, Juniperus pinchotii, Ziziphus obtusifolia, Prosopis glandulosa, and also allowed for the establishment of Pinus ponderosa in some northern areas.

This system typically contains grass species such as Bouteloua curtipendula, Schizachyrium scoparium, Andropogon gerardii, Hesperostipa comata, Sporobolus heterolepis, and Bouteloua gracilis, although the majority of the associations within the region are dominated by Pascopyrum smithii or Schizachyrium scoparium. Isolated patches of Quercus macrocarpa also can occur. This system represents the common prairie type in the Rolling Plains of Texas (Elliott 2011). This prairie often has Schizachyrium scoparium as a dominant, with Bouteloua curtipendula, Bouteloua hirsuta, Bouteloua gracilis, Bouteloua dactyloides (= Buchloe dactyloides), Andropogon gerardii, Pascopyrum smithii, and Nassella leucotricha also commonly encountered. Grazing tends to favor shortgrass species such as Bouteloua dactyloides and Bouteloua gracilis. This system is frequently invaded by juniper (primarily Juniperus pinchotii) and Prosopis glandulosa (Elliott 2011).

Differences in topography and soil characteristics occur across the range of this system. It is often characterized by gently rolling to extremely hilly landscapes with soils developed from loess, shale, limestone or sandstone parent material, including Pennsylvanian formations of the Red Rolling Plains (Elliott 2011). Mollisol soils are most prevalent and range from silt loams and silty clay loams with sandy loams possible on the western edge of the range. The Red Hills region of Kansas and Oklahoma, which contains examples of this system, contains somewhat unique soil characteristics and has developed from a diversity of sources including red shale, red clay, sandy shale, siltstone, or sandstone. These soils have developed a characteristic reddish color from the primary material. These soils can consist of silt, loam, clay loam, or clay and can have textures ranging from a fine sandy loam to a more clayey surface. Ecological Sites include Clay Slopes, Loamy Prairie, Clayey Upland, Claypan Prairie, Sandy Loam, and Clay Loam (Elliott 2011).

Fire, grazing, and drought are the primary processes occurring within the system. The diversity in this mixedgrass system likely reflects both the short- and long-term responses of the vegetation to these often concurrent disturbance regimes (Collins and Barber 1985). Fire is not as common as in more fertile, well-watered tallgrass prairies further east but is still important. Fire-return intervals have been estimated at 5-10 years (K. Kindscher pers. comm.), but fires burn patchily across the landscape, consuming vegetation in some areas and missing others. This combined with the differential responses of species to burning results in greater diversity across the landscape (Wright 1974). Grazing by native ungulates, primarily bison (Bos bison) and small mammals, principally prairie dogs (Cynomys spp.) added a further degree of patchy disturbance to the mixedgrass prairie (Whicker and Detling 1988, Weltzin et al. 1997). Long-term precipitation variance affects diversity of the mixedgrass prairie, creating conditions more favorable to shortgrass species during droughts while allowing mixedgrass species to spread during wetter years (Albertson and Tomanek 1965).

In addition to loss through direct conversion to crop fields, farmland development has fragmented the natural landscape and has eliminated the large-scale processes of fire and grazing by native ungulates and small mammals that were necessary to maintain this system. Lack of fire, grazing, or mowing results in a decrease in productivity as sites accumulate more litter and native warm-season grasses become more dominant. Lack of fire allows tree cover to increase rapidly, especially on lower, more mesic slopes. This system is well-adapted to moderate grazing over time or heavy grazing for short periods, but when used as long-term pasture and with high stocking rates, many of the dominant native grasses are reduced or eliminated (Branson and Weaver 1953). Heavy haying or grazing, or if those are done consistently during the mid-summer months, negatively affects the dominant warm-season grasses by removing their biomass before they have flowered. Cool-season grasses and forbs which set seed earlier are favored by these activities. Native and non-native forbs, woody species, and C3 grasses increase in the absence of fire, especially when combined with grazing by livestock. Drier sites on hilltops or rocky soils persist longer, but mesic sites on lower slopes can be invaded by trees and shrubs after just several years without fire. Non-native grasses have been planted for forage on some sites, as well.

This system is found throughout the central and southern areas of the western Great Plains ranging from southern South Dakota into the Rolling Plains and western Edwards Plateau of Texas.