South Central Mixed Grass Prairies

The South Central Mixed Prairies occur midway along a rainfall gradient, with tallgrass prairies dominating at the wetter, eastern end of the continuum and shortgrass prairies in the drier west. It is called “mixed” because it is an overlap zone, or ecotone, and includes plant species from both prairie types.

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Soil Organic Carbon Profile at Leasburg Cemetery, near Atwood, KS (Klopfenstein et al. 2015)

Plant production and Species Richness near Atwood (Tomanek and Albertson, 1957)

SOILS:         Soils were sampled in a western Kansas cemetery established prior to 1906, and therefore uncultivated for at least a century (Klopfenstein et al. 2015). Soil texture at the Leasburg cemetery site located near Atwood, KS was a silty loam with 18.7% sand, 70.8% silt and 10.4% clay. Average organic carbon concentration was a maximum of 3.5% at the surface, averaging 2.5% at 5 cm depth and then decreasing to 1.5% at 20 cm depth. Bulk density was 0.9 g/cm³ in the surface 10 cm, increasing to 1.2 g/cm³ at 20 cm in depth.

Similar soil properties had been documented 32 km (20 miles) eastward at Oberlin, KS where Honeycutt et al. (1990) measured soil profiles in a “never cultivated” mixed grass prairie at four positions along a topographic gradient. Soils were silty loam in the surface 40 cm with higher clay concentrations deeper in the profile. Soil organic carbon was ~2.0 % in surface 10 cm, with 1% at ~30 cm and decreasing with depth. They also reported that pH averaged 6.1 in the top 10 cm of the soil and increased to 7.5 at one meter depth. Total Phosphorus concentration was ~ 10 g/kg in the surface 20 cm, but then increased to a maximum at ~ 40 g/kg at  70 cm before declining again.

LITTER: Aboveground biomass varied along the topographic gradient; and was complemented by a “heavy mulch” or litter layer.

PLANTS:    The natural vegetation community of the south-central mixed prairie was documented in detail by Tomanek and Albertson (1957) at Atwood, KS. Sites were selected at three topographic positions in an ungrazed, mixed prairie, including uplands, slopes, and lowlands. These were compared to moderately and heavily grazed sites nearby. 12 species of grasses plus several sedges, were identified along transects in the ungrazed prairie (Table 1). Tall grasses formed a 1.5m (5 ft) canopy layer with a mixed grass understory 25cm (10 in) in height, above a lower layer of short grasses. Three-quarters of the forage was produced in the first half of the growing season.

Uplands:         Blue grama was the dominant grass on the ungrazed upland, with buffalo grass in the understory. The upper story also included side oats grama, big bluestem, little bluestem, red three awn, purple three awn, western wheatgrass and sand dropseed. Basal cover averaged only 28.6% but the interspaces between grasses were filled with a heavy mulch to a depth of 5cm (2 in) or more. The two short grasses comprised 60% of the plant cover.

Slopes:           Little bluestem and blue grama grasses increased considerably in abundance in midslope, with sideoats grama at 44.4%. Three awn grass was scattered throughout and buffalo grass only present in isolated small areas.

Lowlands:      Lowlands are the moistest habitat, collecting runoff and accumulating moisture-holding organic matter.  Here big bluestem dominated the vegetation, and the highest diversity of forbs was found. Pure stands of big bluestem were common. Big bluestem, along with sideoats grama made up 69% cover. There was low basal cover of 17.1% due to the  excessive mulch, which reached a depth of more than 30 cm (12 in) some places and average weight of 2.7 kg/m2 (12 tons/acre).

Table 1: Composition of Grass Species (%) on ungrazed sites associated with 3 topographic features at Atwood, KS (Tomanek and Albertson 1957)

Literature Cited

Honeycutt, C.W., R.D. Heil, and C.V. Cole 1990. Climatic and topographic relations of three Great Plains soils: II. carbon, nitrogen, and phosphorus. Soil Sci. Soc. Am. J. 54: 476-483.

Klopfenstein, S.T., D.R. Hirmas, and W.C. Johnson. 2015. Relationships between soil organic carbon and precipitation along a climosequence in loess-derived soils of the central great plains, USA.  Catena 133: 25.

Tomanek, G.W. and F.W. Albertson. 1957.   Variations in cover, composition, production, and roots of vegetation of two prairies in western Kansas.  Ecological Monographs 27(3):  267 – 281.