Date of this Version
Prairie Naturalist (December 1984) 16(4): 145-158.
Detailed studies on soil texture and moisture retention indicate a close association between edaphic features and the distribution and composition of plant communities along topographic gradients at Arapaho Prairie, a typical, semi-arid Nebraska Sandhills prairie. The vegetation characteristics of three major habitat types (ridge, slope, and valley) and several minor subtypes (swale, stable ridge, and eroding ridge) are recognized and quantitatively described. Texture analysis indicates that the soils of dune slopes and ridges are largely azonal and are very coarse with substantially lower fine fractions (silt-clay ~ 13-15%) than soils of the more lowland swale and valley sites where surfact silt-clay fractions can reach 20-25 %. Soil moisture contents at 0.1 bar (10-14% by volume) and 15 bars (3-4 %) are similarly lower for dune sands than the surface soils at the lowland sites (0.1 bar ~ 19-23% and 15 bars ~ 5-9%). These moisture characteristics result in potentially 50-100% more moisture stored in the surface lowland soils which is rapidly utilized by the shallow-rooted, drought-tolerant grasses Stipa comata, Agropyron smithii, and Bouteloua gracilis. Deeprooting grasses (Andropogon hallzi, Panicum virgatum, and Sorghastrum avenaceum), forbs (Helianthus rigidus and Petalostemon purpureum) and shrubs (Yucca glauca, Prunus besseyi, and Amorpha canescens) are more abundant on the coarse-textured dune sands where soil moisture is stored deep in the profile. On dune ridges Sorghastrum and Stipa appear to be mutually exclusive with their distributions closely related to small-scale variations in topographic relief and surface soil texture and moisture characteristics. For all vegetation types, gradient analysis of major grasses, forbs, and shrubs shows systematic replacement of individual species along topographic gradients which, in many cases, can be related to rooting morphology, physiological differences in water use, and spatial and temporal variation in soil moisture.