Grasslands of the World.pdf - Disasters and Conflicts - UNEP

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Figure 6.5
Average January and July
temperatures for various sites
on a south-north transect in
the Great Plains.
Grasslands of the world
tall-grass prairie occurs mostly as isolated tracts, such as the Osage Hills in
Oklahoma and the Flint Hills in Kansas.
TOPOGRAPHY AND SOILS
Topographically, the Great Plains are relatively level, but minor topographic
variations are important in influencing plant species distributions and other
inclusions within the grassland . Often, poorly drained sites in depressions,
either with or without standing water, offer habitats for plants and animals not
found in adjacent grassland.
Grassland soils have developed from a variety of parent materials: limestone,
sandstone, shale, metamorphic and igneous outwash, and loess (Buol,
Hole and McCracken, 1980; Dodd and Lauenroth, 1979; Miller and Donohue,
1990; Sims, Singh and Lauenroth, 1978). The major soils are mollisols, deep
soils with dark A horizons and high (>50 percent) base saturation (especially
calcium) (Miller and Donohue, 1990). Surprisingly, the A horizon has a clay
content nearly equal to that of the B horizon (Baxter and Hole, 1967). Several
soil organisms including a common prairie ant (Formica cinerea) are apparently
involved in translocation of clays from the B horizon to the A (Buol, Hole
and McCracken, 1980). Buol, Hole and McCracken (1980) describe the soil
forming process for mollisols as “melanization”. This process consists of five
specific processes (Hole and Nielsen, 1968):
1. Growth of plant roots into the soil profile.
2. Partial decay of organic material in the soil.
3. Mixing of the soil by soil micro-organisms.
4. Eluviation and illuviation of organic colloids and some inorganic colloids.