Geoscience Reference
In-Depth Information
Fig. 6.1. Ponderosa pine grows on ridgetops adjacent to this
mixed-grass prairie on the western Great Plains; cottonwood
sometimes grows along creeks. the tan grass in the fore-
ground is little bluestem. other common plants are blue
grama, Sandberg bluegrass, western wheatgrass, junegrass,
needle-and-thread grass, and fringed sagewort. this grass-
land is near the Black Hills, at an elevation of about
4,500 feet.
herbivores, nitrogen enrichment from atmospheric fall-
out in the form of dust and rain, and elevated levels of
carbon dioxide in the atmosphere. 4
Wherever grasslands occur, plant species composi-
tion varies with changes in topographic position, such
as from hilltops to valley bottoms. Valley bottom soils
are deeper, finer-textured, moister, and more fertile than
soils on hilltops. topography also affects snow drifting,
with more snow accumulating in ravines and on leeward
slopes, causing them to be wetter than the surrounding
area. Depressions or playas with little or no drainage are
common, and typically they have saline or alkaline soils.
Differences in soil depth, salinity, and texture cause con-
siderable variation in what sometimes appears as a uni-
form vegetation cover—monotonous during mid-day but
inspiring when the sun is low in the sky.
other patterns occur on a finer scale. For example,
cloning from root crowns, rhizomes, and stolons some-
times causes the formation of patches that are one to
several yards across (such as those for blue grama, prairie
sandreed, and yucca). Another cause of patchiness is the
burrowing of animals, such as badgers, ground squir-
rels, harvester ants, pocket gophers, and prairie dogs.
their small disturbances favor some plants over others.
Also, “fairy rings” may develop—an interesting but
poorly understood phenomenon thought to be caused
by saprophytic fungi (fig. 6.2). 5
Surviving in the Grassland Environment
Most of the grassland ecosystem is hidden in the
soil, where 75 percent or more of the plant biomass
is found and, not surprisingly, most of the herbi-
vores also (fig. 6.3).6 6.3). 6 extensive root systems, required
to obtain water for the relatively small aboveground
shoots in this semi-arid environment, provide food
 
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