Page:Popular Science Monthly Volume 80.djvu/219

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THE GREAT AMERICAN DESERT

A continuous association composed of hunch-grasses is the typical vegetation of the whole Sand Hill region. This covers the hills and ridges over thousands of square miles, being absent only from the "blow-outs" and the moister valleys. Once established in the sandy soil the bunch-grasses cope very successfully with the fury of the wind and the shifting sand. However, if fire or over-grazing seriously reduces the bunches in size and vitality, subsequent winds may uproot and carry them away. But on the whole the bunch-grasses are very effective sand binders, and it would be a great calamity indeed if they

PSM V80 D219 Land damaged by overgrazing.png

Fig. 2. A frequent Sight: Over-grazing often Results in Bare Hills and Blowing Sand.

were to be removed and nothing substituted. It is due to them more than to any other single type of plants that the vegetation of the hills is enabled to persist. Within the shelter of the bunch-grass association scores of valuable species thrive that in its absence would never have found access to the region.

The bunch-grass par excellence is the little blue stem (Andropogon scoparius), but associated with it are others, such as sand grass (Calamovilfa longifolia), and needle grass (Stipa comata). Andropogon scoparius is the dominant species throughout the region, the other species being present only occasionally. It is the little blue stem that gives the first greenish hue to the sand hill landscape in the spring, and it is the same species that clothes the hills with the rich reddish-purple in the autumn and through the winter. Hall's blue stem (Andropogon hallii), common on the upper slopes of the hills and the tops of ridges, is usually of secondary importance. Its few tall whitish or bluish stems