. Carnegie Institution of Washington publication. THE PRAIRIE-PLAINS GRASSLAND FORMATION. 15 Winner in South Dakota, and Long Pine and McCook in Nebraska. Along the west it occurs from near Calgary, Alberta, southward to Lewiston and Billings, Montana, Douglas and Laramie, Wyoming, and Colorado Springs and Trini- dad, Colorado. Beyond the eastern limit, Bouteloua and Bulbilis merely per- sist as alternes in xerophytic situations in the midst of the ; (Cf. Pound and Clements, 1900; Shantz, 1906, 1911, 1917; Pool, 1914; Weaver, 1919; and Clements, 1920.) In concluding this summary s
. Carnegie Institution of Washington publication. THE PRAIRIE-PLAINS GRASSLAND FORMATION. 15 Winner in South Dakota, and Long Pine and McCook in Nebraska. Along the west it occurs from near Calgary, Alberta, southward to Lewiston and Billings, Montana, Douglas and Laramie, Wyoming, and Colorado Springs and Trini- dad, Colorado. Beyond the eastern limit, Bouteloua and Bulbilis merely per- sist as alternes in xerophytic situations in the midst of the ; (Cf. Pound and Clements, 1900; Shantz, 1906, 1911, 1917; Pool, 1914; Weaver, 1919; and Clements, 1920.) In concluding this summary statement of the grassland associations, it may be well to point out that while the major factor is water-content, precipitation and evaporation are so greatly modified by edaphic con- ditions that two types of vegetation are often found in juxtaposition. Thus at Limon, Colorado, native and crop plants in adjacent fields have been excavated, one lot from typical short-grass land, the other in sandy soil in typical mixed prairie. Even in the midst of the prairie region, at Lincoln, Nebraska, an epitome of decreasing rainfall and con- sequent mixed prairie and short-grass plains vegetation may be found on a single hillside (Weaver, 1919). The base is clothed with tall prairie grasses and herbs, while the gravelly crest, which is of glacial origin, is covered with a nearly pure growth of Bouteloua gracilis and B. hirsuta. On the upper slopes the short-grass layer is overtopped by Koderia cristata, Stipa spartea, and Andropogon scoparius. Here, as everywhere throughout the formation, root relations are important and root competition plays a decisive role. In "Ecological Relations of Roots" it has been pointed out that every plant association has a rather definite community root habit. The peculiar set of edaphic conditions of an association, especially its water-content, leaves its impress upon root distribution. Roots of true prairie show distinct differences in habit fro
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