138 CLIMAX FORMATIONS OF WESTERN NORIH AMERICA. 



or five dominants are often found over large stretches also. The most common 

 are Stipa-Agropyrum-Bouteloua-Bulhilis and Agropyrum-SHpa-Koeleria-Bou- 

 teloua-Carex. Stipa viridula, as a sod-former, is the typical dominant of 

 broad swales and shallow valleys. It is more or less subclimax in habit and 

 hence is usually associated with Agropyrum. 



Sequence of dominants. ^The factor correlations of the dominants have 

 received little attention. For the present it must suffice to infer them from 

 the topographic and successional relations, and to check this by reference to 

 the behavior of the tall-grasses in the prairie and the short-grasses on the 

 plains. This establishes a fairly definite and practical sequence, though it 

 is impossible to assign accurate values to the correlations with water-content, 

 evaporation, and light. In addition to succession and topography, range, 

 growth-form, and subclimax dominants, such as Andropogon, Calamovilfa, 

 and BoiUelaua racemosa, are all in agreement in indicating that Stipa viridula 

 and Agropyrum are the most mesophytic and Bouteloua the most xerophytic. 

 The actual sequence is (1) Stipa viridula, (2) Agropyrum, (3) Koeleria, (4) 

 Stipa comaia, (5) BuUnlis, (6) Carex stenophylla, (7) C. filifolia, and (8) Bou- 

 telona gracilis. While the rainfall is 5 to 10 inches less and the evaporation 

 rate correspondingly greater than in the prairies, the water-content is but 

 little lower, owing chiefly to the lessened transpiration resulting from a 

 smaller population and a shorter season. This appears to be confirmed by the 

 deep roots, even of the short-grasses, indicating a fairly adequate water- 

 supply. It is the similarity of the root behavior which explains the close 

 equivalence of the seven dominants, as shown by the fact that four or more 

 frequently occur in the most intimate mixture and that practically every one 

 has been found with each of the others. This also explains why pure consoci- 

 ations are rare (plate 25). 



While the existence of the Stipa-BouUloua association has not been recog- 

 nized before, it is now clear that the bunch-grass formation and the grass 

 formation of high prairies and plains of Pound and Clements (1898; 1900: 

 354, 380-386) are essentially this association. Even at that time the close 

 similarity with the true prairies was clearly recognized, as the following shows: 



"The foothill grass formation has much in common with the prairie forma- 

 tion of region II. As one looks at the high rolling prairies in region IV cov- 

 ered with Stipa comata, from a distance the carpet of Stipa, variegated with 

 the profusely flowering astragali, lupines, and psoraleas which abound in it, 

 appears to be a piece out of the famiUar prairie of the eastern portion of the 

 State." 



This similarity is further emphasized by the fact that the societies are 

 largely the same, as shown by the following list. 



SOCIETIES OF THE MIXED PRAIRIE. 



By far the major number of subdominants is the same for the Stipa-Koeleria 

 and the Stipa-BouteUma associations. This would be expected from the 

 dominance of tall-grasses in both, indicating a deep penetration of water and 

 roots and hence a favorable soil for deep-rooted herbs. The mixed prairies 

 naturally lack such societies as Phlox pilosa, Baptisia leucophaea, Anemone 

 canadensis, etc., which are typically eastern, and they have added a few from 

 the west. The chief difference lies in the fact that the societies are less lux- 



