THE TRUE PRAIRIE. 125 



SOCIETIES. 



Nature. The societies of the grassland formation are constituted by 

 perennial herbs which give a distinct impress to large areas of the grass cover. 

 As already indicated, they show a dominance which is subordinate to that of 

 the grasses, and hence are termed subdominants. Originally the term was 

 employed for all conspicuous subdominants of wide range (Clements, 1905). 

 As the importance of the distinction between climax and developmental 

 communities became manifest, the society was restricted to the climax, and 

 the corresponding term socies was used for the successional subdominants. 

 In the superficial study of an association, subdominants of all sorts will be 

 found to alternate and mix with each other. All such communities appear to be 

 societies, until a study of succession reveals the fact that some are relatively 

 permanent while others are temporary, and many indeed persist for only a 

 few y?ars. Where disturbance is continuous or recurrent, as in grazing, 

 temporary societies or socies persist as long as the disturbance lasts, and their 

 real character can be determined only by protected quadrats or by com- 

 parison with undisturbed areas. In the majority of such cases, however, 

 socies can be recognized by the fact that they are composed of species of 

 annual or biennial habit. 



Control of dominants. The subdominance of a society is necessarily limited 

 by that of the grass dominants. In grassland, water is the primary limiting 

 factor, and determines the competition between dominants and subdominants 

 as well as within the corresponding communities. The fact that the two 

 belong to distinct vegetation-forms means that they avoid competition in so 

 far as possible by making different demands and at different times. Theoreti- 

 cally, the grass dominants should gradually gain the advantage over the sub- 

 dominants and replace the latter completely. This is especially true of legume 

 societies, the reaction of which greatly stimulates the growth of grasses. Such 

 an outcome is frequent in the Bouteloua gracilis, Bulbilis, and Agropyrum 

 glaucum consociations, wherever the dense turf holds the water in the upper 

 soil layer. In such cases, the grass roots absorb practically all of it and leave 

 little or none for the deeper-rooted herbs (Shantz, 1911 : 51; Weaver, 1919 : 

 51). As a consequence, the number and extent of societies depend primarily 

 upon rainfall. Where the rainfall is from 25 to 40 inches and the evaporation 

 correspondingly low, societies will usually be so numerous and luxuriant as to 

 conceal or at least obscure the dominant grasses during much of the growing 

 season. As the rainfall decreases and the evaporation increases to the- west- 

 ward, the dominants will take more and more of the water-content, and the 

 number and extent of the societies will steadily diminish. The result is that 

 the true prairies (Stipa-Koeleria association) show the best development of 

 societies. The wealth of subdominants is partly due also to the fact that 

 the prairies have been able to draw almost equally upon the eastern and the 

 western floras. The mixed prairies have the same societies for the most part, 

 but they are reduced in number and even more in extent and density. This 

 is due partly to reduced rainfall and partly to the presence of the lower layer 

 of short-grasses and sedges. 



The bunch-grass prairie is much poorer in societies, on account of a low 

 winter precipitation. The poorest of all is the short-grass plains {Bulbilis- 



