120 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



though many of them are more characteristic of some associations than others. 

 The number of societies common to the whole formation or the major portion 

 of it is several times greater than the number peculiar to any one association. 

 The behavior of the subdominants seems fully as significant as that of the 

 dominants, when their much greater number and plasticity are taken into 

 account. 



Ecological relations. — The ecological relationship is indicated primarily 

 by the vegetation-form. All of the grass dominants are sod-formers, with the 

 exception of those of the bunch-grass association. These are all bunch- 

 grasses, and appear to be correlated with a winter precipitation which is 60 

 to 80 per cent of the total annual. The dominants of the prairie associations 

 possess a tall growth-form, usually 2 to 3 feet and often 3 to 5 feet high. As 

 the name indicates, the short-grass association consists of dominants regularly 

 1 to 2 feet high. These heights refer to the flowering stems, and the difference 

 between the tall-grass prairie and short-grass plains is even more striking 

 when the significant growth relations of the leaves are concerned. The leaves 

 of the buffalo-grass, Bulbilis dactyloides, are normally within 4 inches of the 

 soil, and those of grama, Bouteloua gracilis, within 4 to 8 inches. This applies 

 likewise to Carexfilifolia and Car ex stenophylla, which are often very important 

 constituents of the short-grass association and are sometimes more abundant 

 than the grasses. On the other hand, the basal leaves of Stipa and Agropyrum 

 are usually 8 to 15 inches high, and the leafy stems reach a height of 2.5 to 3.5 

 feet. This difference appears to be primarily one of water-content, more or 

 less emphasized by grazing. The height and leaf-length of Bouteloua in par- 

 ticular can be doubled or trebled under irrigation. In nature, the height of 

 the stems has been found to vary 100 per cent from a wet to a dry year. 

 Striking as this difference between short-grasses and tall-grasses appears to 

 be in the Great Plains, it disappears to a large extent in the desert plains of 

 New Mexico and Arizona, where Bouteloua and Aristida regularly reach 

 heights of 18 to 40 inches. The general ecological equivalence of the two 

 forms is also well shown in the Stipa-Bouteloua association, where Bouteloua 

 is frequently associated with Stipa as a layer, and Bulbilis with Agropyrum. 

 As would be expected, the tall-grasses tend to have deep roots and the short- 

 grasses shallower ones. In both cases this is largely determined by the depth 

 of available water and by the compactness of the soil (Shantz, 1911 : 40; 

 Weaver, 1915 : 274; 1917 : 56, 1919). 



Subdominants. — The subdominants are practically all long-lived perennial 

 herbs, in which the shoot and root have solved the problem of successful com- 

 petition with the grasses. Four fairly well-defined types may be recognized. 

 Perhaps the commonest is the type with tall bushy stems, such as Psoralea 

 tenuiflora, Amorpha canescens, Glycyrhiza lepidota, and Carduus undulatuS, 

 which both shade and overtop the grasses in some degree. A second type is 

 shown by such species as Petalostemon candidus, P. purpureus, Solidago 

 rigida, Lepachys columnaris, etc., in which several tall strict stems come from 

 one root. A third is illustrated by Eriogonum annuum, Helianthus rigidus, 

 and Gilia aggregata, with a single slender shoot overtopping the grasses. In 

 the fourth type, the stems form a tuft or mat-like mass, which dominates the 

 grass shoots; this is seen in Astragalus crassicarpus, Aragalus lamberti, Arte- 

 misia frigida, and Opuntia polyacantha. In Balsamorhiza, Solidago, Carduus, 



