THE SAGEBRUSH CLIMAX. 



153 



give the sagebrush a varied aspect, and seem to call in question its value as a 

 distinct formation, especially in hilly and mountainous country, where it 

 mixes or alternates constantly with fragments of other climax communities. 

 As is shown below, however, the great central mass of the community leaves 

 no doubt as to its formational unity and rank. 



Unity of the formation. — The geographical unity is greater than that of 

 most other climaxes in that the sagebrush occupies a natural physiographic 

 unit, the Great Basin. While the most representative species, Artemisia 

 tridentata, extends far beyond the limits of the latter, the formation proper 

 does not. The Great Basin is likewise a climatic unit, and hence naturally 

 corresponds to its climax. It is hemmed in by the high mountains, and con- 

 tains by far the most extensive area with 5 to 10 inches of rainfall to be found 

 on the continent. The general rainfall limits are from 5 to 15 inches in the 

 interior, though to the eastward sagebrush mixes with or yields to grass as the 

 rainfall rises above 12 inches (fig. 5). 



Boise, Idaho 

 13 in. 



Fillmore, Utah 

 15 in. 



11 



Fig. 



5. — Monthly and total rainfall for representative localities in the Basin sage- 

 brush association. 



With respect to the component species, the unity of the climax is proved by 

 such widely ranging dominants as Artemisia tridentata, Chrysothamnus 

 nauseosus, Atriplex confertifolia, A. canescens, Gutierrezia sarothrae, and 

 Eurotia lanata. Of the 17 dominants, only 4 fail to occur throughout the 

 central mass of the formation as indicated by the limits of the Great Basin. 

 As to origin, the formation is characteristically southwestern. The main 

 body of dominants, which constitute the Atriplex-Artemisia association of the 

 Great Basin, seem to have moved northward at an early period, perhaps 

 before the Pleistocene, though they have probably undergone considerable 

 differentiation since that time. A more recent lateral development has pro- 

 duced the Salvia-Artemisia association of southern California and Lower 

 California. The latter found itself between the chaparral on the one hand 

 and the rapidly desiccating desert on the other, and has covered but a limited 

 area in comparison with the main association. Its relationship, however, is 

 clearly indicated by its frequent contact with Artemisia tridentata, and espe- 

 cially by its occupying the same position between the desert scrub or grass- 

 land and the chaparral formations that the Atriplex-Artemisia association 

 does. The floristic unity of the formation is conclusively indicated by the 



