A. TRIDENTATA. 149 



prairie of several of the great parks. The mictium of sagebrush and mixed prairie is 

 a characteristic feature of northwestern New Mexico and northeastern Arizona. 



While the greatest contact of sagebrush is with the bunch-grass and mixed prairies, 

 it touches the desert scrub in Nevada and in a few places in desert California. It makes 

 occasional contacts with the coastal sagebrush in southern California and northern 

 Mexico, but usually as more or less isolated communities. It frequently constitutes 

 parks with the pinon-cedar woodland from Nevada to New Mexico, and these are 

 usually connected by a kind of sagebrush-cedar savannah. It sometimes bears a similar 

 relation to the lower portion of the yellow-pine woodland, and on the west slope of the 

 Rocky Mountains is often mixed with Petran chaparral. 



Fia. 23. — Inner bracts of the involucre of Artemisia tridentata holandtri and A. cana: a, b, e, from the type collection 

 of bolanderi (29805 UC); d, e, froaa typioil cana (70518 UC); /, g, also from typical cana (51692 UC). All X 8. 



The most frequent associates of A. tridentata are the other shrub dominants of the 

 sagebrush association. Chief among these is Atriplex confertifolia, followed closely by 

 Chrysothamnus nauseosus and viscidiflorus. Other important associates are Atriplex 

 canescens, Grayia spinosa, Tetradymia spinosa, and Eurotia lanata. Among the dwarf 

 shrubs the most important are Gutierrezia sarothrae and Kochia vestita, the former indi- 

 cating disturbance in some degree and the latter the presence of alkali. The sagebrush 

 proper is also frequently associated with one or more of its variads, such as A. <. arbus- 

 cula, nova, and trifida, as well as with other species of Artemisia, such as cana, rigida, 

 and spinescens. In practically all these cases, the different forms alternate rather than 

 mix intimately, corresponding to some factor difference, usually of water. For example, 

 A. cana is usually found below tridentata in valley or depressions, while the latter oc- 

 cupies small valleys and ravines below trifida. 



The typical form of the sagebrush is an indicator of deep soils of somewhat greater 

 water-content and largely free from alkali. While it occurs frequently with such halo- 

 phytes as Kochia and Sarcobatus, it either alternates with them or its working roots 

 probably occupy a less saline layer of the soil. It is found also on rocky slopes and 

 ridges, but usually in the deeper pockets of soil, and as a result of the higher water- 

 content due to the lack of competition. In the mixed prairie and especially in the 

 Bad Lands, it is an indicator of greater water-content. While the sagebrush does form 

 root-sprouts to some extent, this is not sufficient to make it an indicator of fire as a 

 rule, except when a dwarf form is produced. 



Of the subspecies of tridentata, one, parishi, resembles it in practically all ecological 

 respects, while the others, arbuscula, bolanderi, nova, rothrocki, and trifida, are char- 

 acteristically dwarfed, as is true also of the typical form in less favorable conditions. 

 They occupy the thinner or drier soils, with the exception of rothrocki, which prefers 

 meadows and depressions in the subalpine region, while typica occupies the drier slopes. 

 Two dwarf forms of the latter, one of recent and the other of more remote origin, are 

 typical indicators of fire and overgrazing, to which they doubtless owe their origin. 



