148 GENUS ARTEMISIA. 



in the mountains. While apparently always at high altitudes, it commonly occupies the 

 belt between the grassland and the higher, better drained, but less fertile slopes where 

 typica abounds. 



The least known of all the subspecies is bolanderi. This was based upon specimens 

 said to be gathered at Mono Pass, California, but perhaps they came from well down 

 the eastern side of the Sierra Nevada. It is most like trifida in shape and lobing of the 

 leaves, but has larger heads with more flowers and the white tomentum is loose, not 

 closely appressed as in that. The type collection consists only of flowering twigs and 

 the leaves are nearly all entire. The only other collection is one from 2,400 meters 

 altitude at Sand Flat, south of Mono Lake, California {Clements and Hall 11702). In 

 this the leaves are largely trifid, and it therefore seems likely that the lower ones of'the 

 type were also more generally cleft than in the portions preserved. The Sand Flat 

 plants were growing in the main belt of typical tridentata, but on an exceptionally cold 

 and bleak plain where they were competing with a low, turf-forming Sporobolus. It is 

 conceivable that these conditions are responsible for the modification of tridentata into 

 this form and that trifida was similarly produced along the northern limits of the range 

 of the species. A nearly identical form has been noted as minor variation 4 oi A. cana. 

 Except for the preponderance of entire leaves, which are also somewhat longer, this 

 duplicates bolanderi and indicates the possibility of the origin of morphologically equiva- 

 lent plants from quite different, although related, stocks. Figure 22 shows the manner 

 in which leaves of trifida and bolanderi sometimes come to look much alike (except 

 in the loose, white pubescence of the latter) and also the similarity of both to occasional 

 leaves of the variation from A. cana just mentioned. 



A recent suggestion by Smiley (Univ. Calif. Publ. Bot. 9:396, 1921) that bolanderi 

 may be a hybrid between typical tridentata and rothrocki will be further investigated in 

 the field. It is not believed, however, that the latter grows in the neighborhood of the 

 Sand Flat station cited above. 



A study has been made of variation in the shape of the inner bracts of the involucre 

 in subspecies bolanderi, since in the original description Gray described these as "nar- 

 rowly oblong" as contrasted with the "broad" bracts of A. cana, and this difference 

 has been used by later writers as a specific criterion. However, when dissected out 

 and carefully compared, it is found that the inner bracts of heads from the type col- 

 lection, although smaller, are not essentially different in shape from those commonly 

 found in A. cana (see fig. 23). 



ECOLOGY. 



Artemisia tridentata is typically a low shrub, but it ranges from a dwarf form less 

 than a foot high to a small tree 20 feet high. It is the characteristic dominant of the 

 sagebrush climax of the Great Basin, where it often forms a pure consociation over large 

 areas. It ranges far beyond the climax area into the mixed prairie and bunch-grass 

 prairie, especially where overgrazing has given it the advantage in competition with the 

 grasses. This is especially true of the bunch-grass association in eastern Oregon, north- 

 eastern California, and southern Idaho, which has been almost completely replaced by 

 sagebrush. The latter seems to have all the marks of a climax community, but the 

 abundance of the bunch-grass dominants in protected places makes it clear that the 

 sagebrush has become controlling only in the historical period as a result of overgrazing. 

 On the east the sagebrush makes a broad mictium with the mixed prairie in Wyoming, 

 in which it is favored by the overgrazing of the more palatable grasses. In the western 

 edge of the Dakotas and Nebraska it becomes a subclimax community of the more 

 stable valleys of the Bad Lands. The sagebrush scarcely reaches the plains of Colo- 

 rado, owing to the mountain barriers, but it is more or less abundant in the mixed 



