THE BASIN SAGEBRUSH. 159 



equivalence and sequence, and the instrumental results as of secondary 

 importance. The latter are indispensable but never paramount. 



Successional sequence. — A question naturally arises as to the possibility 

 of succession in a region of such low rainfall and in basins of such high salt- 

 content. Shantz (1916 : 235) has shown conclusively that succession is a 

 normal process, even in the most saline areas about Great Salt Lake: 



" Two lines of development are initiated by the Allenrolfea association. The 

 more natural line is brought about largely by the gradual lowering of the 

 ground-water level. As a result water is less and less supplied from the ground- 

 water, and more and more from the surface as rain. Allenrolfea, when the 

 ground-water is not too close, is gradually replaced by Sarcobatus, Suaeda 

 moquinii may follow Allenrolfea and be replaced in turn by Sarcobatus. As a 

 rule, Sarcobatus and Suaeda are mixed, the former being the most important 

 plant. Sarcobatus, which often forms a pure association in this valley, usually 

 forms a scattered growth, the interspaces being occupied by Atriplex. This 

 mixed association finally gives way to pure Atriplex when the ground-water is 

 no longer within the reach of Sarcobatus roots. The Atriplex association is not 

 readily replaced in the Tooele Valley. The soil is rather strongly alkaline 

 and is very slowly leached. No permanent type of vegetation stands between 

 this and the alkali-avoiding Artemisia in this Valley. Artemisia and Atriplex 

 are not sharply separated at the ecotone, and, although Artemisia is never 

 luxuriant along this line, there is no doubt that it is gradually replacing the 

 Atriplex as the conditions become more favorable for plant growth. 



"Kochia, which occurs on land of unusually heavy texture, represents the 

 most extreme conditions in the Valley in regard to the shortage of water, and 

 indicates the presence of 0.5 to 1 per cent salt below the first foot. The run-off 

 in this land is very great, and it is very slowly leached. If a salt flat could be 

 lifted above the level influenced by ground-water, and slightly leached, 

 especially in the surface foot, the conditions would be very similar to those in 

 the larger Kochia areas of the Valley. Since such conditions are not markedly 

 different from Atriplex land, Atriplex is slowly advancing along the broad 

 ecotone. In time, Atriplex will probably replace much of the Kochia. The 

 ecotone between Kochia and Artemisia is very sharp, and a great change occurs 

 in salt-content and the physical texture of the soil. When water drains over 

 land of this type, and where unusual leaching occurs, Artemisia enters directly 

 on Kochia land. This is due to the proximity of the Artemisia and Kochia 

 areas. A more natural change would be from Kochia to Atriplex, and from 

 Atriplex to Artemisia.' 



This account conforms essentially to the course of the halosere throughout 

 the sagebrush association. Sarcobatus and Chrysothamnus n. glabratus are the 

 chief subclimax dominants in saline valleys, though this role is usually taken 

 by Atriplex corrugata and A. nuttallii over the extensive gumbo plains derived 

 from such deposits as the Mancos and Steele shales. These are followed by 

 Tetradymia spinosa and this by Atriplex confertifolia, or by Chrysothamnus 

 nauseosus. The latter is next invaded by Grayia in some regions, and by such 

 low Artemisias as A. trifida or A. arbuscula in others. In still other areas, 

 Atriplex confertifolia is followed directly by Artemisia tridentata, often with 

 more or less Eurotia, Atriplex canescens, Chrysothamnus viscidiflorus, or 

 Gutierrezia. The general sequence, more or less modified by local conditions, 

 recurs in hundreds of valleys throughout the association. It not only con- 

 firms the successional movement, but explains the characteristic mixing 

 throughout (plate 34). 



