DESERT CLIMAXES. 235 



30 feet of the lake level. Records show a fluctuation of 16 feet in the lake 

 level during the last forty years. 



"The gradual recession of the lake in the later stages produced great salt 

 flats in which the salt content of the soil exceeds 2.5 per cent. On these salt 

 flats one of the three following types of vegetation has developed. Salicornia 

 rubra often forms a pure association over rather extensive areas. The estab- 

 lishment of these annual plants is doubtless greatly favored by an unusual 

 supply of fresh water in the form of rain. The established plants withstand 

 excessive amounts of salt, the average salt content of the soil being about 2.5 

 per cent. Salicornia utahensis forms the first perennial stage in many places 

 and occurs on extremely saline soil (2.5 per cent). Usually this plant occurs 

 as low hummocks, but at other places forms a continuous even cover. It is 

 succeeded by Allenrolfea as the soil becomes somewhat less strongly saline. 

 Allenrolfea in many places enters directly on the salt flats, representing the 

 initial stage of the succession. In such places the salt content is somewhat 

 lower than where Salicornia enters. Allenrolfea occurs on land which has a salt 

 content of about 1 to 1.1 per cent, while Salicornia occupies land of more than 

 2 per cent salt. 



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

 The more natural line of development 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. Allerir 

 rolfea, when the ground-water is not too close, is gradually replaced by Sarco- 

 hatus. 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 Atri- 

 plex association is not readily replaced in Tooele Valley. The soil is rather 

 strongly saUne 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 replac- 

 ing the Atriplex as the conditions become more favorable for plant growth. 



"Kochia, which occurs on land of unusually heavy texture, has not been 

 included in the sketch here presented. It represents the most extreme condi- 

 tion 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 on this land is very 

 great and it is consequently very slowly leached. If a salt flat could be lifted 

 above the level influenced by ground-water, and shghtly leached, especially 

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

 Kochia areas of the valley. Since 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 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 proxinuty of the Artemisia and Kochia areas. A more natural 

 change would be iiomKochm to Atriplex, and from Atriplex to Artemisia 



The whole vaUey hes below the shore Une of Lake Bonneville, and as the 

 water gradually receded the land now above the lake level was graduallv 

 exposed. Bonneville was not a salt lake, and it is probable that much of the saee 

 land m the vaUey was exposed before the lake became salt. Much of the sage 



