196 University of California, Puhlications in Botany V^oh. 7 



area Rabbit-brush is confined to certain definite tracts ranging in 

 size from a few square feet to about 800,000 acres. It is prevented 

 from growing in other places, sometimes by unfavorable soil, moisture, 

 or climatic condition ; sometimes by the more aggressive habits of other 

 species with which it is obliged to compete. For example, when the soil 

 is very strongly alkaline it is either devoid of vegetation or supports 

 such strongly alkali-tolerant species as Grease-wood (Sarcohatus), 

 Pickle-weed {Salicornia) , Sea Blite {Suaeda), and Spirostachys. 



Where the alkali is somewhat less in amount but still too abundant 

 for upland plants certain varieties of Rabbit-brush, particularly the 

 green forms such as viridulus, graveolen<f, and consimilis become dom- 

 inant and with Salt-grass (Distichlis) , Poverty -weed (Iva axillaris), 

 and similar plants cover areas of considerable extent. Here are found 

 the most extensive and important stands of rubber-producing shrub 

 and the percentage of rubber is fairly higli. Tlie soil is somewhat 

 moist but too strongly alkaline for standard agricultural crops and 

 since the native grasses and other forage and browse plants are of but 

 little use to stockmen the value of the land is very slight. The removal 

 of the Rabbit-brush would favor the growth of grazing plants, as has 

 been demonstrated by our experiments. However, the brush would 

 again assert itself in a few years unless held in check. If it is found 

 practicable to grow Rabbit-brush for its rubber, these extensive tracts 

 could be had at a very low price and without serious interference to 

 any established industry. 



Passing upwards from the alkaline flats just described one often 

 comes to better drained and scarcely alkaline slopes. Here the Sage- 

 brush {Artemisia tridcntata) is usually dominant and forms the ex- 

 tensive brush-lands of the Great Basin area. It mixes more or less 

 with the Rabbit-brush varieties mentioned above, but it is less alkali- 

 tolerant and so yields to them on the lowlands. On the slopes and 

 higher plains and especially in loose soil, or along dry water-ways, 

 the Sage-brush is often mixed with or replaced by another series of 

 varieties of Chrysothamnus. These are the gray forms, particularly 

 nmiseosus, speciosiis, gnaphalodcs, hololeucus, and frigidus. The 

 green and the gra}^ forms intermingle slightly where they meet in soils 

 of moderate alkalinity and moisture, but the latter are much more 

 intimatel.y associated with Sage-brush. In the terminology of the 

 ecologist, the Sage-brush is climax, the gray Rabbit-brush is subclimax. 

 In other words if the former is disturbed b}' fire, trampling, washout, 

 etc., it is replaced by the Rabbit-brush, which, however, is in turn 



