62 A Study of the Vegetation of 



Its shrubby growth-form, numerous, erect, fascicled, and dense, 

 white-woolly branches and white-woolly leaves, combined with a 

 greater height-growth than most of its associates, make it a con- 

 spicuous plant in the landscape. The non-woolly variety, Chryso- 

 thamnus nauseosus graveolens, not less conspicuous, is also very 

 abundant. Both plants extend well up the warm valleys and 

 canyons but are practically absent from the high prairies eastward. 



Tetradymia canescens, with a growth form similar to the pre- 

 ceding half-shrubs, likewise lends a decidedly xerophytic tone to 

 the dry scab-lands. It reaches its eastward extension in this 

 region. 



A plant which forms a rather distinct consocies is Elymus 

 condensatits. On moist flood-plains this grass reaches a height 

 of 3-5 feet. In such situations the stout stems are densely 

 tufted and areas of many acres may be dominated by this species. 

 In deep soils on the dry hillsides it is not infrequent, but here it 

 usually occurs in more or less isolated clumps. 



In the eastward extension of the Agropyron community along 

 the rim-rock of the streams into higher altitudes, many of the 

 species being confined to the warmer valleys, drop out. In such 

 situations Balsamorhiza sagittata is often abundant between the 

 bunches of Agropyron. 



In conclusion it may be said that the plants of this consocies 

 live typically not only in a climate of lower rainfall but also one in 

 which the period of drought, owing to the warmer climate with a 

 growing season several weeks earlier, is correspondingly increased 

 as compared with that of the well developed prairies of the 

 higher altitudes. 



In the preceding pages I have treated the Festuca and Agro- 

 pyron communities as climax units of vegetation. Without ques- 

 tion, favorable changes of climatic conditions, such as a slight in- 

 crease in rainfall, would cause an extension of the Festuca com- 

 munity into the area now occupied by the Agropyron consocia- 

 tion. The latter in turn would extend its area downward and 

 westward into the desert scrub formation. Indeed, such move- 

 ments of plant populations are easily traced in the new areas of 

 rim-rock and scab-land where they are brought about as a result 



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