PLANT SUCCESSIOlSr AND RANGE MANAGEMENT. 15 



As in the case of the turfed area, the moisture content on tlie bunch- 

 grass area became unavailable, except for a few days in the super- 

 ficial layer between August 20 and September 1. 



THE EFFECT OF DISTURBING FACTORS. 



The opening up hj excessive grazing or otherwise of a well-estab- 

 lished stand of small wheat grass, as well as of congeneric turfed 

 species, is congenial to the immediate establishment of a rather scat- 

 tered growth of other plants, both deep-rooted and shallow-rooted, 

 provided, of course, that seed is available and growth is not seriously 

 hampered. Where the fertility of the soil is not appreciably changed 

 as a result of the destruction of the wheat-grass turf, several shallow- 

 rooted species and a few aggressive deep-rooted plants soon make 

 their appearance. Obviously, the shallow-rooted species, as a result 

 of both aerial and subterranean competition, sooner or later yield to 

 the invasion of the more permanent and luxuriant deep-rooted plants. 



Among the deeper-rooted perennials which gain a foothold early in 

 the destruction of the matlike growth of wheat grasses where the fer- 

 tility of the soil is not appreciably decreased, yellow brush (Chryso- 

 thamnus lanceolatiis) is the most characteristic. Of the perennial 

 grasses, small mountain porcupine griss {Stipa minor) is the most 

 characteristic. These two plants are among the first of the deep- 

 rooted- perennials to signify the waning of the wheat-grass cover. 

 Where the wheat-grass type is relatively young it often supports an 

 occasional plant of yellow brush which may struggle along for several 

 years in competition for water with the superficial roots of the wheat 

 grass.^ In due time the j^ellow brush, approximately 90 per cent of 

 whose root-absorbing surface is below that of the wheat grass, gives 

 away; but when the stand is opened up any remaining straggling 

 yellow-brush specimens quickly regain their luxuriance of growth. 

 (Fig. 6.) 



Section A of figure 6 pqrtrays a relatively young stand of small 

 wheat grass where yellow OTUsh was conspicuous prior to the estab- 

 lishment of the wheat grass subclimax. Section B shows the incom- 

 ing of small wheat grass from seed and the unhampered growth of 

 yellow brush. Should the wheat-grass subclimax (section A) again 

 be destroyed, or the stand sufficiently opened up to favor the perco- 

 lation of a considerable portion of the rainfall to a depth corre- 

 sponding to the location of the main feeding roots of the yellow 

 brush, the surviving specimens of the latter would immediately show 

 a remarkable response in growth. An increasing abundance of yel- 



1 While yellow brush is persistent in competition with other species, it is not believed 

 to be a very long-lived species. Numerous stem examinations have shown that it seldom 

 attains an age of much more than 10 years. A single specimen with 12 annual rings was 

 found. 



