PLANT SUCCESSION AND RANGE MANAGEMENT. 15 
As in the case of the turfed area, the moisture content on the 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 by 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 ^Ohryso- 
thamvnus lanceolatus) 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. 1 In due time the yellow 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 portrays a relatively young stand of small 
wheat grass where yellow brush 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. 
