PLANT SUCCESSION AND EANGE MANAGEMENT. 27 
SOIL WATER CONTENT. 
Specific measurements of the water content of the soil have brought 
out two interesting facts. First, the water-holding capacity in the 
upper foot of soil on a well-established stand of porcupine grass 
and yellow brush is less than on similar areas where the wheat-grass 
type is equally well established. The average of 15 soil samples 
obtained in 1915 of soils supporting a turfed cover of wheat grass 
was 11.2 per cent higher than the average of the same number of soil 
samples on the porcupine-grass-yellow-brush area previously oc- 
cupied by turfed wheat grass. Soil samples taken on the same areas 
in 1916 and 1917, as in the preceding year, gave practically the same 
relative figures. Likewise, the same number of soil samples, repre- 
senting the bunched wheat-grass cover showed an average of 4.6 per 
cent more moisture than that of the porcupine-grass-yellow-brush 
cover. Second, the average available water content of the soil when 
saturated was less on the porcupine-grass-yellow-brush areas than on 
the wheat-grass lands ; and, as might be expected, the available water 
content was exhausted correspondingly earlier in the season. There- 
fore, on an average, growth is arrested somewhat earlier on fully 
stocked porcupine-grass-yellow-brush areas than on fully stocked 
areas of the wheat-grass type. 
THE EFFECT OF DISTURBING FACTORS. 
The most reliable indication of the presence of conditions adverse 
to the perpetuation and maintenance of the highest development of 
the porcupine-grass-yellow-brush consociation, including its less 
stable cover of secondary species, is the replacement of one or both 
of the dominant species by other aggressive plants, chiefly nongrass- 
like species. As shown in figure 9, there is normally present on the 
porcupine-grass-yellow-brush areas a more or less scattered stand 
of plants of the second- weed stage, of which yarrow (Achillea Ian- 
ulosa), sweet sage (Artemisia discolor), and blue foxglove (Pent- 
stemon procerus) are the most typical. These species are almost in- 
variably among the first of the more permanent nongrasslike plants 
to increase in abundance as the porcupine grass and yellow brush 
are killed out. Because they reproduce almost entirely by vegetative 
means from long rootstocks, these nongrasslike plants probably 
increase more rapidly than any other perennial nongrasslike 
species. Accordingly, they may be declared the most reliable indi- 
cators of the presence of some factor, or combination of factors, ad- 
verse to the porcupine-grass and yellow-brush stand with its many 
desirable associated species. For a time the dead or dying porcupine- 
grass-yellow-brush cover is replaced by plants of the same species. 
As the unfavorable conditions continue their play, however, the 
