PLAE'T SUCCESSIOi^r AISTD RANGE MAXAGBMENT. 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-yeliow-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 bmiched 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-gra.ss-yellow-brush areas than on fully stocked 

 areas of the wheat-gi-ass 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 s]Decies 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 IcuTb- 

 idosa), sweet sage {Ai^temisia discolor), and blue foxglove {Pent- 

 stemon frocei^Ji) 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 entirel}" 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 



