26 BULLETIN/ 791, U. S. DEPARTMENT OF AGRICULTURE. 
turn 
from the surface to a deptli of 3 feet or more that the substra- 
usually becomes desiccated from a few inches below the surface to 
the average depth of the longest roots at approximately the same 
time in the season. 
CONDITIONS OF GROWTH AND REPRODUCTION. 
Porcupine grass feeds in approximately the same soil stratum 
(fig. 9) as turfed wheat grass, the average maximum depth of 
the roots being about one-third that of the bunched wheat grass. 
Yellow brush, on the other hand, extends its roots to about the same 
depth as the bunched wheat grasses. Porcupine grass and yellow brush 
therefore enter into serious competition for water only where the 
porcupine-grass tufts occur so densely as to prevent ready percola- 
tion of water to the lower depth of soil, a condition which occurs 
somewhat commonly only on the older and fully stocked areas. 
Generally, the porcupine-grass-yellow-brush consociation is more 
open, at least below ground, than the bunched wheat-grass lands 
(compare figs. 7 and 9) ; hence it is characterized by a more 
rapid percolation of water through the soil than occurs in the 
bunched wheat-grass cover. For this reason there is less variation in 
the distribution of the water from the surface downward on a porcu- 
pine-grass-yellow-brush area than on an area supporting a normal 
stand of bunched wheat grass. The depth to which the precipitation 
penetrates on the sodded wheat-grass area is extremely shallow as 
compared with the depth of penetration on a bunched wheat-grass 
area or on a porcupine-grass-yellow-brush area. Therefore, it is 
clear that so far as the available soil water supply is concerned, con- 
ditions are far more favorable for the establishment of species of 
variable length and character of root system on the porcupine-grass- 
yellow-brush type than on the turfed wheat-grass areas. Likewise, 
owing to the more open stand and the shallow feeding roots of por- 
cupine grass, the soil water content, between 1 and 4 feet in depth, 
is available to a greater variety of plants other than grasses on this 
consociation than on a fully developed area of bunched wheat grass, 
the moisture supply for which must be obtained from the same soil 
depth as for the support- of other deep-rooted plants. 
While a relatively large proportion of the precipitation is ab- 
sorbed on the porcupine-grass-yellow-brush consociation, this cover, 
as in the case of the wheat-grass type, never occupies soils that re- 
main too moist for the promotion of vigorous- growth. During un- 
usually dry years, growth slows down markedly, a condition which 
results in the temporary disappearance of many of the secondary- 
species. Small mountain porcupine grass and yellow brush, how- 
ever, are persistent, though } T ellow brush yields more readily to the 
effects of soil desiccation than does its grass associate. 
