26 BULLETIN "791, U. S. DEPARTMENT OF AGRICULTUIIE. 



from the surface to a depth of 3 feet or more that the substratum 

 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- 

 j3ine-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-gi'ass- 

 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-yeliow-bnish 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, grovfth 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 yellow brush yields more readily to the 

 effects of soil desiccation than does its grass associate. 



