28 BULLETIN 791, U. S. DEPARTMENT OF AGEICULTUKE. 



soil with its decreased moisture supply becomes unfavorable to the 

 maintenance of the original plant cover, and reproduction, both by 

 vegetative means and by seed, is greatly curtailed; but the condi- 

 tions produced strongly favor the rapid invasion and establishment 

 of the formerly suppressed blue foxglove, sweet sage, and yarrow, 

 and these with certain other plants soon become established. The 

 majority of these invading species feed in approximately the same 

 soil stratum as small mountain porcupine grass — that is, chiefly in 

 the upper foot or so of soil. 



The safest indications pointing toward the maintenance or per- 

 haps the progressive or higher development of the depleted and 

 thinned porcupine-grass-yellow-brush consociation, is an increas- 

 ing density and luxuriance of certain blue grasses and fescues, all of 

 which are shallow-rooted, and a decreasing abundance or entire ab- 

 sence of the brome grasses and other deep-rooted species character- 

 istic of earlier successional stages.^ This gradual elimination of the 

 deep-rooted species is, of course, accounted for by the fact that the 

 available moisture supply in the lower soil depth decreases in some- 

 what the same proportion as on the wheat-grass areas. Among the 

 blue grasses characteristically associated with the highest developed 

 stands of the porcupine-grass-yellow-brush type, Nevada blue grass 

 and little blue grass are the most conspicuous. In less abundance, 

 but in approximately the same stage in the succession, occur Buck- 

 ley's blue grass {Poa huckleyana) , and Fendler's blue grass {P. 

 fendleriana) . Malpais blue grass, on the other hand, usually reaches 

 its maximum abundance prior to the highest development of the 

 porcupine-grass-yellow-brush cover. Like many of the perennial 

 nongrasslike species, it has all but disappeared when the porcupine- 

 grass and yellow-brush stand has attained its maximum density.^ 



When the porcupine-grass-yellow-brush consociation has prepared 

 the way for the invasion and establishment of the wheat-grass type, 

 porcupine grass is usually more abundant than yellow brush, and 

 competition of a more or less serious character occurs between the 

 porcupine-grass and the yellow-brush plants. As the bunches of 

 porcupine grass increase in number and size, the rate and depth of 

 the percolation of rainfall into the soil greatly decrease. This re- 



1 In general the brome grasses are relatively low in the cycle of succession. They 

 usually precede the blue grasses, fescues, and porcupine grasses. Likewise, the blue grasses 

 and fescues usually precede the porcupine grasses, though this varies somewhat with the 

 species. Because of the exceptionally strong seed habits of porcupine grass and the fact 

 that the seeds are self-planted, and a good stand of seedlings is therefore assured under 

 favorable conditions of soil and moisture, a somewhat general belief prevails that 

 porcupine grass may precede the brome grasses, the blue grasses, the fescues, and certain 

 other grasses in the succession. Detailed quadrat data have proved this belief erroneous. 



2 The approach toward the highest development of the porcupine-grass-yellow-brush type 

 can usually be recognized by the presence of at least a scattered cover of wheat grasses, 

 of which violet wheat grass is usually the first to appear. 



