258 MINNESOTA BOTANICAL STUDIES 



occurs in eastern Colorado is great and that even during periods of 

 more than normal rainfall available soil-moisture is limited to a few 

 inches of the surface soil. He feels that it is because of this fact 

 that "the vegetation is composed of short grasses which have a great 

 number of roots limited to the surface foot or so of the soil. Most 

 deep-rooted species are then shut out by lack of soil-moisture in the 

 deeper layers of the soil and later-season plants are excluded because 

 available moisture is usually lacking, even in the surface layers, dur- 

 ing later summer and autumn." The surface soil becomes extremely 

 dry and hot shortly after midsummer so that from this time there 

 are very few species but the dominants to be found in the formation. 

 All of the more conspicuous principal and secondary species except 

 the cacti have disappeared by the fourth week in July. 



THE GRAMA-BUFFALO-GRASS ASSOCIATION 



The most typical form of the short-grass formation is rep- 

 resented by the grama-buffalo-grass association. If that portion of 

 the sandhills covered by the bunch-grasses is monotonous, then this 

 association presents a case of extreme monotony. This is the most 

 typically closed association of the sandhill uplands and this condi- 

 tion, which prevents the establishment of many secondary species, 

 is the cause of the extremely monotonous appearance of the short- 

 grass formation. The dominant plants are low and closely matted 

 in many places so that the ground is completely covered. Fre- 

 quently, however, the carpet-like cover and close tenacious sod are 

 broken here and there and within these light-colored soil areas 

 certain additional species become established or bare patches alter- 

 nate with the low patches of grasses. The grama-grass when 

 growing alone occupies somewhat more sandy soil and produces a 

 more open carpet than does the buffalo grass. When the latter 

 species occurs alone, as it often does in characteristically circum- 

 scribed patches, the ground is covered by a mat so close that 

 extremely few secondary species are enabled to establish themselves 

 within such areas. It is probably because of this close growth of 

 shallow-rooted species together with a lower available soil-moisture 

 that the secondary species of this association and of the formation 

 as a whole are much fewer in number than is true for the prairie- 

 grass formation. 



