Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 251 



lated with the bunch-grass association, the chief association of the 

 prairie-grass formation as it occurs over the Nebraska sandhills. 



THE SPEAR-GRASS ASSOCIATION 



This association is one of the minor sandhills associations. In 

 its most clearly defined condition it is well represented in the "Stipa 

 Formation" of Pound and Clements (57) which reaches its highest 

 degree of development north and west of the sandhills. The asso- 

 ciation is therefore an eastern modification of the "Grass Formation 

 of the High Prairies" of the Phytogeography of Nebraska which is 

 met at frequent intervals in the western, northern, and eastern por- 

 tions of the sandhills region. It is also found as isolated patches in 

 dry valleys and upon stable hills in all parts of the region. In the 

 latter form this association is broken up to such a degree that it 

 frequently appears as a locally dominant type within the bunch- 

 grass association. 



The tendency toward the development of a more open associa- 

 tion, somewhat simulating the bunch-grass condition, is the com- 

 monest expression of this association as it is found in the sand-hills 

 as contrasted with the more distinctly sod-forming habit that Pound 

 and Clements (57) noted farther westward. 



Developmentally the spear-grass association is to be regarded 

 as a later stage than the bunch-grass association of the sandier up- 

 lands. This stage may pave the way by still further accumulation 

 of litter and humus for the later occupation of the hills by the short- 

 grass formation toward the west, or toward the east the final stage 

 may be one of the more distinctly sodded associations of the prairie- 

 grass formation. This intermediate position is very evident in 

 many localities. 



The dominant species of the spear-grass association is Stipa 

 comata. This western Stipa, with Koeleria cristata as a common 

 associate, is controlling in many parts of the hills where wind action 

 has been more or less conquered and where the bunch-grass associa- 

 tion has been completely displaced over many square miles of rolling 

 hills. The lessened wind action is shown by the relative scarcity of 

 blow-outs. As a rule the substratum of this association is less 

 sandy and contains more organic matter than that of the bunch-grass 

 association. Andropogon scoparius and A. hallii are occasionally 

 seen as relicts in the more sandy portions and along the borders of 

 this association. 



