Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 233 



and general edaphic conditions to those of the bunch-grass associa- 

 tion. The most perfect degree of development and the greatest 

 area of the Muhlenbergia association at present are to be found 

 over the uplands of central Garden County and western McPherson 

 County. However, the association has been seen in various other 

 portions of the sandhills where the extreme severity of certain eco- 

 logical factors has brought it into existence. Within the above cen- 

 ter this association dominated by Muhlenbergia pungens occupies 

 hundreds of acres of the uplands from which Andropogon sco- 

 parius and many of its numerous associates are all but completely 

 excluded. 



Composition of the Muhlenbergia Association 

 dominant species 

 Muhlenbergia pungens 



PRINCIPAL SPECIES 



Bouteloua hirsuta Prunus besseyi 



Lathyrus ornatus incanus Psoralea lanceolata 



Phaca longifolia Yucca glauca 



SECONDARY SPECIES 



Allionia linearis Opuntia humifusa 



Andropogon hallii Rosa arkansana 



Andropogon scoparius Senecio plattensis 



Calamovilfa longifolia Stipa comata 

 Lygodesmia juncea 



The Muhlenbergia association is very commonly found as scat- 

 tered relict patches about blow-outs and sand-draws. This is the 

 case especially in those places where the latter association has been 

 seriously impaired by fire or continued overpasturing. In fact we 

 shall see that Muhlenbergia pungens is one of the regular members 

 of the blow-out association. Whenever blow-out conditions are 

 brought to bear over extensive upland tracts by either of the above 

 agencies, blow-outs are regularly generated in the most exposed 

 situations. These expanses of overgrazed land are furthermore 

 characterized by much surface which is not directly exposed to the 

 extreme erosion which results in the formation of blow-outs or sand- 

 draws. Yet such broad, sloping areas are often so open, dry, and 

 wind-swept that the bunch-grasses are slow in invading, and ac- 

 cordingly they may be controlled by an altogether different type of 



