Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 219 



possible, a type of vegetation with which the prairie can not favor- 

 ably compete. Because of the peculiar rigors that the forest frontier 

 meets, tree species invade the prairie sod with extreme slowness 

 notwithstanding the fact that there may be sufficient soil-moisture in 

 such situations for the maintenance of a woodland plant cover par- 

 taking of the nature of the Ohio-Mississippi valley forest complex. 

 The prairies quite effectively restrict the westward march of the 

 great hardwoods forest and conversely the forest reduces to a great 

 degree the possibility of the eastward extension of the prairies. 



Shantz (60) has contributed evidence to show that within the 

 Prairie-Great-Plains grassland the distribution of the prairie-grass 

 and short-grass formations is clearly a natural delimitation brought 

 about by precipitation and available soil-moisture. The prairie 

 grasses and their characteristic associates can not invade to any 

 great distance the short-grass region because of too low water 

 content of the soil. On the other hand the eastward dispersal of the 

 short-grasses and their associated species is restricted by the pres- 

 ence of deeper-rooted species of the prairie-grass formation which 

 are not only able to hold the ground against, but actually to repel, 

 their shallower-rooted relatives. 



The prairie-grass formation as above defined is by far the most 

 extensive type of vegetation within the sandhill regions of Nebraska, 

 Stretching as it does in almost unbroken continuity over the uplands 

 in practically all parts of the hills and extending well down into the 

 drier valleys, the area occupied by the associations of this formation 

 constitutes at least sixty per cent of the total 18,000 square miles. 

 The typical vegetative background of the sandhill landscape is that 

 of a prairie in which the bunch-grasses are the most frequent con- 

 trolling species. 



THE BUNCH-GRASS ASSOCIATION 



The bunch-grass association is to be found to a greater or less 

 degree in practically every portion of the typical sandhills, and with 

 the blow-out association is to be considered as the most characteristic 

 type of vegetation within the region. The bunch-grasses are absent 

 from the uplands only where fire or grazing has ushered in one of 

 the associations nearer the initial stage in the succession, or, as on 

 the outskirts of the region and in certain valleys, where sod-forming 

 species become controlling. The bunch-grass association as here 



