Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 217 



(2) a Mississippi-Ohio valley element, also from the east, crossing 

 the prairie as slender tongues, (3) a foot-hill element whose alliance 

 is westward, or great plains and montane in nature, and (4) a 

 "proper" element or that which the vicissitudes of the region have 

 called into existence. The western montane element is seen especially 

 well-developed along the northern portion of the sandhill region 

 where it is represented by Piniis ponderosa scopulorum, a conspicu- 

 ous species of the hills and canyons where Loup Fork Beds are 

 exposed along the Niobrara River and its sandhill tributaries from 

 the south. 



The most powerful or controlling influence here is exhibited by 

 the eastern prairie element. The sandhills of our state appear to 

 present a western type of the vegetation of the Prairie Province 

 existing under the extreme environmental rigors of a dune region. 

 To this must be added the observation that this prairie variation is 

 augmented and considerably modified, first, by the presence of cer- 

 tain species that do not appear in the prairies farther eastward, and, 

 second, through changes in the habits and structure of typical 

 prairie species that have enabled such plants to succeed under the 

 peculiar conditions imposed by a sand dune region within a semi- 

 arid climate. 



As would perhaps be anticipated from the intermediate posi- 

 tion of the sandhills, it has been found that prairie species decrease 

 in frequence and abundance rather rapidly from east to west, and 

 that the frequency and abundance of Great Plains species rapidly 

 diminish as one passes eastward from the western limits of the 

 region. Two vegetational frontiers are thus seen to meet within 

 the region covered by the present investigation. 



UPLAND FORMATIONS 

 The Prairie-Grass Formation 

 Under the prairie-grass formation I mean to include all those 

 uplands lying in general between the Mississippi River and the 

 Rocky Mountains that are dominated by grasses of a considerable 

 height and possessed by either the sod-forming or bunch-grass 

 habit. It is appreciated that this is a very broad conception and 

 that the formation thus characterized covers a vast territory and is 

 composed of numerous associations. The Prairie-Grass Formation 

 of this nature embraces practically all of that great stretch of land 

 included within the prairie and sandhill "regions" of Pound and 



