Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 305 



eastward, constitutes a prophetic index of the possible culmination 

 of the upland series. Furthermore, the encroachment of the spear- 

 grass association upon the domain of the bunch-grasses is another 

 indication, in the form of a connecting link, of the most evident 

 relationship between the bunch-grass association and the other 

 types of vegetation represented in the great prairie province. The 

 ready adaptability of Stipa comata to a substratum slightly more 

 stable than that occupied by the bunch-grasses, and its ability in 

 following with the sod-forming habit as the soil becomes more 

 stable and harder, brings forth the suggestion that the spear-grass 

 association is probably destined eventually to supplant the bunch- 

 grasses over the greater portion of the sandhills region. The path- 

 way may thus be prepared for the occupation of the region by some 

 of the more distinctly sodded associations of the prairie-grass forma- 

 tion. Evidences of such an intermediate position held by the spear 

 grasses are especially strong in the sub-sandhills of the northern 

 and eastern portions of the region. 



Toward the drier western limits of the region other possibili- 

 ties are presented. Here the wire-grass transition association is 

 seen grading on one hand into bunch-grass land and on the other 

 merging upon the more flat clayey land into the short-grass forma- 

 tion. The evidence, therefore, appears to favor the conclusion that 

 at least in some cases the bunch-grasses may be regularly succeeded 

 by a short-grass cover. This is taking place on the western border 

 of the hills and in some of the less sandy outlying sandhill areas as 

 in Chase and Dundy counties. 



Since it is an apparently unquestionable fact (Shantz 60) that 

 the short-grass formation is correlated especially with a relatively 

 low rainfall and low available soil-moisture as compared with the 

 prairie-grass formation, it may be that the area of short-grass land 

 is to become greater in these particular portions of the sandhills. 

 At any rate the short-grasses have come into complete control of 

 sandhill areas in Chase and Dundy counties and Andropogon sco- 

 parius remains merely in the form of relicts. 



In short, the present state of rather unstable climatic and 

 edaphic equilibrium which exists along this semiarid short-grass 

 and prairie-grass frontier appears to favor the short grasses, while 

 farther eastward a higher normal precipitation and a higher avail- 

 able soil-moisture in a sandier soil and at greater depths, makes 

 possible the continued supremacy of the prairie grasses and their 



