Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 257 



and climatic conditions that to a large degree govern the distribu- 

 tion of this typical Great Plains plant formation. Within the sand- 

 hills the formation occurs in widely scattered islands and tongues in 

 various parts of the region where it has succeeded in pushing east- 

 ward among the prairie-grass associations. The total area occupied 

 by this formation in the sandhills is very small as compared with 

 that covered by the prairie-grass associations. In mixture with the 

 wire-grass transition association short-grass is to be found far east- 

 ward along the northern border of the sandhills and in many places 

 here the fairly typical short-grass formation is seen. Shantz has 

 pictured in an admirable manner the eastward extension of these 

 outposts of the short-grass formation of the Great Plains Area to 

 central Nebraska where they meet the prairie-grass formation, the 

 nature of which, as it occurs in the sandhills, has already been dis- 

 cussed. A very good conception may be gained of the situation 

 with which we are dealing if we picture a broad vegetative back- 

 ground composed chiefly of the bunch-grass and blow-out associa- 

 tions over the uplands in almost all portions of the sandhills in prac- 

 tically unbroken continuity except for the relatively small and 

 widely scattered islands, tongues, and belts of short-grasses, the one 

 with a strong eastern affiliation and the other with a decidedly west- 

 ern tone. We have as it were the meeting and overlapping of two 

 divergent vegetative frontiers whose foci are widely separated. 

 Naturally then we find that the short-grass formation becomes 

 increasingly noticeable as we pass westward through the hills until 

 we finally leave the sandhills with their prevailing prairie-grass land- 

 scape and finally enter the arid short-grass region of the high plains. 



The soil-moisture content of short-grass land is typically lower 

 than that of any of the prairie-grass series and the non-available 

 water (as determined by the wilting method) is much higher than 

 in the bunch-grass land. Preliminary studies have been made upon 

 the penetration of precipitation into the two kinds of soil and it has 

 been found that this process is very much more rapid in bunch-grass 

 land than in short-grass land. Run-off from these soils differs in 

 the reverse relation. Shallow-rooted species are doubtless favored 

 by these facts which may also account for the absence of many of 

 the deeper-rooted secondary species that are characteristic of the 

 bunch-grass association where soak-in is great and where there is a 

 perpetual supply of moisture below a depth of a few inches. 



Shantz has shown that the run-off from this association as it 



