THE PRAIRIE-PLAINS CLIMAX. 219 



relatively high. In both cases the development of the community reduces the 

 evaporation. Succession, therefore, does not depend upon evaporation; it is 

 a cause of the reaction, and not an effect of it. 



Pool (1912 : 209; 1914 : 189) has made a thorough study of the xerosere in 

 the sand-hills of Nebraska. The blow-out stage is dominated by Calanwvilfa, 

 Redfieldia, and Psoralea lanceolata, the Muhlenbergia stage by M. pungens, the 

 bunch-grass associes by Andropogon hallii and A. scoparius, the spear-grass 

 associes by Stipa comata and Koeleria cristata, the wire-grass stage by Aristida 

 basiramea, A. purpurea, and Sporobolus cuspidatus, and the short-grass 

 climax by BoiUeloua and BuUrilis. The course of development is as follows : 



"The progress of the series has been described at some length in connection 

 with the prairie-grass and short-grass formations. The first stage in the 

 regular succession is always represented by the blow-out association which, 

 by means of a number of phases which can not be satisfactorily delimited, 

 gradually passes over into the bunch-grass association. This association is 

 to be regarded as the temporary climax in the upland cycle. The association, 

 because of the extreme rigors of its habitat and the very slowly changing sub- 

 stratum, appears to represent a long persistent type of vegetation and at the 

 same time to pass over, under certain conditions, to a more typically sodded 

 prairie-grass or possibly short-grass cover. 



"While the bimch-grass habit of the dominant species appears to be the 

 perfect solution of these particular environic conditions, yet the frequency 

 and abimdance of many interstitial species with a strong relationship to the 

 more extensive prairie-grass types farther 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 bimch-grass association and the other types 

 of vegetation represented in the great prairie province. The ready adap- 

 tability of Stipa comata to a substratum slightly more stable than that occu- 

 pied 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 pathway 

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

 distinctly sodded associations of the prairie-grass formation. 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 re^on other possibilities are pre- 

 sented. 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 formation. 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 sand-hill areas as in Chase 

 and Dundy counties." 



Vestal (1914 : 363) has given the following sunamary of the developmental 

 relations of a black-soil prairie in Illinois. 



"The county hne station, a half-hour's ride west of Chicago, contains areas 

 of prairie, forest, and forest border in still fairly good conation. Preserva- 

 tion of the forest from former prairie fires is suggested by its location on the 



