306 MINNESOTA BOTANICAL STUDIES 



many deep-rooted associates. Such are the conditions that mark 

 the tension Hne between these two broad types of grassland. 



Disturbing factors sometimes interrupt the course of a regular 

 vegetative cycle. The influence of grazing is especially noticeable 

 in the sandhills during these later years when the range has been 

 fenced to a degree that was unknown to the older cattlemen. A 

 comparison of the vegetation on opposite sides of a fence often 

 pictures in a vivid manner the destructive effect of too severe pas- 

 turage. The degree of denudation produced by the stock is some- 

 times so great as to bring about a reversion to blow-out conditions 

 and the subsequent development of the plant association charac- 

 teristic of such conditions. Prairie fires often initiate the same 

 retrogressive cycle. Grazing and fire sometimes combine in bring- 

 ing about the subjugation of the bunch-grasses or other grassy 

 associations and the reestablishment of the blow-out association. 

 Working thus together or separately these forces often eradicate 

 the great majority of the deeper-rooted secondary species of the 

 sandy uplands. During the earlier stages of bunch-grass disinte- 

 gration Muhlenbergia pungens is liable to become very abundant 

 and controlling over wide areas. This species forms an associa- 

 tion at such times that is almost as pure and exclusive as the short- 

 grass cover of dry, hard soils. The Muhlenbergia association may 

 appear as a stage interpolated between the blow-out association 

 proper and the bunch-grass association proper, although quite 

 naturally at one time it may partake more especially of the nature 

 of the one association and at another time the other association. 

 There is thus some evidence to indicate that perhaps Muhlenbergia 

 pungens once played a significant role in upland successions which 

 has, however, now all but completely disappeared. 



No evidence has been gathered which indicates that the destruc- 

 tion of the bunch-grass association by either fire or grazing animals 

 ever results in the establishment of a "pure short grass cover" as 

 Shantz records (60) for eastern Colorado. The typical bunch-grass 

 land in our sandhills is far too sandy and exposed to too great wind 

 action to permit of this succession. The regular process in this con- 

 nection is, as has been shown, a reversion to the blow-out association 

 and the probable subsequent reestablishment of the bunch-grass 

 association in a number of years if the denuding influences are re- 

 moved. 



When bunch-grass land is "broken" one of two different prob- 



