THE PBAIEIE-PLAINS CLIMAX. 223 



check. In spots where the grass was thin, seedling Prosopis and Quercus 

 were not killed by the flames. In a year or two they began to shade the 

 ground, kiUing out the grasses and gaining protection against fire. Even 

 where the tops were killed by later fires, the roots sprouted repeatedly. With 

 more intensive grazing the quantity of the grass was also greatly lessened, with 

 the result that fires were fewer and less severe. As a consequence, the mes- 

 quite or the oak became well established and maintained an area in which 

 other shrubs were able to succeed. By this time the grass had become so 

 sparse that burning was out of the question, and the chaparral became perma- 

 nent. 



Pool (1914 : 306) smnmarizes the course of the subseres in the sand-hills of 

 Nebraska as follows: 



"Disturbing factors sometimes interrupt the course of a regular vegetative 

 cycle. The influence of grazing is especially noticeable in the sand-hills 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 

 pasturage. The degree of denudation produced by the stock is sometimes so 

 great as to bring about a reversion to blow-out conditions and the subsequent 

 development of the plant association characteristic of such conditions. Prairie 

 fires often initiate the same retrogressive cycle. Grazing and fire sometimes 

 combine in bringing about the subjugation of the bunch-grasses or other 

 grassy associations and the reestabUshment 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 disintegration Muhleribergia pungens is liable 

 to become very abundant and controlling over wide areas. This species forms 

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

 grass cover of dry, hard soils. The Mufdenbergia 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 

 Muhleribergia pungens once played a significant r61e in upland successions 

 which has, however, now all but completely disappeared. 



"No evidence has been gathered which indicates that the destruction 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 for eastern 

 Colorado. The typical bunch-grass land in our sand-hills is far too sandy and 

 exposed to too great wind action to permit of this succession. The regular 

 process in this connection 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 removed. 



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

 invited. If the site is exposed to wind action the almost invariable conse- 

 quence is the immediate generation of blow-out phenomena. But if wind 

 action is not so severe, breaking regularly leads to the early establishment of 

 the various ruderal combinations. These 'weed stages' are most commonly 

 represented by the species noted under the discussion of the ruderal vegetation 

 of the region. Through a series of changes which have not been thoroughly 

 studied the ruderals eventually yield their position to the bimch-grasses. 



"The spear-grass association may also be caused to revert to the blow-out 

 association after breaking, or such areas may be dominated for a number of 



