79 



Prairie Province; the other is between two provinces, leads to the 

 permanent replacement of the prairie vegetation, and consequently 

 affects the area and the boundaries of both the Prairie and the De- 

 ciduous Forest Pro\ inces. 



Because of the larg^e area occupied, its resistance to succession by 

 associations of the same province, and its ability to reoccupy the 

 space where it has been destroyed by wind action, the bunch-g'rass 

 association must be regarded as a temporary climax. 



The succession caused by wind will be described first. It begins 

 with the development of the Panicinn pseudopnhescens association, 

 and is followed by a number of associations representing the blowout 

 formation. 



THE PANICUM PSKUDOPUBESCENS ASSOCIATION 



Notwithstanding the resistance offered by the bunch-grasses to 

 remo^'al of sand by the wind, the exposure of from 20 to 50 per cent, 

 of the surface gives considerable opportunity for aeolian action. 

 Large bunches are not destroyed, and probalily not seriously injured, 

 by the removal of sand, but the smaller bunches may be killed. With 

 eveiy subtraction from the veg'etative covering" more sand is exposed 

 and the effect of the wind correspondingly heightened. One species 

 of bunch-grass, Panicum psciidopuhcsccns, can not only endure the 

 removal of sand from beneath it, but seems to thrive better under 

 such conditions than when mixed with larger grasses on more stable 

 sand. As the blowing proceeds, an increasingly larg^er portion of the 

 surface is occupied by it, until finally it becomes dominant, and the 

 bunch-grass association is thereby converted into the Paniciiui psciido- 

 piibcscens association. Just where the dividing line between the two 

 should be drawn is cjuestionable. It has been arbitrarily decided that 

 the bunch-grass association must have at least half the surface occu- 

 pied to be considered typical, and it may also be arbitrarily con- 

 sidered that, in the Panicum pscttdopubcsccns association, the char- 

 acteristic species should constitute at least three fourths of the plant 

 covering. When the vegetation does not comply with these condi- 

 tions it may be regarded as representing transitional stages of this 

 succession or of other successions. 



The best development of this association is in the almost original 

 conditions of the Hanover area, but it also occurs in the Oquawka, 

 Dixon, and Havana areas, presenting the same essential characters 

 in each. 



Since the development of the association depends primarilv upon 



