304 MINNESOTA BOTANICAL STUDIES 



grassland. The woodland vegetation of the region is derived, as I 

 have shown, from two widely separated forest centers and appears 

 to play an unimportant role in the characteristic plant successions 

 as they occur within the region under study. There is evidence in 

 some quarters of a slight displacement of grassy associations by 

 woodland forms, but on the whole this tendency is so slight that it 

 may be passed with mere notice. There is every evidence that the 

 uplands, as well as the most of the lowlands, will never become 

 completely occupied by the eastern type of woodland, at any rate as 

 long as the present control remains. Furthermore, it appears that 

 the frontier of a western forest complex represented by Pinus 

 ponderosa scopulorum is slowly losing ground within this region. 



All of the associations (except the woodlands) as delimited 

 above are stages, or closely related to stages, in one of two great 

 successional series. One of these begins with the appearance of 

 certain plants in blow-outs or similar situations leading to the pro- 

 duction of relatively stable conditions and to the invasion by other 

 slightly less wind-tolerant plants and a still further decrease in 

 blow-out dynamism, and, eventually, with the coming of additional 

 wind-resistant species, ends, at least temporarily, in the establish- 

 ment of the bunch-grass association. The other, beginning when 

 the first water-plants make their appearance in the ponds and lakes 

 of the region, culminates, at least temporarily, in the establishment 

 of the hay meadow association. 



The progress of the first series has been described at some 

 length in connection with the prairie-grass and short-grass forma- 

 tions. The first stage in the regular succession is always repre- 

 sented 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 asso- 

 ciation, because of the extreme rigors of its habitat and the' very 

 slowly changing substratum, 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 bunch-grass habit of the dominant species appears 

 to be the perfect solution of these particular environic conditions, 

 yet the frequency and abundance of many interstitial species with a 

 strong relationship to the more extensive prairie-grass types farther 



