218 



CLIMAX FORMATIONS OF NOETH AMERICA. 



"In conclusion, the conditions in Bur Oak Grove serve to indicate the last 

 three periods in the vegetational history of the state: 



"1. Period of forest advance, leading to a great development of forests in 

 areas of physiographic diversity. 



"2. Period of prairie fires, following the advent of man and leading to the 

 restriction of the forest to protected areas and the corresponding extension 

 of the prairie. 



"3. Period of civilization and the virtual cessation of the struggle between 

 forest and prairie." 



'Lake Michigan 



\ 



Lower Beach 



I 



Cakile-Xanthium Assoc. 



Ammophila Dune 



\i 



Calamovilfa Dune 



Prunuspumila Dune \^^v 

 Populus candicans Dune t\ 



Juncus balticus 



i 



Potentillaanserina 



Assoc. 



Salix syrticpla Dune 

 I 



Elymus canadensis Dune 



Liatris scariosa 

 Assoc. 



Artemisia-Panicum Assoc. 



T 



Bunchgrass Assoc 



^ Poa compressa 

 Assoc. \ 



/ ^^ Blowout Assot, 



Prunus Thicket 

 Assoc. 



Quercus vejutina Assoc, 

 Quercus-Carya: Assoc. 



Ulmus-Acer Assoc, 

 Acer saccharum Assoc. 



Fig. 8. — ^Diagram of development of maple-beech climax about Lake Michigan, 

 showing course of xerosere. After Gates. 



Gleason and Gates (1912 : 478) have determined the rate of evaporation for 

 several consocies in central Ilhnois, namely, the river-bank, the bottom-lands, 

 the bunch-grass community, the blow-out, and the Quercus velvtina community: 



The invasion of the bunch-grass by Q. velviina begins in the normal com- 

 mimity where the rate of evaporation is high. Species of the mixed forest 

 also appear in the oak consocies while the evaporation rate there is still 



