CHICAGO AND VICINITT. 



49 



above, it is scarcely probable that it is anything more than a very 

 slowly passing forest stage. The fact that in all directions from Chi- 

 cago the ultimate forest type on morainic uplands is not the oak- 

 hickory but the maple-beech forest leads us to expect that here. This 

 latter type seems to be of a higher order in all respects. It is found 

 in richer soil, where the humus content is very great. Seedlings of the 



Fig. 29. — Typical forest of low morainic clay soil, made up chiefly of bur oaks [Quercus 

 macrocarpa) . 



beech or maple can easily grow in the relatively light oak forest, 

 whereas oaks cannot grow in the denser shade of the maple or beech. 

 Furthermore, oak forests have been seen with a pronounced under- 

 growth of beech. It would seem that one of the chief factors in 

 determining the order of succession of forests is the light need of the 

 various tree species, the members of the culminating forest type being 

 those whose seedlings can grow in the densest forest shade. There 

 are evidences that the oak forests about Chicago are being succeeded 

 by the beech or maple. The best instance of this which the author 

 has seen is on the low moraines along the Desplaines river west of 

 Deerfield. The sugar maple {Acer saccharinum) has already been 

 mentioned as a character plant of the temporary mesophytic forests of 

 ravines. Here we see it in the more permanent forest of the morainic 



