129 



SUCCESSIONS FROM THE BI.ACK OAK ASSOCIATION 



In Illinois, succession between different types of forest is usually 

 caused by the disturbance of some feature of the physical environ- 

 ment by physiographic changes. It therefore becomes possible to 

 correlate the succession of forests with the physiography (Cowles, 

 ipoi). In the sand areas of the state, however, physiographic proc- 

 esses are primarily not concerned, and the whole process is due 

 to the reaction of the plant upon its habitat, by which there is de- 

 veloped a different habitat, adapted to a different type of vegetation. 

 The chief feature of the vegetation by which these environmental 

 changes are caused is the general density of the plant covering. This 

 leads to the partial exclusion of sunlight, heat, and moisture from, the 

 soil, and to the addition every autumn of a large quantity of vege- 

 table matter. In this way the soil moisture is conserved, the trans- 

 piration of the plants is diminished, and a layer of leaf-mold is slowly 

 formed. The leaf-mold also aids in the conservation of moisture 

 in the soil and at the same time increases its capacity for hold- 

 ing water. All of these changes go on simultaneously, and each 

 is correlated with the others. The whole is in a mesophytic direc- 

 tion and, as a result, the xerophytic black oak association is succeeded 

 by vegetation of a more mesophytic type, consisting of at least two, 

 and possibly more, distinct associations. For the preseut, that of the 

 Winnebago and Amboy areas will be considered as one, termed the 

 bur oak association, and that of the Hanover, Havana, and Oquawka 

 areas as another, the mixed forest association. 



THE BUR OAK ASSOCIATION 



In the Winnebago area the sand is distributed in irregular ridges, 

 separated by irregular depressions of various sizes. The latter are 

 occupied by an association characterized by bur oak, Qncrcus macro- 

 carpa, and white oak, Qiiercus alba, together with a distinct type of 

 herbaceous flora. None of the narrower upland ridges has as yet 

 been succeeded by the bur oak association, although some show in- 

 dications of it in the presence of scattered plants of choke-cherry. 

 Primus znrginiana, black cherry. Primus serotina, and hazel, Corylus 

 americaiia. The bracken fern, Pteris aquilina, also becomes more 

 abundant near the bur oak association (PI. XV, Fig. i), and its pres- 

 ence in large quantities may in some degree be considered as one in- 

 dication of the approaching succession. On some ridg'es there is a 

 well-defined zone of Pteris along the slopes, extending neither into 

 the xerophytic black oak timber above, nor into the mesophytic bur 



