135 



B. Species more typical of preceding associations 



Pteris aqiiilina Ceanothiis americanus 



Poa pratensis Apocynuni androsacmifoliuni 



Tradescantia rcflcxa Monarda mollis 



Salix tristis Synthyris Bidlii 



Quercus velntina Gerardia grandiflora 



Fragaria virginiana, Pediciilaris canadensis 



var. illinoensis Antennaria sp. 



Rosa hinnilis Helianthiis strumosus 



Anwrpha cancscens Coreopsis palniata 



Bnphorbia corollata Cacalia atriplicifolia 



THE MIXED FOREST ASSOCIATION 



In the Hanover, Oquawka, and Havana areas the succession from 

 the black oak association is dependent primarily upon a general and 

 gradual increase in the water content of the sand and a correspond- 

 ing decrease in the light, without the concurrence of hinorical fac- 

 tors as in the two areas previously described. The succeeding vegeta- 

 tion is derived wholly from the surrounding associations. Since 

 other types of vegetation are developed best near the rivers which 

 border these sand areas, invasion begins near the river and gradu- 

 ally extends back toward the center of the sand deposits, so that 

 the most advanced stages in the succession are always found near 

 the river. The development of this succession is greatest in the Ha- 

 vana and least in the Hanover area; a feature which is perhaps cor- 

 related with the general southeastern origin of the forest fonnatioii. 



In the Havana area, as in the Winnebago deposits, the narrower 

 ridges of sand are the last to be affected by this succession, while the 

 broader ridges or those near the river generally show some indica- 

 tion of it. Certain species are soon recog'nized as the normal pio- 

 neers in the succession, and while their order of appearance is not 

 constant, their presence is always connected with the development 

 of a thin, fibrous laver of leaf -mold over the surface of the s^nd. 

 It is frequently possible to observe nearly all stages in the succession 

 in a distance of a mile, passing from the edge of the forest toward 

 its center. In some places, adjacent ridges of sand represent differ- 

 ent stages of the succession, and permit an easy comparison of the 

 vegetation. This is especially well shown on the first two forested 

 ridges east of Havana, about two miles (3 km.) from that city. The 

 first of these represents an advanced stage of the succession, while 

 the second is occupied by a nearly typical black oak association. 



