341 



be no particular arrangement of the dominant species when they oc- 

 cur together, except in the more pronounced ridges. Here Populus 

 occupies the crest and Salix and Coniiis the slopes. With them are 

 a number of secondary species, many of which are either invaders 

 of the forest type of vegetation or relics of the prairie. 



Succcssional Relationships. — On sandy ground this association is 

 very frequently introduced by the invasion of cottonwood {Populus 

 dcltoidcs), followed by species of Salix and Cormis. In black soil, 

 species of Salix or Cormis are more usuall_Y the pioneer invaders. 

 Succession is accomplished by the cutting-off of the hght supply from 

 the vegetation below as soon as the shrubs attain sufficient size. In 

 due course of time some of the species of Salix and Populus become 

 trees, with almost the same assemblage of secondary species, but ul- 

 timately the thickets occurring on the beach plain will be replaced 

 by the Ouercus velutina association, while those near the base of the 

 bluffs will be replaced by the oak-hickory woods. 



UST 01^ THE SPECIES OE THE POPUEUS-SAEIX-CORNUS 

 THICKET ASSOCIATION 



Dominant Species 

 Cormis stolonifeva 

 Populus treuiuloidcs 

 Populus deltoides 

 Rosa Carolina 

 Salix amygdaloid cs 

 Salix cor data 



Secondary Species 



Aster innhellatus 

 Aster novae-angliae 

 Betula alba papyrifera 

 Betula pumila 

 Brouius incanus (i plant) 

 Dloscorea paniciilata 

 Bquisetuin arz'ense 

 Helianthus Occident alls 



f. illinoensis 

 Helianthus grossescrratus 

 Lactuca canadensis 

 Lechea leggettii 



Relic Species 



Achillea niillcfoVunu 



Salix discolor 

 Salix longifolia 

 Salix lucida 

 Salix pedirillaris 

 Salix serissinia 



Lechea zdllosa 

 Lonicera dioica 

 Prunus serotina 

 Ribes sp. ? 

 Rhus toxicodendron 

 Rhaninus alnifolia 

 Rubus ocddentalis 

 Spiraea salicifolia 

 Solidago canadensis 

 Solidago serotina 

 Sam bucus canadensis 

 Silphiuni integrifoliuni 



Lespedcza capitata 



