334 



sociation which has been driven from the region. Usually the Os- 

 mimda is between the Calamagrostis and the prairie, but it is also be- 

 tween the Querciis velutina and the Calamagrostis, and less fre- 

 quently is preserved as a relic in the midst of willow thickets which 

 have been developed in boggy ground. Osmunda cinnamoinea is a 

 very characteristic species of this association, but it is entirely absent 

 from the Beach region. The only associates that have been noted 

 with the Osmunda regalis are Geraniiun inacidatnm, Fragaria virgini- 

 ana, Polytrichiim sp., and Zizia aurea. 



THE POTENTII.LA FRUTICOSA ASSOCIATION 



This northern association occurs on sandy soil which is usually 

 moist, although only exceptionally flooded. The association typically 

 follows the destruction of the pines in a soil which can support an 

 association genetically higher than bunch-grass prairie, but not yet 

 sufficiently mesophytic for the blazing star {Liatris spicata) prairie. 

 It often occupies the lower ground between the ridges on which the 

 pines are growing. 



The Association. — The wide-spread growth of the dominant spe- 

 cies, Potcntilla fruticosa, a low bushy plant, is the characteristic fea- 

 ture of the association. Few or no characteristic secondary species 

 occur, since this association is a boreal relic. Other species that may 

 occur are usually relics or invaders of former or succeeding associa- 

 tions. The composition of the invaders depends almost entirely upon 

 the proximity of the associations likely to succeed. In the southern 

 part of the region, especially towards Waukegan, this association is 

 so intermingled with the Liatris spicata prairie that it is difficult to 

 separate them. Throughout most of the year this association pre- 

 sents a dull, monotonous color-tone, but in the late summer it is re- 

 lieved by the bright yellow flowers of the Potentilla, which occur in 

 profusion. 



Successional Relationships. — Shrubby cinquefoil {Potentilla fru- 

 ticosa) has more ability to invade and take possession of bunch-grass 

 prairie than Liatris spicata prairie, but in turn the Potentilla is al- 

 most immediately followed by Liatris spicata. Near the pines Po- 

 tentilla fruticosa. easily invades the heath and prepares it for subse- 

 quent prairie invasion. Potentilla fniiticosa readily takes possession of 

 the moister places where pines have been removed, while the heath 

 is characteristic of the drier places. Seedling pines {Pinus strobus) 

 occasionally obtain a foothold in the Potentilla fruticosa, while seed- 

 ling oaks {Querciis velutina) are less liable to do so. In general, 

 however, oaks will obtain dominance quicker in cut-o\-er pine land 



