PLANT ASSOCIATIONS OF NORTHERN MICHIGAN 157 



is occupied by numerous species of introduced weeds. Among 

 them Poa pratensis, sometimes associated with Phleum pratense, 

 soon becomes dominant, the species and individuals of other weeds 

 are reduced in number, and in a short time the blue grass meadow 

 is developed. Table 7 exhibits in parallel columns the structure 

 of the same field in 1914 and 1917. This field was plowed in 1913 

 and has since then been abandoned. 



It is not known how long the blue grass association may persist. 

 It is still well developed in numerous places in the aspen region, 

 which was formerly occupied by pine forest and was lumbered 

 over thirty years ago. The introduction of the blue grass was 

 doubtless contemporaneous with the logging, and the association 

 now marks the site of feeding grounds and logging roads. Numer- 

 ous other species still occur wdth the blue grass along roads now in 

 use, but isolated areas frequently contain no other foreign species, 

 or Chrysanthemum leucanihemum alone. Other species, which 

 may have been associated with the blue grass formerly, have suc- 

 cumbed under the competition with the blue grass sod. 



The association still presents a considerable number of second- 

 ary species around an old mill, abandoned at least seven years 

 ago. Here they are in general restricted to the sawdust heaps, to 

 the base of walls, and in some cases to the inside of abandoned 

 stables. Thirty-six species were listed about the premises in 

 1917, and two additional species occurred within the stables only. 



The ultimate fate of the association depends entirely upon its 

 relation to continued human activity. Usually the logging camps 

 and roads are abandoned when the lumbering is completed. A 

 young forest develops rapidly at the margin of the blue grass 

 association and gradually encroaches upon it. At the same time 

 scattered individuals of forest species succeed in establishing 

 themselves within the main body of the association, and by their 

 shade contribute to the destruction of the blue grass and the 

 eventual reappearance of the forest. Among these pioneers 

 Rubus idaeus, var. aculeatissimus and Acer saccharum are conspic- 

 uous in the succession by the beech-maple forest, and Rhus 

 glabra and Prunus pennsylvanica in the succession by aspens, 

 leading ultimately to a pine forest. Indications of the former are 



THE PLANT WORLD, VOL. 21, NO. 6 



