ON THE DEVELOPMENT OF TWO PLANT ASSOCIATIONS 

 OF NORTHERN MICHIGAN' 



HENRY ALLAN GLEASON 

 University of Michigan, Ann Arbor, Michigan 



It has been pointed out in an earlier paper- that the first stages 

 in the development of a plant association are the immigration of 

 species from the surrounding plant population and the selection 

 from among them of those particular species which are adjusted 

 to the local environment at that time. In the same paper it was 

 also shown that the control of the physical environment by cer- 

 tain species of the selected immigrants is always an important 

 feature of the environment and that ultimately the number of 

 individuals of each species in the association is an index to the 

 completeness and exactness of their adjustment to local condi- 

 tions. Those species which exert the greatest measure of environ- 

 mental control, thereby favoring their own multiplication and 

 restricting that of other plants, are known as the dominant species 

 of the association. 



The development of an association on denuded ground there- 

 fore presents the following four stages : (1) immigration of numer- 

 ous species, (2) selection^ of those species adjusted to the physi- 

 cal environment, (3) the development of dominancy among cer- 

 tain species, and (4) the restriction in number or even complete 

 extinction of the remaining secondary plants. These stages occur 

 in the order stated, but the inception of one does not await the 

 completion of the preceding. In fact, immigration and selection 

 continue through the whole existence of the association. Any 

 further changes in the association, aside from seasonal variations 



1 Publication no. 170 from the Botanical Laboratory of the University of 

 Michigan. 



2 Bull. Torr. Bot. Club 44: 463-481. 1917. 



' It is easily understood that selection takes place by the elimination of unad- 

 justed species, rather than by any sort of environmental choice. 



151 



