April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 177 



working out the detailed structure and ecological relations of the various 

 plant associations. In either case the ultimate unit with which we work in 

 the field is the association-concrete — the piece of vegetation which we can 

 literally place our hands on. 



A Concrete Example. — The manner in which this three-fold scheme of 

 classification may be applied to the vegetation of any particular climatic region 

 can be best illustrated, perhaps, by a concrete example. The outline presented 

 below, in slightly different form, has been successfully used in the classifica- 

 tion of the plant associations of northern Cape Breton Island (Nichols, '18). 



I. The climatic climax association-type : the climax forest. 

 II. The physiographic formation-complex of the region. 



A. The primary formations of xerarch and mesarch successional series. 



1. The formations of ordinary well-drained uplands. 



a. The association-types of rock outcrops. 63 



b. The association-types of talus. 



c. The association-types of glacial till. 



d. The association-types of sand plains. 



2. The formations of well-drained uplands along streams. 



a. The association-types of rock ravines. 



b. The association-types of clay ravines, 

 f. The association-types of open valleys. 



d. The association-types of boulder plains. 



e. The association-types of flood plains. 



3. The formations of well-drained uplands along the seacoast. 



a. The association-types of bluffs and headlands. 



b. The association-types of beaches and dunes. 



B. The secondary formations of xerarch and mesarch successional series. 



1. Formations due to the influence of lumbering. 



2. Formations due to the influence of fire. 



3. Formations due to the influence of cultivation. 



C. The primary formations of hydrarch successional series. 



1. The formations of inland lakes and ponds. 



a. The association-types of well-drained lakes. 



b. The association-types of undrained lakes. 



2. The formations of inland lake- and seepage swamps. 



a. The association-types of well-drained swamps. 



b. The association-types of poorly drained swamps. 



3. The formations in and along rivers and streams. 



a. The association-types of ravines. 



b. The association-types of flood-plains. 



4. The formations along the seacoast. 



a. The association-types of salt marshes. 



b. The association-types of brackish marshes. 



D. The secondary formations of the hydrarch successional series. 



63 Under subdivisions a, b, c, etc., the associations are considered, both specifically and 

 in their successional relations. 



