April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 165 



terms of physiography. Thus we may speak of a particular rock ravine for- 

 mation, or a particular flood plain formation, or a particular salt marsh 

 formation. 



Like a climatic formation, a physiographic formation may be a very 

 heterogeneous vegetation-unit. Here as there, however, there is a tendency, 

 through the phenomenon of succession, for the vegetation to progress toward 

 a common climax. The nature of this climax, in so far as it differs from the 

 climatic climax, is determined primarily by the habitat conditions already 

 referred to which characterize the area as a whole. 40 



As with the association and the climatic formation, the present concept also 

 is capable of both concrete and abstract interpretation. Thus we can refer 

 in the concrete to the vegetation of a particular flood plain or a particular 

 salt marsh as a flood plain formation or a salt marsh formation, but we can 

 also speak in the abstract of the flood plain formation along the Connecticut 

 River or of the salt marsh formation in New England. 46 



D. Interrelations of the Various Geographical Vegetation-units 



The General Situation. — To sum up briefly, a climatic formation comprises 

 a complex of associations linked together by climate ; a physiographic forma- 

 tion comprises a complex of associations linked together by physiography. 

 These geographically defined subdivisions of vegetation represent ecological 

 vegetation-units of a higher order than the association. Fundamentally the 

 two are logically and clearly differentiated both from one another and from 

 the association. The climatic formation, in general, is the more compre- 

 hensive unit, and a particular climatic formation ordinarily includes numer- 

 ous physiographic formations. 



Apparent Exceptions to Rule. — When it comes to the interpretation of 

 concrete cases in the light of these abstract concepts, there are certain apparent 

 discrepancies between fact and theory which deserve a few words of comment. 



To begin with, the line of demarcation between climatic formation, physi- 

 ographic formation, and association is not always sharply defined. A physi- 

 ographic unit-area such as a sand plain, for example, may exhibit very little 

 diversity in either topography or soil, with the result that the vegetation is 

 essentially uniform throughout. In such a case it seems obvious that no dis- 



40 See further discussion of physiographic climax on p. 172. 



40 The abstract use of this term may well be extended to include ecologically similar 

 physiographic formations the world over. Generally speaking, if the term is so used, the 

 same physiographic formation might be expected to occur in regions which are similar 

 to one another in climate and physiography, but it would not be looked for in regions 

 which are climatically different. Formations which occupy corresponding physiographic 

 unit-areas in climatically different regions, however, might be referred to a common 

 physiographic formation-type. Thus a rock ravine formation in the Arizona mountains 

 and one in Connecticut might both be referred to the rock ravine formation-type. 

 The use of the term type in this connection would serve to emphasize the physiographic 

 parallelism of the formations concerned. 



