156 



GEORGE E. NICHOLS Vol. IV, No. 2 



of view, associations may be classified with reference to their physiognomy 

 and ecological structure (section V). In the second place, the influence of 

 environment is seen in the manner in which plant associations are distributed 

 over the face of the earth in relation to various features of climate and physi- 

 ography. From this point of view, associations may be classified with refer- 

 ence to their geographic relations (section VI). Finally, the influence of 

 environment is seen in the changes in vegetation which ensue in the course 

 of time. From this point of view, associations may be classified with refer- 

 ence to their successional relations (section VII). The field covered by the 

 first point of view might be termed physiognomic plant sociology, that covered 

 by the second geographic plant sociology, and that covered by the third 

 dynamic plant sociology. 



V. Classification Based on Physiognomy and Ecological Structure 



The Association-type. — It is a matter of common observation that plant 

 associations which differ from one another more or less markedly in their 

 floristic composition may be essentially identical in their physiognomy and 

 ecological structure. To illustrate: Throughout most of the eastern United 

 States the prevailing type of natural vegetation on the better soils of ordinary 

 well-drained uplands 28 is some sort of a mesophytic deciduous broad-leaf 

 forest. This forest may vary greatly from place to place with regard to its 

 floristic composition — i.e., it may include various floristically different associ- 

 ations — but it is essentially uniform throughout with regard to its physi- 

 ognomy and ecological structure. Again, the Chamaecyparis association of 

 swamps along the Atlantic coastal plain and the Thuja association of swamps 

 farther north seem essentially identical as regards their physiognomy and 

 ecological structure. The same is true of the pitch pine association in Con- 

 necticut and the jack pine association in Michigan, of various associations of 

 marshy or reedy swamps, and so on almost ad infinitum. This brings us to 

 the idea of the association-type. Briefly stated, all associations which resemble 

 one another in physiognomy and ecological structure, regardless of their 

 floristic composition, may be referred to a common ecological association- 

 type. 29 



21 The term " upland " is one which I have heen accustomed to use, for convenience, 

 to include all types of topography other than depressions occupied by lakes, swamps, etc. 

 Under " ordinary uplands " I have included all areas of this description that are not 

 so situated as to be influenced by the proximity of rivers, streams, or seas. 



M The term formation is employed by various continental European ecologists in the 

 same sense that I have here used the term ecological association-type, and in this pro- 

 cedure they are followed by certain American writers. Following Schimper ('03), Cle- 

 ments ('07, '16), Moss ('10), and Tansley ('11, '20), I have applied the term formation 

 to certain geographically defined groups of associations (see section VI), and the term 

 obviously can not be applied in both senses without confusion. 



The term forest-type is in general use among American foresters, but by them it is 



