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GEORGE E. NICHOLS Vol. IV, No. I 



grows in — and in this respect it may be that a subtle distinction can be drawn 

 between habitat and environment; but, if so, it is practically a distinction 

 without a difference. The use of the term habitat is sometimes restricted to 

 areas in which all the environmental conditions are essentially homogeneous 

 throughout, 12 but the habitat idea is capable of much broader application. In 

 a general way, it can be applied to areas of any description, such as north- 

 facing slopes, ravines, flood plains, or salt marshes, which are characterized 

 by uniformity with regard to certain particular environmental conditions. 13 



General Habitat Relations of the Association. The plant population of 

 any particular piece of ground has originated through immigration from 

 surrounding areas. " Of all the immigrants into an area, only those may 

 establish themselves which find in it environmental conditions within the limit 

 of their own environmental demands. The actual mature immigrant popu- 

 lation of an area is therefore controlled by two sets of factors : the nature of 

 the surrounding population, . . . and the environment " (Gleason, '17). 



In so far as the ecological characteristics of the vegetation are concerned, 

 a plant association is to be looked upon as an effect of which the habitat 

 factors are the cause. And since the vegetation is essentially homogeneous in 

 these particulars, it follows that the habitat must also be homogeneous through- 

 out the area which an association occupies. This may be accepted as a logical 

 corollary to the definition of plant association ; in fact, this correlation carries 

 so much weight in the minds of many ecologists that habitat uniformity is 

 specifically included in their definitions of the term association. 14 



Habitat Variations within the Association. A distinction must be made 

 between the general habitat relations of the association as a whole and the 

 specific habitat relations of its constituent elements. To begin with, each 

 particular life-form which enters into the composition of an association may 

 have its own particular habitat relations. Thus, in a forest the small trees 

 differ in their habitat relations from the dominant trees in whose shade they 

 grow, on the one hand, and from the low shrubs or herbs which carpet the 

 ground, on the other. At the same time, for each of these life-forms the 



12 Such as the area occupied by a specific association : see observations in later para- 

 graphs. This is the sense in which I have interpreted the term in an earlier paper ('17). 



13 Interpreting the term in its more restricted sense, areas of this description would 

 have to be regarded as habitat-complexes (see Nichols, '17). 



14 Riibel ('12, p. 93), for example, defines the association as "a plant community 

 of definite floristic composition, uniform habitat conditions and uniform physiognomy," 

 and I myself ('17) have stated that "uniformity of habitat affords the criterion of the 

 association." The elimination of the habitat idea from the definition of the association 

 has been urged more especially by Du Rietz, Fries, and Tengwall ('18), and, it seems 

 to me, on good grounds. The association is a vegetation-unit which is naturally defined 

 by certain inherent characteristics of form and structure. As soon as we attempt to 

 introduce the habitat as a criterion in defining it we are departing from the facts presented 

 by the vegetation : we are considering the causes of the facts. 



