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GEORGE E. NICHOLS Vol. IV, No. 2 



afforded by the dry cliffs, the moist slopes, the wet ledges, the inundated rocks, 

 etc., in a rock ravine. From this point of view a physiographic unit-area 

 might be described as a physiographically determined habitat-complex: it 

 represents a complex of habitats linked together by physiographic develop- 

 ment, and therefore to be regarded as a unit in its relation to the physiographic 

 features of the region in which it occurs. 



Demarcation of Physiographic Unit-areas in the Field. — Generally speak- 

 ing, when it comes to practical field studies it is quite possible to demarcate 

 physiographic unit-areas with reference to conditions of either topography or 

 soil. On the whole, and especially in regions of rugged relief, the topographic 

 conditions, through their influence on ground water relations and exposure, 

 seem to be of the greatest ecological importance ; but soil structure and com- 

 position may be nearly as influential in determining the nature of the habitat — 

 even more so in regions of gentle relief. In a physiographic classification of 

 plant associations, then, both topographic and edaphic conditions must be taken 

 into consideration, although necessarily either one or the other must be selected 

 as the primary basis. 



With regard to the treatment of specific cases, it is not possible to lay 

 down rigid rules. Thus from a purely geographical standpoint it may often 

 be possible to distinguish a larger number of physiographic divisions than is 

 necessary from an ecological point of view. Take, for example, two areas 

 of rocky upland which are alike in their topographic relations, but unlike in 

 the character of the rock concerned. Physiographically these may be differ- 

 ent, but it is only as such differences affect the character of the vegetation 

 that they become of ecological consequence. For this reason, in the case of 

 uplands in particular, it is often most practicable to group several areas, which, 

 strictly speaking, may be physiographically distinct, under one head. Thus for 

 ecological purposes it might be quite sufficient to distinguish the two upland 

 areas just mentioned as ordinary rocky uplands (in contrast to sandy uplands, 

 or to rocky uplands along the seacoast or along streams), taking no account 

 of the more detailed physiographic subdivisions based on geological dissimi- 

 larities. The difficulty of making any definite rules by which to recognize 

 individual physiographic unit-areas in the field is increased by the fact that 

 for various reasons (suggested on page 166) it may be advisable to interpret 

 physiographic unit-areas more broadly in some cases than in others. After 

 all, it is the vegetation in which we are primarily interested, and the sharpness 

 with which we attempt to draw physiographic boundaries is best governed 

 accordingly. 



PJiysiograpIiic Formations. — Taken in its entirety, the vegetation of any 

 specific physiographic unit-area may be designated a physiographic forma- 

 tion: 44 it comprises a complex of associations which are geographically linked 

 with one another by physiography, and it may therefore be referred to in 



"The same as edaphic formation, in part, of Nichols ('17). 



