574 PLANT GROWTH AND PLANT COMMUNITIES 



loosely) into a given stand (or association)? The preliminary condi- 

 tion is that they be within dispersal range thereof, which is not neces- 

 sarily so of all members of the regional flora. 



The figures that can be obtained for a stand or an association will 

 therefore indicate in each case what percentage of the regional species 

 is able to take at least some advantage of this particular ecosystematic 

 complex. It gives no clue whatsoever to the relative number of dom- 

 inants (concomitant or alternative) nor indeed of any exclusivity. In 

 this respect the flora-vegetation ratio may be quite characteristic of an 

 area. For instance, in Central Baffin the striking ecological amplitude 

 of very many species makes for a high percentage of regional flora in 

 several communities. This also coincides with poorly defined bound- 

 aries between communities. In Central California, where the vegetation 

 mosaic is quite complex, it is rather remarkable how short the species 

 lists in many stands can be. 



3. Indicators. The very presence of a certain species, especially if it 

 is somewhat abundant, is the traditional starting point of all plant soci- 

 ology. Therefore, quite apart from the sheer numbers of identifiable 

 species, some individual characterization is due each and every one 

 that occurs at least once in a quadrat, in a stand, or in the association as 

 a whole. Three principal qualities can be sought in this respect: the 

 floristic groups into which the species fall, as suggested by their general 

 geographical distribution; the indicator groups which their ecological 

 amplitude casts them into; and finally their frequency and/or con- 

 stancy in the particular unit considered. 



a. Floristic elements. There are not too many plant communities 

 whose component species all belong to the same floristic element. An 

 "element" can be defined as a group of species (usually taxonomically 

 unrelated ) which have a fairly long common history. It is in that sense 

 that phytogeographers have spoken of "boreal," "amphi- Atlantic," "Ap- 

 palachian," "Mediterranean," and other elements. Such designations are 

 usually not based on actual knowledge of past migration from fossil 

 evidence but more generally on the present general distribution and 

 form of area, on the one hand, and on the geographic location of the 

 closest of kin, on the other. Since the days of Humboldt, Asa Gray, and 

 Engler, such gradings have been widely apphed: WulfiF (1943), Cain 

 (1944), and Good (1953) have abundantly discussed the justification 

 and consequences of these recognized floristic affinities. No doubt the 

 Scandinavians have used it more thoroughly than others, especially 

 Hulten (1950), whose theory of equiformal areas (1937) deserves to 

 be tested in other parts of the world. 



It must be emphasized that the total geographic area of a species 

 is a bioclimatic unit of a certain magnitude, usually exceeding that of 

 one or more communities to which it belongs and almost always ex- 



