THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 573 



consists in determining the "minimal area"— i.e., the smallest areal unit 

 likely to contain most of the characteristic species of a given associ- 

 ation. This is done by cumulative sampling of quadrats which are 

 doubled a number of times— for instance, 10 cm.^, 20 cm.^, 40 cm.^, etc. 

 It may be immediately apparent that plant communities that include 

 the larger life forms (such as trees) will of necessity have a greater 

 areal requirement. But it does not follow that the latter is necessarily 

 proportionate to mass-development. Cain and Castro (1959) have very 

 recently reviewed the manifold aspects of this question and abundantly 

 documented its methodology. 



b. Stand and association. The constancy of a given species is de- 

 duced from the percentage of its occurrence in the total number of 

 stands sampled, whereas its frequency is its percentage of occurrence 

 in the quadrats of a single stand. ( See Braun-Blanquet, 1932, 1951, for 

 original definitions; see Dansereau, 1957a, for applications. ) These no- 

 tions, which first come to mind, will be considered below, although 

 it must be remembered that they cannot well be dissociated from the 

 various calculations meant to show relative richness. In all the pub- 

 lished phytosociological charts, considerable variation occurs in the per- 

 centage of species that recur in any given stand. The charts themselves 

 are designed, in the first place, on the basis of this recurrence (con- 

 stancy ) and on the basis also of other criteria ( such as fidelity. ) Table 

 I shows some of the extremes and ratios obtained with respect to the 

 stand/association relationship. 



c. Association and regional flora. The richer the regional flora, the 

 greater the availability of building blocks for any given community. 

 But it does not follow that the association will be richer, for a number 

 of factors will counter this probability. For instance, some of the ri- 

 parian associations along the Amazon are very poor in species, whereas 

 the regional flora is the richest in the world. On the other hand, it has 

 long been recognized that the upland rain forest comprises some of the 

 richest stands of vegetation ever recorded ( Schimper, 1903; Warming, 

 1909). It has often been said that this richness is such as to preclude 

 any proper delimitation of associations on a floristic basis, detectable by 

 the means commonly employed by phytosociologists. More careful 

 study has shown that dominance does indeed occur in some cases 

 (Beard, 1944; Veloso, 1945), although it may be more shifting than 

 elsewhere (Aubreville, 1949). Recent studies (Schnell, 1952; Cain, 

 Castro, Pires, and da Silva, 1956; Veloso and Klein, 1959) all tend to 

 show that it is quite possible to apply current phytosociological con- 

 cepts and methods to this richest of all vegetations. 



In cooler and drier regions the relationship of regional flora to 

 association flora assumes more manageable proportions. The real ques- 

 tion is: How many of the available species are incorporated (however 



