38 



pearance of the vegetation and does not affect the growth of the sec- 

 ondary forms. There are frecjuently no successional relations be- 

 tween these local areas, or consocies. Areas characterized by dom- 

 inant species of widely different appearance can not be regarded as 

 belonging to the same association unless it can be shown that the 

 areas represent transitory stages of development, as described below 

 for the stabilization of blowouts. If the dominant species have the 

 same general form, but do not tend to mix, except in the tension zone 

 between them, and are accompanied by different groups of secondary 

 species, the occurrence of different associations is sug-gested, as in 

 the black oak and bur oak forests. 



The secondary species occupy a comparatively small area in the 

 associations and their number usually depends in some way upon the 

 habits of the dominant species. This is well illustrated by the bunch- 

 grass association, in which the secondary species are absolutely de- 

 pendent upon the dominant bunch-grasses. More species are con- 

 cerned and their distribution is frecjuently irregular. These irreg- 

 ularities, however, are seldom coincident with any variation in the 

 dominant plants, but are caused chiefly by competition for space reg- 

 ulated by seed dispersal and seasonal climatic fluctuations. A con- 

 siderable variation in their quantitative distribution may be expected, 

 unless they belong to the derived element of the association (see be- 

 low), in which case they may indicate the beginning- or the end of a 

 succession or some local change in the environment which is never- 

 theless not sufiicient to induce a change in the dominant species. 



Two areas of vegetation dominated by different species are ac- 

 cordingly probably consocies of the same association if (i) there 

 is no obvious difference in their environments; (2) if there is no evi- 

 dence of succession between them; (3) if the secondary species are 

 the same for each; (4) if the dominant species are of the same 

 vegetative form or (5) tend to mingle in other areas with the same 

 environment and secondary species. 



A slight deviation from these criteria may be neg'lected if there 

 is a preponderance of agreement with them, while a radical deviation 

 would indicate that the areas represent distinct associations. In most 

 cases (in the sand areas, at least) their application in the study of the 

 vegetation leads to definite and unquestionable results. 



Whether small or large, associations usually contain some species 

 which are more characteristic of other areas. This derived clement 

 can be recognized only bv comparison with neighboring associations, 

 where the species in question are more numerous, more general in 

 distril)ution, or more luxuriant in growth. The^' are least abundant 



