GROUND ASSOCIATION 17 



heterogeneous composition, while closed formations have a 

 definite uniform stamp. Willkomm (t896:29) made use of 

 the same distinction in classifying the plant formations of 

 Spain, and Pound and Clements (!898 : 321, 405) related the 

 concept more closely to association. "What is true of for- 

 mations is equally true of vegetation, so that association may 

 be regarded as open or closed with reference to the densitv 

 and thoroughness with which the plants occupy the ground. 

 In open association (assoaiatio aperta), the ground is 

 slightly or partially occupied, readily permitting the entrance 

 of new plants without the displacement of those already 

 present. Such an arrangement is characteristic of the early 

 stages of a formation, or of a succession of formations. It pro- 

 duces unstable open formations (sporadophytia), ' which arise, 

 usually after denudation, in sandhills, blowouts, gravel slides, 

 dunes, flood plains, burned areas, etc. In closed associ- 

 ation {associatio clausa ), occupation of the ground is com- 

 plete, and the invasion of new species can occur only through 

 displacement. Closed association results in stable, closed 

 iormatioias (pycnophytia), such as forest, thicket, meadow and 

 prairie. As open association characterizes the early stages 

 of a succession of formations, so closed association is 

 peculiar to the later or last stages of all such successions. In 

 short, open formations represent certain phases of the 

 dynamic condition of vegetation, while closed formations 

 correspond to the relatively final static conditions of vege- 

 tation. It is a fundamental principle of association that 

 every succession from denudation, or from newly formed 

 soils begins with open formations, and ends with a closed 

 formation. The causes leading up to open and closed associ- 

 ation are intimately connected with development, and will 

 be considered under invasion and succession. 



Species guild association (associatio peregrina). Drude 

 (1885) has distinguished a kind of association pecaliar to in- 

 vasion, in which there is a successive or concomitant 

 movement of certain species of a formation into another 



'Clements, F. E. A System of Nomenclature for Phytogeography 

 Engler31:bl 1902. 



