SYXTHETIC CTfAEACTERS OF THE COMMUNITY 58 



tion of an association. All sample stands examined in the study of an 

 association should be at least of the size of the minimal area and must 

 have the characteristic combination of species. Minima of space and 

 number of species differ for each and every association, and their 

 fixation can be accomplished only by empirical methods. 



Degrees of Presence. — The presence grades of the species of a com- 

 munity are conveniently expressed by a 5-degree scale : 



5 or V = constantly present (in 80 to 100 per cent of the stands); 

 4 or IV = mostly present (in 60 to 80 per cent of the stands). 

 3 or III = often present (in 40 to 60 per cent of the stands). 

 2 or II = seldom present (in 20 to 40 per cent of the stands). 

 1 or I = rare (in 1 to 20 per cent of the stands). 



In accounts of a rather general nature the words "constantly," 

 "mostly," etc., may be used instead of the figures. 



Presence may also be expressed as a fraction C^Hsf %)• The 

 denominator gives the number of stands examined; the numerator, the 

 number of occurrences. The species of the highest presence classes — 

 those in at least four-fifths of the stands examined — may be designated 

 as the "constants" of the association. 



Determination of Presence. — The determination of presence is made 

 upon normal mature stands (association individuals). It tells in how 

 many of the stands examined the particular species occurs as a com- 

 petitor, Ilvessalo and others have shown that the group of species 

 showing the highest degree of presence varies greatly with the age of a 

 timber stand. Initial stages, therefore, must not be mixed up with 

 mature phases of an association. Further, the stands recorded should 

 be distributed as evenly as possible over the region studied ; but they 

 must not be taken from stations with wide differences of altitude. 

 Sub-associations and facies must be discussed separately. 



The question of how many stands are necessary to conclude a study 

 of presence cannot be answered by any general rule. Ten records often 

 give a satisfactory picture ; in those very rare cases of associations which 

 are ecologically very strictly specialized throughout, fewer records will 

 do. A few well-developed stands are better for the determination of 

 presence than a large number of floristicaUy heterogeneous examples. 



Presence and Constancy. — There is no difference in principle between 

 "presence" and "constancy." Practically, however, it is advisable 

 to designate as studies of constancy those investigations of presence 

 which are made with plots of sharply limited area {e.g., 0.5, 1, 2, 10 sq. 

 m.). Only such studies can be treated by mathematical-statistical 

 methods. When so conducted they have a real value for deepening 

 our understanding of the structure of plant communities. 



