THE STRUCTURAL CHARACTERISTICS OF THE COMMUNITY 31 



species on each sample plot is recorded. The density (distance between 

 individuals) of a species is equal to the number of square units of the 

 total area divided by the sum of the individuals (r/. Martinet, 1898). 

 The exactness of the result obtained increases with the number of 

 sample areas examined and with the evenness of the distribution of 

 the species throughout the entire community. ^ The size of the sample 

 plot chosen may vary within wide limits and naturally depends upon 

 the kind of vegetation. In a Mediterranean therophyte community 

 areas of 0.5 or 1 sq. m. are appropriate ; in the beech or spruce woods at 

 least 500 sq. m. must be taken for the tree layer. This counting 

 method is especially indicated in mature timber of uniform age, in 

 bush steppe, and in rock-slide communities. IMuch more difficult is its 

 application in grassland communities, with tussock or sod-forming 

 plants, where it is further complicated by the difficulty of distinguishing 

 the individuals and counting the shoots. If density can be determined, 

 the determination of abundance would obviously be superfluous. 



Cover, Space, Weight (Deckungsgrad, Dominanz). — The question of 

 the space demands of a species is a wholly different concept from that of 

 the number or density of the individuals. In close connection with the 

 former stands the question of the cover (Deckungsgrad) of the species. 

 In clinging communities (algae, hchens, mosses) the degree of dominance 

 is identical with the relative participation of the species in the cover- 

 ing of the surface (cover, areal percentage). In several-layered tree 

 and shrub communities the degree of dominance of a species is given by 

 its share in the different layers of the vegetation. 



Determination of Cover. — The degree of dominance of clinging and 

 rooted species is expressed in all field work by the cover, that is, the 

 area covered by the individuals of one species. The entire shoot 

 system of all the individuals of a species is thought of as projected on 

 the ground, and the area covered thereby is estimated. In distinctly 

 layered communities each layer must be estimated separately. The 

 significance of cover was recognized by Von Post (1867), but it was 

 first clearly distinguished as "area! percentage" in 1914 by Lagerberg. 

 For the determination of cover, Lagerberg (1915) used small quadrats 

 of 0.5 or 0.1 sq. m. which were arranged in straight lines regularly 

 spaced over the area studied. The estimate of the surface covered by 

 each species in the quadrats was indicated by means of the fractions 

 ■^i, H, %, }i, and each species received the number nearest to the 

 fraction of the area actually covered by it. Species with very small 



' It sliould therefore be noted whether the mode of distribution is regular 

 (hypodisperse) or irrefT'diir and (-rowded in places (hyperdisperse) (rf. Sehustler, 

 l<r2;i, and p. 36). 



