APPLICATION OF PHYTOSOCIOLOGICAL TECHNIQUES 285 



159 cm., 1; 130 to 149 cm., 1; 110 to 129 cm., 3; 90 to 109 cm., 7; 70 to 89 

 cm., 25; 50 to 69 cm., 32; and 10 to 49 cm., 1,101 stems, from a total of 

 1,170 trees. This number of re-measured stems differs from the original 

 number because of trees that had died in the interval and ones that proved 

 not to be as large as 1 dm. 



No counts were made of smaller trees than 10 cm. d.b.h., but there is 

 no doubt that most of the density and coverage in the lower strata of the 

 forest, including the ground layer occupied by herbs in temperate forests, 

 is formed by reproduction of trees capable of attaining the forest canopy. 

 As will be shown later in the life-form study, rain forest is overwhelmingly 

 phanerophytic. 



Coverage is commonly taken as a measure of dominance. Cover is the 

 combined foliage measured as proportion of ground area shaded when the 

 canopy is assumed to be projected vertically onto the ground, as if there 

 were a direct overhead source of light. In forests generally it is difficult to 

 estimate the coverage of species of trees, and in rain forest this is impossible 

 because the density of the crown and the multiple layers prevent one making 

 accurate observations. It is, therefore, common practice to determine dom- 

 inance not by means of coverage, as for herbs, but by means of basal area. 

 The basal area of a species of tree in a forest is the sum of the cross-section 

 areas of the stems of trees of the species, measured at the standard height 

 of 4.5 ft. above the ground. Having exact measurements of all the stems on 

 the plot, it was easy to compute the basal areas. 



For the entire plot the combined basal area was 65.2 sq. m. for trees 10 

 cm. d.b.h. or over. This is equivalent to 32.6 sq. m. (350.8 sq. ft.) per 

 hectare and 13.04 sq. m. (140.3 sq. ft.) per acre. Vochysm guianensis, with 

 981.2 sq. dm. of basal area on the plot, was the leading species. Goupia glabra 

 was second, with 704 sq. dm. and 10.8 per cent. The first five species in im- 

 portance, according to basal area, had a total of 40.65 per cent of the basal 

 area. Altogether there were only 25 species with individual basal areas of 

 1 per cent or more of the total, and together they made up 71.6 per cent of 

 the total basal area. This leaves nearly 150 species to make up the remain- 

 ing 28.36 per cent. There is, then, in this type of forest no approach to dom- 

 inance by one or a small number of species as the concept is customarily used 

 to apply to temperate forests by foresters and plant sociologists alike. 



Frequency. Frequency is the phytosociological concept concerned with 

 pattern of occurrence of members of a species population within a community. 

 It is determined on a basis of presence or absence of plants of a species on 

 sub-samples taken within a stand of a community. It is commonly expressed 

 as percentage of sub-samples containing the species in relation to the total 

 series of sub-samples. Frequency data have an obvious relationship to size 

 of sub-sample inasmuch as the larger the sample area the greater the chance 

 a species has of being included in it. 



