ume of 22 species of dicotyledons growing in an area of 0.25 m^ 

 was 164 cc, while that of 6 monocotyledons in the same area was 

 65 cc — a ratio of 2.5 to 1,^ but the ratio of numbers was 3.7 to 1. 

 The volume occupied by plants cannot be inferred from weight 

 measurements because the weight per unit of volume, the 

 density, varies for different parts of plants and for different 

 species; for example, clovers weigh more per unit of volume than 

 do grasses (Figure 1-15). 



Frequency 



Frequency is concerned with the degree of uniformity of the 

 occurrence of individuals of a species within an area. It is meas- 

 ured by noting the presence of a species in sample areas which 

 are distributed as widely as possible throughout the stand, the 

 results being expressed as a percentage, the frequency index or 

 percentage frequency. For example, if one or more individuals 

 of a species is found in each of 15 of a total of 25 samples, its fre- 

 quency is 60 per cent. 



The distribution of a species is rarely regular or uniform in a 

 stand (Figures 3-1 and 3-3). Variation is caused by many in- 

 fluences, such as microhabitat conditions of topography or soil 

 (Figure 2-4), vegetative propagation, quantity and dispersal of 

 seeds, time of invasion of one species as compared to others, graz- 

 ing by livestock, activity of rodents, and depredation by insects 

 or diseases. As a result, patterns may be present, with centers of 

 higher frequency and greater abundance separated by areas of 

 lower frequency and abundance. A pattern of alternation or 

 interdigitation is more pronounced where the topography is 

 irregular or where the soil varies within short distances. For ex- 

 ample, in an abandoned field in Louisiana the frequency of John- 

 son grass {Sorgum halepense) was 28 per cent in the old furrows and 

 per cent on the 6-in. higher ridges, and that of paspalum {Pas- 

 palum conjugatum) 4 per cent in the furrows and 22 per cent on 

 the ridges; while the dominant, goldenrod {Solidago hirsutissima), 

 was high in both — 98 and 84 per cent, respectively. ^^° 



Frequency determinations by means of sample areas are often 

 needed in order to check general impressions about the relative 



112 • THe Community 



