species, and in stratification. Another example is seen in western 

 wheatgrass and cheatgrass which form stands of single- or few- 

 stalked plants growing close together with a high degree of 

 uniformity, very different from the clumped bluebunch-wheat- 

 grass stands. 



Physiognomy, although useful in recognizing and delineating 

 different kinds of communities, cannot be substituted for analytic 

 characters in sociological analysis, but it is valuable in prelimi- 

 nary investigations or reconnaissance, both on the ground and by 

 airplane, to be followed by sociological analysis. Physiognomy is 

 also useful in broad and general descriptions of vegetation as an 

 introduction to thorough understanding of a stand. Complete 

 sociological analyses, accompanied by physiognomic descriptions, 

 are still needed for most vegetation types in North America. 



Pattern in vegetation occurs in the form of groups or clumps 

 of individuals, or in any other nonrandom arrangement of 

 plants. ^^ Physiognomic contrast between groups, such as shrubs 

 in a grassland (Figure 1-8), or zones around a lake, accentuates 

 pattern and is readily seen; but slight differences in density, 

 cover, or frequency often require quantitative methods of deter- 

 mination before the pattern is detected. If the pattern is small in 

 scale, sampling with a small quadrat is necessary to determine it, 

 but if the pattern is both large in scale and high in intensity, such 

 as patches of dense aggregations of individuals of a species 

 separated by areas where they are absent, a large quadrat is 

 needed. If, however, the pattern is large in scale but consisting, 

 for example, of patches of one or more species of higher and 

 lower density in a mosaic, a much smaller quadrat size is required. 

 In analyzing vegetation for pattern, the qualities, density, cover, 

 and frequency are most serviceable criterions. 



Causes of pattern may be grouped under three headings: (1) 

 morphological, in which the growth of a propagative organ such 

 as a rhizome is very important; (2) sociological, in which com- 

 petition and association of species are of great import; and (3) 

 physiographic (Figure 1-4), in which topographic variations in 

 soil moisture, concentration of nutrients, soil texture and struc- 

 ture, and others, are concerned. ^25,126 jj^ ^j^g initial colonization 



Symmthetic Characteiristics of the Coxnmiaimity • IS! 



