that the exact combination and sequence of events are seldom 

 if ever repeated,^'* yet, as in any kind of classification, exact 

 duplication or likeness is not essential for grouping stands 

 into abstract categories. The degree of similarity needed for 

 placing stands in the same community-type or association re- 

 quires careful judgment; but, as in evaluations of many bio- 

 logical phenomena such as stages in mitosis, different investigators 

 have been found to agree closely in their decisions. In placing 

 stands in community-types, an increase in objectivity is secured 

 by employing quantitative criteria such as the coefficient-of- 

 similarity.25'62127.103 While similarity is the chief criterion in 

 classifying stands, a certain degree of relationship, or affinity, is 

 also present because of the common evolutionary origin of in- 

 dividuals of a species and because of interactions such as pollina- 

 tion, and obviously the greater the number of species in common, 

 the greater will be the affinity. 



In some places, very small units, such as a square meter or only 

 a few square centimeters, of various kinds of vegetation occur re- 

 peatedly. These variations in species composition are caused by 

 changes in the environment, particularly in the substratum. For 

 example, the top of a mound in a bog or marsh may have a very 

 different plant cover from that in the depressions, and the north 

 and south sides of a single mound may vary greatly, especially in 

 the Arctic and Subarctic. Such small units are microstands, each 

 kind with its inherent characteristics and specific habitat condi- 

 tions. The aggregate of such microstands in an area may be 

 considered as a community-complex when there is some order 

 or pattern in distribution as on the borders of lakes or ponds, on 

 sand dunes, around ant-hills, and on snow-bed areas in alpine 

 regions. When the microstands are intermixed without any ap- 

 parent pattern, as commonly occurs in marshes and bogs, the 

 conglomeration may be considered as a community-mosaic.^ ^7 

 The vegetation on mounds in the tundra may consist largely of 

 wind-resistant lichens and low shrubs, while in the depressions 

 mosses, other kinds of lichens, and taller shrubs grow. It is often 

 desirable to treat the entire mosaic or complex as a unit, but each 

 of the various kinds of microstands composing it must be an- 

 alyzed to secure a full understanding of the vegetation. Similar 



1.74 • Classification of Comsnunities 



