200 CLASSIFICATION OF PLANT COMMUNITIES 



127. Plant Formations.— In dealing with any group of objects, 

 li\ing or otherwise, it is often convenient, or even necessary, to 

 classify them in some logical way. The particular kind of classifica- 

 tion that is best depends largely upon the use that is to be made of it. 

 For example, we may classify seed plants according to growth form, 

 or on the basis of relation to water supply, or into natural families 

 and orders, and each of these classifications has valuable uses. 



There has been a considerable amount of discussion and disagree- 

 ment among ecologists concerning wdiat is the best scheme of classi- 

 fication of plant communities. If, however, we recognize that plant 

 communities are individuals we should be able to classify them in 

 any way that may suit our purpose, just as we do in the case of indi- 

 vidual plants, without being subjected to undue criticism. 



One of the most used as well as most useful of the methods of 

 classifications is that of grouping the plant associations into plant 

 formations. A plant formation consists of all of the associations 

 within a climatic region. Since it is a product of the climate and is 

 controlled by it all of the component associations present similar 

 physiognomies and have similar climatic requirements. For exam- 

 ple, all of the deciduous forests of the eastern portion of the United 

 States are considered as belonging to one formation although the 

 various deciduous forest associations differ from one another con- 

 siderably floristically. Likewise, the various grassland associations 

 of the central United States may be grouped along with bunch grass 

 association of California and that of the northern portion of the 

 Great Basin into one grassland formation. 



Unfortunately it cannot be said that all ecologists agree that the 

 above conception of plant formations is the correct one. Many 

 consider the formation as the individual of the third type and the 

 associations as morphological parts of the formation. It seems more 

 logical, however, to consider the formation a group of individuals 

 rather than a single organic entity. The pinyon-jimiper woodland 

 of the western states, for example, is all considered as belonging to 

 the same formation but there is a pinyon-juniper community in 

 the Chisos Mountains of southern Texas, another in northern New 

 Mexico, and still others in several other western states. These are 

 all separate, individual plant associations and not parts of a single 

 individual but they may all be grouped together into one woodland 

 formation. 



128. Successional Series of Communities.— The vegetation of a 

 region cannot be thoroughly studied without taking into considera- 



