CHAPTER XV 

 THE CLASSIFICATION OF COMMUNITIES 



A system which is scientifically sound presupposes a knowledge 

 of the material to be classified. The study of plant communities is not 

 at present far enough advanced to supply the minute details for a 

 strictly natural and therefore permanent classification, but the funda- 

 mental outlines of a system that will express natural affinities may be 

 pointed out. These outhnes can even now be used to advantage in 

 treating the communities of a well-studied area. 



Attempts at a classification of plant communities reach well back 

 into the last century. The changes this classification has suffered 

 remind us of the history of systematic botany. The first grouping was 

 based upon obvious but purely superficial characteristics : physiognomy. 

 Later Warming based his division upon one of the most cogent causes 

 of physiognomy: the water relation. He distinguished three great 

 classes : 



Hydrophytes, communities with a high water balance. 



Mesophytes, communities with a medium water balance. 



Xerophytes, communities with a low water balance (Diels, 1918). 



The individual plant communities are arranged under these 

 principal classes. 



Schimper's classification (1898) is based more upon the develop- 

 ment of vegetation. The climatic terminal communities are distin- 

 guished from the beginning and transition stages and are grouped under 

 the headings: forest, grassland, and desert. 



Following out the dynamic-genetic principle of classification, 

 Clements (1916) worked out a system of plant communities, but it 

 was overloaded with hypothetical assumptions. 



Of the newer classifications, mention should be made of Graebner's 

 division of communities according to the nutrients of the soil and also 

 the physiognomic-ecologic divisions of Brockmann-Jerosch and Riibel 

 (1912), Vierhapper (1921), Du Rietz (1921), and Riibel (1930). 



Physiognomic-ecologic Systems. — Physiognomic-ecologic systems 

 aim to arrange the floristically defined associations, or other natural 

 and regularly recurring combinations of life forms in a series of exter- 

 nally related (physiognomic) "formations" (c/. p. 302). They do not 



361 



