300 PLANT SOCIOLOGY 



thorough monographic presentation of individual plant communities. 

 The biological spectrum of an association sometimes reveals interest- 

 ing and unsuspected ecological relations. In the warm regions of the 

 earth it may become necessary to make subdivisions of the life-form 

 groups. In doing this, care should be taken to make the subdivisions 

 in harmony with Raunkiaer's general outline. 



Each plant community consists of a definite group of life forms. 

 Each habitat favors certain groups of life forms and almost excludes 

 others. The more extreme the habitat conditions the sharper the 

 selection and the more pronounced are the ecological characteristics of 

 the life forms. For this reason biological spectra for decidedly pioneer 

 associations best reflect the ecological relations of the habitat. 



The Loiseleurietum cetrariosum, an exceedingly wind-resistant 

 dwarf-shrub community of snowless ridges in the Alps, is composed of 

 about two-thirds chamaephytes, mostly fruticose lichens, and one-third 

 hemicryptophytes. The rock-cleft association of the Androsacetum 

 helveticae is similarly resistant to wind and frost with 50 per cent 

 chamaephytes (45 per cent Ch. pulvinata, 5 per cent Ch. velantia), 

 45 per cent hemicryptophytes (30 per cent H. rosulata, 15 per cent H. 

 caespitosa), and 5 per cent rhizome geophytes. The adjoining snow- 

 protected Curvuletum, on the other hand, contains 64 per cent hemi- 

 cryptophytes, 30 per cent chamaephytes, 4.5 per cent geophytes, and 

 1.5 per cent therophytes. 



If the biological spectra of two or more regions are closely similar, 

 one may assume that growth conditions are similar. A comparison of 

 the spectra of the Agrostidetum tenuis of the south Cevennes (44° N.) 

 and of the Trisetetum flavescentis of the Schanfigg (47° N.) proves that 

 these two rich meadow communities are nearly ecologically equivalent, 

 as shown by the similarity of their biological spectra (Table 35), in 

 spite of their very considerable floristic differences. 



Table 35. — Biological Spectra of Two Rich Meadow Associations of the 



Arrhenatherion Elatioris and of the Fagetum of the South Cevennes 



Association F Ch H G Th 



Agrostidetum of the south Cevennes 



Trisetetum of the Schanfigg 4.5 89 4.5 2 



Fagetum of the south Cevennes 2 4 51.5 40.5 2 



The above percentage of types in the biological spectra can give 

 only a very vague idea of the total physiognomy of a community, 

 unless accompanied by records of mass and cover of the species. In the 



