CHAPTER XIV 

 THE DISTRIBUTION OF COMMUNITIES 



The aim of the study of synchorology is the concise description of 

 the occurrence and distribution of plant communities, the deHmitation 

 of their areas, and the grouping of them into regional units. Conclu- 

 sions concerning the origin, descent, and ecology of communities may 

 be drawn from their areal configuration. These regional units are 

 important indicators of climate and soil conditions and are indispensable 

 for the biogeographic characterization of the great divisions of the 

 earth. In order to understand the details of the distribution of 

 vegetation, the smaller units (associations, alliances) or at least the 

 association complexes must be included within the scope of the 

 investigation. 



Geographic Variants. — Every widely distributed association shows 

 characteristic modifications in floristic composition in the different 

 parts of its range. These may be termed geographic variants or races. 

 These variants may be due to distributional or historical causes, as 

 when the ranges of the species of the companion plants of the com- 

 munity are not coterminous with the range of the community. Under 

 certain circumstances the interplay of external factors in two adjacent 

 areas gives a similar but not identical sociological effect. Absolute 

 identity between all the individual stands of an association is extremely 

 rare, occurring only between communities which have few species or are 

 very narrowly localized. 



Different geographic variants of an association are often character- 

 ized by parallel species with similar ecological requirements. Senecio 

 unijlorus, a characteristic species of the Pennine Curvuletum, is 

 replaced by S. carniolicus in the Rhetic Curvuletum. The central 

 Swiss Xerobrometum has Anemone montana and Potentilla puberula; 

 that of the upper Rhine and northern Switzerland has A. Pulsatilla 

 and P. arenaria, with similar ecology. 



Where the areas of two geographic variants come into contact, their 

 boundary is often indistinct : they merge into one another as in the case 

 of the Xerobrometum of Swabia and northern Switzerland and that of 

 the upper Rhine. If they are separated by barriers, transitions are 

 lacking, and the floristic individuality is much better preserved. This 



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