THE DISTRIBUTION OF COMMUNITIES 



345 



are best to be considered (following Koch, 1928) as impoverished 

 altitudinal stands of the Caricetum infiatae-vesicariae. 



Topographic Arrangement of Units of Vegetation.— The topo- 

 graphic arrangement, the interdigitation, the mutual penetration of 

 plant communities are occasioned primarily by external factors, 

 secondarily by competition of the species and communities. Such 

 arrangements are either zonal or mosaic. In both cases the alternation 



Fig. 172.— Round hillock complex at 2.010 m. in the Tatras, showing somewhat 

 diagrammatically a girdle arrangement of vegetation due to duration of snow cover; 

 association of Polytrichum sexangulare (a) , Salix herbacea (6) , Luzula spadicea (c) , and the 

 Trifidi-Distichetum {d). (Braun-Blanquct.) 



or mixture of communities follows regularly. It is continually the 

 same plant communities which meet, interpenetrate, and compete with 

 one another. We speak of them as contact communities or contact 

 associations. 



Zonation.—A girdle or beltlike arrangement of the units of vegeta- 

 tion, whether on a large or on a small scale, is caused by similarly 

 arranged differences of important factors of the habitat: temperature, 



E S£ 



ff^Nff 



Fig. 173. — An inversion of the girdle arrangement of vegetation due to drifting of 

 snow in the Tatoas at 2,160 in. Associations of Calamagrostis-Luzula spadicea {A) , Salix 

 herbacea (B), and the Trifidi-Distichetum (C). 



soil moisture, salt- or nitrogen-content of the soil, duration of snow 

 cover, wind, etc. (Figs. 172, 173). The concentric vegetation zones of 

 the earth (not to be confused with ''regions of vegetation") are due to 

 the gradual increase in temperature from the poles to the equator. 

 Decrease in humidity and precipitation toward the interior of con- 

 tinents causes zonation into belts of forest, grassland, steppe, and 

 desert. 



