350 PLANT SOCIOLOGY 



lower. According to Beck, Karstdolinen that are long covered with 

 snow have at top a spruce forest and farther down Pinus pumilio and 

 scrub, then Rhododendron and alpine rock plants, and finally moor. 



Community Complex. — Among dunes, on high moors, on recent 

 alluvium, and on snow land the vegetation often appears as a promiscu- 

 ous mixture of associations and fragments of associations. On closer 

 examination there is an unmistakable regularity in the arrangement, in 

 that very definite social groups occur repeatedly. The whole com- 

 munity complex, topographically limited, often forms a genetic unit 

 (sere) and has a pronoimced physiognomic unity. Moors and swamps 

 were among the first community complexes to be studied (Fig. 176). 



The "Pozzines" of the alpine levels of Corsica present a very 

 characteristic land-forming complex. Their sociological composition 

 and development were explained by De Litardiere and Malcuit (1926) 

 (Fig. 177). 



Various complexes are to be distinguished in the high mountains, 

 depending on the altitude and the substratum. There are wind- 

 erosion complexes, snow-soil complexes, flood-plain complexes, etc. 

 Well-defined community complexes of considerable extent are readily 

 shown by means of maps. This method is especially useful to 

 geographers. 



Migrations of Plant Communities. — Natural displacements of 

 area proceed very slowly. They are due either to changes in the 

 habitat or to plant migrations still in progress. The work of Scan- 

 dinavian scholars shows that the spruce forest is gradually creeping 

 over the watershed from the Swedish to the Norwegian side of the 

 Scandinavian mountains. The investigations of Lewis and others 

 seem to demonstrate that in the park land of Alberta, willow scrub and 

 stands of Populus tremuloides have recently invaded the adjoining 

 grassland. 



Displacements of the area of whole plant communities are often 

 related to the advance or retreat of constructive or destructive species 

 of high dynamogenetic power. The spread of a Fagus association is 

 due to migration of Fagus itself. A Curvuletum can appear only where 

 Car ex curvula has become established. The contact of communities in 

 well-vegetated regions gives rise to an intense struggle the outcome of 

 which determines the entrance or the disappearance of numerous 

 satellites. 



The sudden appearance of natural associations, as in quarries, on 

 floating rafts of debris, on the bottoms of drained ponds, and in artificial 

 water reservoirs, is always evidence of a feeble interrelation of the 

 species and very little mutual dependence among the different members 



