352 PLANT SOCIOLOGY 



Swiss area on the northwest, this community covers large areas. Here 

 it is mixed with more distinctly Atlantic species such as Galium 

 hercynicum and Digitalis 'purpurea. But in northwestern France it 

 reaches its full development as the Q. sessiliflora-H . pulchrum associa- 

 tion, as described by Gaume (1924) from the forests of Orleans. 



Along with this pronouncedly Atlantic outlier there are in south- 

 central Europe other very fragmentary communities of distinctly 

 northern origin, such as the Carex limosa, the C. lasiocarpa, the Sphag- 

 num fuscum, and the Isoetes echinospora associations. 



On the other hand, there are everywhere in northern Europe, 

 according to Cajander (1921, p. 26), many associations which occur 

 only in the most favorable soil conditions and are derived from com- 

 munities that are much better developed farther south. 



This is true also of the forest communities dominated by Fagus 

 silvatica. 



The data of geology, paleontology, and pollen analysis all indicate 

 that the beech forest in relatively recent geologic time penetrated into 

 central and northern Europe. Fossil leaves show that the beech was at 

 home in central and southern France both in late Pliocene time and at 

 the close of the glacial period. Nowhere do we find the beech associa- 

 tions better developed as climaxes and richer in important 

 characteristic species than in southwestern and southeastern Europe. 

 These were the postglacial centers of distribution of the beech associa- 

 tions. The north German and Scandinavian beech forests, with their 

 raw humus and meager companion flora, give a wholly inadequate idea 

 of the vigor, the richness, and the abundance of species of the original 

 southeastern and southwestern beech associations. Whether the 

 beech forests of southern Sweden are really climax communities is 

 therefore very doubtful (cf. Tamm, 1921, p. 246). 



Pioneer and Relict Communities. — From the standpoint of distri- 

 bution in both time and space we distinguish aggressive, advancing 

 pioneer associations and restricted, retreating relict communities. 

 In regions of young topography pioneer associations predominate 

 (c/. Cooper, 1923). 



Among relict associations are those described by W. Christiansen 

 from the oak scrub of Schleswig-Holstein and the remnants of high 

 moor in southern Germany and in southern Europe generally. Such 

 also are the communities of geologically ancient regions, like Mada- 

 gascar (Perrier de la Bathie, Humbert), Cape Town (Marloth, Adam- 

 son), the mountains of the Sahara and the Sahara Atlas (Battandier 

 and Trabut, Maire), and Corsica (De Litardiere and Malcuit, 1926). 

 Geographic and climatic isolation are essential for the preservation of 



