THE DEVELOPMENT OF COMMUNITIES 319 



of Glacier Bay (Cooper, 1923). Rhacomitrium canescens takes the same 

 important place in the successional series on the fine-grained glacial 

 and alluvial gravels of Alaska as in the Swiss and French Alps; Cyperus 

 capitatus functions in the dune sands of Agadir exactly as it does on 

 the Mediterranean shores of France (Fig. 160). In general, there 

 are many parallel (homologous) developmental series in regions of 

 similar climate, whose similarity is to be ascribed to similar changes in 

 the habitat. 



Constructiveness of Species in the Alpine Gravel Associations. — 

 Scarcely anywhere is the constructiveness of species so unmistakably 

 clear as in the gravels and the dune associations. The moving lime- 

 stone talus of the eastern Alps is colonized by the "open" Thlaspeetum 

 rotundifolii. Although the distance between individual plants is 

 usually more than one meter, root competition is present. In the 

 Thlaspeetum there appear constructive tufted plants such as Festuca 

 pumila and Sesleria coerulea or creeping shrubs such as D. octopetala. 

 The spreading Dryas carpets become destructive to the Thlaspeetum, 

 but at the same time they are constructive for the following association : 

 the Seslerieto-Semperviretum or the Firmetum. Humus accumulates 

 in the dense mat of twigs of Dryas and is kept from washing or blowing 

 away by the covering of the leaves. This forms a favorable seed bed 

 for grasses. Thus the deeply anchored Dryas carpets become the 

 centers of tuft formation on the talus. With the advent of the tuft- 

 forming grasses and sedges, however, the creeping shrub is more and 

 more overgrown and finally suppressed. 



The dynamogenetic relations of the species of the Stipion calama- 

 grostidis and the Thlaspeion rotundifohi of the alpine gravel slides are 

 admirably treated by Jenny-Lips (1930); those of the Ammophihon of 

 the Mediterranean strand dunes have been studied by Braun-Blanquet 

 (1921), by Klihnholtz-Lordat (1923), and by Burollet (1922). 

 Kolumbe (1925) and Christiansen (1927) give much information about 

 the constructiveness of the various species of the north German dunes. 



In strand regions and in high mountains it is especially evident that 

 not every plant that functions in the geomorphic cycle as gravel holder 

 or sand binder is also of sociological importance. Thus to none of the 

 species of the Thlaspeetum cited above can be ascribed a constructive 

 value for the association, although some species which hold the gravel 

 may be called "consolidating." 



The high moor furnishes a clear example of species of different 

 constructiveness. The splendid monographs of moors recently 

 published by Mehn, Osvald, Hueck, Booberg, and others have not 

 sufficiently emphasized this viewpoint. 



