THE DEVELOPMENT OF COMMUNITIES 317 



ground with unyielding persistence against fire, pasturing, and erosion, 

 and yet neither shrub is ever important in building up the community. 



Trees are often destructive to a preceding community by changing 

 the light, the aeration of soil, or the moisture conditions. On the other 

 hand, they have great constructive value wherever, in their shade, 

 special communities develop and persist. The primeval beech woods 

 of the Cevennes, the Auvergne, the Pyrenees, and Corsica present 

 combinations of companion species whose existence is wholly condi- 

 tioned by the beech-tree layer (Braun-Blanquet, 1915). The same 

 is true of the Picea excelsa forests of the Alps and the Tatra (Beger, 

 1922; Szafer et al, 1923; Dutoit, 1923). 



Many turf societies are dynamically independent of the dominant 

 forest trees in such communities as the subalpine larch woods, the 

 northern birch woods, and the Mediterranean Pinus halepensis forest, 

 where the trees cast little shade and form little raw humus. Thus 

 there are all transitions from direct causality to complete independence 

 of ground vegetation in the forest. The relation of Pinus halepensis to 

 the association of Rosmarinus and Lithospermum fruticosum (Erica 

 multiflora facies) west of the Rhone is noteworthy. This association is 

 identical both under a tree layer of Pinus and in the open but is con- 

 tinually threatened, especially on steep slopes, with being completely 

 washed away by torrents of rain (cf. Fig. 65). A roof of pine branches 

 benefits the shrub association by affording partial shelter from torrents 

 of rain and by modifying the wind and the temperature. The normal 

 shrub society is therefore better protected in the forest than in the 

 open, as long as the light is not too much reduced by the trees. This 

 could happen only under very special conditions in the P. halepensis 

 forest. Thus the pine, while certainly not constructive, and only 

 rarely destructive, is decidedly conserving and consolidating for the 

 Rosmarinus-L. fruticosum association. 



Further, the constructiveness of a species may change in the course 

 of the development of an association. Certain species which are 

 important constructive members of the early stages are rare or absent 

 in the optimum phase. An example may be seen in Dryas and Festuca 

 in the Seslerieto-Semperviretum. Other species appear only in the 

 optimum phase. The appearance and disappearance of important 

 socially constructive species may be used profitably for delimiting the 

 individual stages of a succession. For fixing the time hmits of suc- 

 cessive associations, however, the exchange in the characteristic 

 combination of species is conclusive. 



The naming of the individual stages of a succession should be 

 according to species of high constructiveness. We speak therefore of a 



