346 



PLANT SOCIOLOGY 



The beltlike arrangement of communities is quite obvious in the 

 filling up of still or flowing waters (decrease of moisture) (Fig. 174), in 

 salt marshes (decrease in salt content) (cf. p. 197), on exposed ridges of 

 high mountains (intensity of wind), in caves (decreasing light), in 

 snowy regions of the north and of high mountains (duration of snow 

 cover), and about bird roosts (intensity of manuring) {cf. p. 241). 

 When species which spread peripherally and have rather different 

 ecological requirements come into competition, they may form sharply 

 bounded zones, since each species excludes the other (Fig. 91). Good 

 examples of zonation are to be found in many recent papers on plant 

 sociology. 



Fig. 174. — Zonation of Lake Chambedaze, Auvergne. The floating Comarum- 

 Menyanthes belt is followed by a Car ex lasiocarpa belt; then follow belts of Salix spp. 

 and Betula pubescens, {Photo by Denis.) 



The zonation of algal associations on rocky seashores has been 

 much studied, since Orsted (1844) called attention to it. It is of 

 universal occurrence and is due principally to the light relation. In 

 general, four zones are recognized {cf. Sernander, 1917; Kylin, 1910; 

 Hayren, 1914, etc.) : 



1. The deepest is the sublittoral or extra-littoral zone as far down as red algae 

 extend {Rhodomela subfusca, Corallina, and Ceramium species, etc.). 



2. The lower littoral zone or zone of brown algae, with the Fucaceae. 



