152 PLANT SOCIOLOGY 



as if torn up by a plow. Re vegetation proceeds extremely slowly. 

 According to soil and altitude, pioneer invasion is made by Dryas 

 octopetala and Salix serpyllifolia spaliers or by Carex rupestris or 

 Loiseleuria procumbens. 



Sand-dune communities show similar effects of wind erosion (Fig. 

 88). 



Dune Formation. — The arrangement of sand masses in regions of 

 scant vegetation is also subject to the influence of wind. If the move- 

 ment of sand comes slowly, vegetation may save itself from burial by 



Fig. 89. — Newly formed dunes, and low dunes with Aristida to the west of Biskra 

 (Sahara). {Photo by Miiller.) 



growing up through the sand and binding it. In inland regions the 

 important sand binders are the rhizome geophytes, Agropyruvi and 

 Calamagrostis (especially C. epigeios); in the north African deserts 

 Aristida pungens and Euphorbia gouyoniana. In sea-strand areas 

 Elyrnus europaeus, Ammophila arenaria, and Cyperus capitatus are of 

 high dynamic value. Since their shoot systems rise as rapidly as the 

 sand accumulates, they initiate dune formation and thus are geomor- 

 phically constructive (Figs. 89, 162). 



They are at the same time sociologically highly constructive, as they 

 furnish a starting point for the more important pioneer associations of 

 moving sand. One of the best known dune associations is the Ammo- 

 phila-Medicago marina association of the Mediterranean coasts 

 (Fig. 90). 



