156 



PLANT SOCIOLOGY 



In the Tatra in place of our Elynetiim is the Trifidi-Distichetum; 

 in the Auvergne (at 1,700 to 1,850 m.), the association of Festuca 

 spadicea and Chrysanthemum delarhrei (Braun-Blanquet, 1926); and 

 in the Pyrenees, an association related to the Elynetum, in which 

 Elyna often dominates. On the wind-swept crests of the high Atlas 

 (Jebel Ourgouz) are found beautiful examples of wind erosion at 2,500 

 m. in a Festuca maroccana-Scutellaria demnatensis association covering 

 the crests. 



/,'■ r % 



-^\.:l v' '■/:: 



^-:i/> 



.^. -Z^;^^ 



FiQ. 92. 



-Pinus montana in the Nardetum on the summit ridge of the Aigoual (South 

 Cevennes), 1,500 m. (Photo by Furrer.) 



Alpine wind communities in the Rocky Mountains of Colorado 

 show splendid examples of the cushion-mat type of vegetation. Char- 

 acteristic of wind-swept slopes and ridges are the Paronychia pulvinata, 

 Arenaria sajanensis, Trifolium, nanum, Carcx rupestris, and Sieversia 

 turbinata associations. Rounded shoulders frequently have all soil 

 and rock fragments stabilized by cushions of Paronychia, Trifolium, 

 and other mat-forming species. Gentle slopes and depressions along 

 which rush the powerful descending wind currents, sometimes gather- 

 ing up and other times depositing the coarse wind-borne fragments, 

 are frequently terraced and more or less stabilized by the dwarf shrub 

 Dryas octopetala and its companion Carex rupestris. Both these 

 species exhibit remarkable powers of adaptation for combatting either 

 degradational or depositional action which alternates according to the 



