CLIMATIC FACTORS 



149 



which are so characteristic of wind-swept regions (Figs. 81, 82). The 

 direction of the prevaiUng wind may be clearly deduced not only from 

 trees and shrubs but even from grassland vegetation. 



Sand, grit, or crystals of salt or snow acting as abrasives accentuate 

 the mechanical effects of wind. This sand-blast action is strongest a 

 few centimeters above the surface of the ground or snow. This cutting 

 zone is often destructive to trees and shrubs (Fig. 83). Branches and 

 twigs within reach of the wind show, in the danger zone, more or less 



Fig. 85. — Effect of sand blast in the desert. 6. Stipa tenacissima (Haifa) from rock 

 ridge of sand dunes. Biskra, Sahara. A dead colony eroded by sand. 7. Andropogon 

 laniger, clump eroded on one side. Tolga, Sahara. 8. Centaurea omphalotricha, growing 

 plant body, three-fourths of which has been worn away. Rock desert between Tolga 

 and Biskra. {Photo by Braun-Blanquet, 1913.) 



distinct marks of the sand or snow blast. They are barked and eroded 

 on one side or on all sides and are often partly cut off. Even closely 

 prostrate shrubs like Dry as, species of Salix, Loiseleuria -procumhens, 

 and cushion plants bear marks of wind erosion. Sand grains and snow 

 crystals produce similar erosion effects, but sand cuts deeper than snow 

 (Figs. 84, 85). In Finnish grazing districts, according to Hayr^n, 

 cushions of Hedwigia albicans and Rhacomitrium species are especially 

 resistant. 



Wind is of especial importance to plant sociology in its r61e as 

 distributor of sand and snow. The local depth and duration of snow 

 cover depend less upon the amount of snowfall than upon the prevailing 



