154 PLANT SOCIOLOGY 



Physiological Effects of Wind. — Along with the mechanical effect 

 of wind goes always the physiological drying effect. 



If the water supply of the plant is inadequate, if water movement is 

 slowed up by frozen ground, strong winds produce the same wilting 

 effects as severe drought. The young succulent shoots dry up, the 

 leaves turn brown and often bend or roll up at tips and margins where 

 water loss is greatest. According to Hansen (1904), the vascular 

 bundles lose their conductivity under the influence of wind, which 

 causes drying and death of the mesophyll. 



The physiological action of wind determines the polar boundaries of 

 forest. According to Kihlman (1890, p. 75), it is neither the mechan- 

 ical force of the wind nor the cold nor the salt content nor the humidity 

 of the atmosphere which sets a limit to the forest but rather the 

 uninterrupted drying out of the young shoots, lasting for months, at a 

 season when replacement of the water lost is impossible. 



On the limits of tree growth in the north and in mountains the 

 physiological drying effect of wind is always accompanied by mechan- 

 ical injury, and arboreal vegetation always shows the combined 

 effect. 



Total Effect of Wind. — Resistance to wind injury, like resistance to 

 cold and drought, is a specific character, only partly referable to 

 morphological and anatomical modifications. Dense cushion-like 

 growth is often induced by wind action and is always to be regarded 

 as a protection against high winds. Cushion plants are characteristic 

 of windy habitats: coast regions, desert steppes, crests of high moun- 

 tains, and the arctic. Cushion plants are not only less sensitive 

 to wind but also less sensitive to variations of temperature and intense 

 insolation. It is therefore difficult to estimate wind resistance of 

 plants uninfluenced by other factors. It would be valuable in forestry 

 to know the specific wind resistance of trees and their relation to the 

 wind factor near the timber line. Many mistakes in reforestation 

 might thus be avoided. All of the many attempts at reforestation on 

 the crest of Mont Aigoual and on the Col de Trepaloux in the southern 

 Cevennes (1,520 to 1,560 m.) have come to naught because of under- 

 estimation of the wind factor. Similar failures have often occurred in 

 the Alps. 



Wind and Plant Communities. — The greatest influence of wind 

 upon the form of vegetation is not by hurricanes which snap off and 

 uproot whole stands of trees but by continuous currents of air. 

 Nowhere is this more readily seen than in mountains, where protection 

 from wind is the necessary condition for the existence of many plant 

 communities. The extremes lie hard by one another and are the more 



