CHOICE OF SPECIES IN ARTIFICIAL REGENERATION 29 



snowbreak to Scotch pine growing at high elevations depends 

 largely upon the origin of the seed. Trees grown from seed col- 

 lected at low elevations are much more subject to injury than 

 trees grown from seed collected at high elevations. 



Sleet or rime, although occurring at infrequent and irregular 

 intervals, often exerts greater mechanical effect upon forest vege- 

 tation than snow, although the manner of injury is the same. 

 This injury is most prevalent on north and east slopes in mountain- 

 ous regions. The degree of injury depends very largely upon the 

 species. Those with slender boles, large crowns, numerous or 

 brittle branches, or abundant winter foliage are most subject to 

 damage. Box elder, gray birch, silver maple, chestnut, and poplar 

 are particularly subject to this manner of injury in New England. 

 Elm, hackberry, hemlock, and hickory are very resistant. 



15. Wind in Its Relation to the Choice of Species. The 

 velocity of the wind in all habitats varies from a perfectly calm 

 condition to that of more or less violent storms. The occasional 

 high winds of all regions cause windfall and windbreak. This 

 mechanical injury, however, is most common in regions which 

 usually experience a calm atmosphere. Here the forest vegeta- 

 tion does not become adjusted to withstand great wind pressure, 

 consequently occasional severe storms often cause great damage. 

 In regions where the mean velocity of the wind is high, as on 

 islands, along the coast or on exposed mountains, and in most 

 prairie regions, the indigenous species develop special adaptations 

 to resist it. Forest trees are affected much more than the lesser 

 vegetation because of the marked increase in the velocity of the 

 wind with the height above the ground. 



The continuous stress occasioned by constant winds results in 

 an increase in the mechanical tissue of the tree where the strains 

 occur, and consequently it is better fitted to resist mechanical 

 injury. Species indigenous to windy regions not only have taken 

 on adaptations which better enable them to resist high winds but 

 are better fitted to survive the marked deviations from the normal 

 shape which they are forced to assume. In such regions, the 

 branches and tree stems are more or less markedly bent away from 

 the normal direction of growth and the twigs and leaves are in- 

 jured by breaking. 



Aside from its direct action, the wind exerts a very important 

 indirect action on tree life, chiefly by increasing transpiration 



