CLIMATIC FACTORS : THE AIR 



105 



tributed to the ease and amount of uprooting and wind throw. 

 Storms of such force and destructiveness are rare in North Amer- 

 ica, but lesser winds may cause considerable damage. When closed 

 forest stands are thinned or selectively cut, the remaining trees are 

 subject to wind throw for a number of years even though wind 

 does not blow with great velocity. 



In addition to physiologically-produced flag forms of woody 

 vegetation, there are those resulting from purely physical effects 

 of wind. A study of asymmetric trees in the Columbia River 

 Gorge 152 showed that, when branches are continually bent in one 

 direction by prevailing winds, the branches become "wind trained" 

 and hold their positions permanently. Some grew completely 

 around the trunk from the windward to the leeward side. Still 

 another cause of asymmetry was found here. Severe winter storms, 

 coming largely from one direction, cause much breakage, espe- 

 cially when accompanied by sleet, and almost complete pruning of 

 branches on the windward side often results. 



Transportation by Wind.— We have already indicated how im- 

 portant to precipitation are the vapor-laden winds moving inland 



FlG. 50. Wind throw often results when trees are uprooted, especially if 

 on shallow or wet soil. Here is shown a giant Douglas fir in Washington 

 whose torn-up root system had a spread of fifty feet— U. S. Forest Service. 



