112 THE STUDY OF PLANT COMMUNITIES ■ Chapter V 



widespread a species is, the more efficient are the mechanisms that 

 facilitate its dispersal, regardless of whether the agent be wind, 

 animals, water, ice, or gravity. 



Wind and Soil— The, slightest air movement shifts dust particles 

 from place to place, and increasing velocity results in the transport 





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(fa£j. 



FlG. 57. "Graveyard forest" near Florence, Oregon. Once a closed stand 

 (probably mostly Finns contorta) growing on the soil layer which is broken 

 through in the foreground, the forest was completely buried by the dune 

 now appearing in the rear which subsequently moved on to uncover it again. 

 The view is to leeward.— Photo by W. S. Cooper. 



of larger particles of soil in increasing amounts. Although fine 

 materials are everywhere being shifted by wind, its greatest effects 

 are noticeable in dry climates where there is a prevailing wind and 

 a minimum of vegetation. During extended droughts, the culti- 

 vated, semiarid regions of our Midwest and Southwest have at 

 times become shifting seas of drifting soil, and the clouds of fine 

 materials carried about in the air have given rise to the term, "dust 

 bowl!' 



Over an extended period of time great quantities of materials 

 may be transported and deposited by wind, as is demonstrated by 

 the enormous deposits of loess in various parts of the world. This 



