10 



WINDBREAKS. 



(6) The sea breezes of the Pacific coast, usually very moderate. 

 In summer, however, their warmth enables them to take a great deal 

 of moisture from fields and orchards. 



The economic value of windbreaks is controlled not only by the 

 prevalence of damaging winds in a region, but also by climatic factors 

 which affect the growth of the trees themselves, and hence the pro- 

 ductiveness of the windbreak as a forest unit. Although wind- 

 breaks are useful and add to the comfort of living, to plant them on 

 an extensive scale for the protection of large areas can be recom- 

 mended only where their combined protective and timber value is 

 equal to the value of field crops whi^h might be grown on the same 

 .area. The relative value of the direct and indirect benefits of timber 



DIAGRAM 1. Direction, origin, and season of winds injurious to agriculture. Lines of equal rain- 

 fall controlling the use of windbreaks. 1 Windbreaks for timber production, and special 

 protection. 2 Windbreaks for timber production, and general protection. 3 Windbreaks for 

 general protection, little timber value. 



units is determined by the moisture conditions of the climate. From 

 this standpoint the area of the United States may be divided into 

 three belts (see diagram 1): 



(1) In the plains region east of the Missouri River, even though 

 winds are prevalent at certain seasons and the level topography per- 

 mits their free sweep across the fields, the abundant rainfall (30 

 inches at Omaha, Nebr., to 35 inches in northern Illinois) and gen- 

 erally high humidity of the air greatly decrease evaporation and its 

 bad effects upon plants. The small value of windbreaks as protec- 

 tion to crops, or even their positively injurious influence surround- 

 ing land where drainage is poor, makes it necessary that they should 



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