4 CONTENTS. 



Measurements o/ piiy.'i'vl factors- Physic-logical effects upon plants and 

 animals Crop yields Continued. 



Results of measurements Continued. Page. 



VI. Heat 55 



Measurements of air temperatures 56 



Measurements of soil temperatures 62 



Effects of heating on crops 65 



Acceleration of growth and increased yield 65 



Effect of low temperatures 68 



Sensible temperatures 70 



VII. Humidity 70 



Summation of physical and physiological effects 71 



The zone of competition 71 



The zone of windbreak protection 73 



PART III. 



Direct results of windbreak planting Timber yields 75 



Relation of windbreak formation to quality production 75 



Calculation of area chargeable to windbreaks 76 



Market values of timbers 77 



Estimates and discussion of species 78 



Cottonwood 78 



Green ash 80 



Silver maple 81 



Willow 81 



Honey locust 83 



Osage orange 84 



Russian mulberry '. 86 



Catalpa 87 



White pine 88 



Scotch pine 89 



PART JV. 



Summary 90 



Calculation of the size of efficient windbreaks 90 



Size of efficient windbreaks determined by their values 90 



Groves and shelter belts 90 



Single-row windbreaks 92 



Windbreaks for various regions 94 



1. The Middle West 94 



2. The northern prairies 97 



3. The Lake States 98 



4. The Eastern States 99 



5. The Southwest 99 



6. The Pacific coast. . 100 



