RESULTS OF MEASUREMENTS. 73 



(a) Loss of sunlight. This amounts to from 50 to 125 per cent of the 

 light which might fall on an area as wide as the height of the trees. 

 This loss is greatest in the case of north-south windbreaks. It is 



: accompanied by a somewhat greater loss of crops on an area of about 

 the same extent as the shadow, the increase arising from the inability 

 of plants to form seed with a limited amount of light, which might be 

 sufficient for vegetative growth. This shaded area should be utilized 

 for forage crops or for trees which are able to subsist on partial light. 



(b) Loss of moisture. A zone varying in width from one to five 

 times the height of the trees is affected according to the species and 

 situation. The width of the strip may be greatly reduced by cultiva- 

 tion. This loss of moisture may, in years of drought, result in a com- 

 plete loss of annual crops in the zone affected. The damage to deep- 

 rooted crops need not be so great. 



(c) Temporary reduction of soil fertility. This accompanies and 

 is due to the loss of moisture in the root zone. 



(2) The zone of windbreak protection. In the wider zone of wind- 

 break influence the protection afforded results in a marked benefit to 

 crops, because of the creation on a large scale of conditions similar to 

 those which obtain in a hothouse. These conditions, all of which 

 result from the ability of the windbreak to check the circulation of 

 air currents, are as follows: 



(a) Less wind movement. As a result there is less damage by lodg- 

 ing of grain, movement of soils, and uneven drifting of snow. 



(&) Less evaporation. This results in the ultimate conservation of 

 the moisture of the soil. In dry years this may mean a partial crop 

 instead of a complete loss. 



(c) Greater heat during the hours of sunshine and lower air tempera- 

 tures at night. In the aggregate this stimulates plant growth during 

 the spring and summer. The value of this factor in the season of 

 1908, on the north side alone, was more than sufficient to offset the 

 damage due to shading, and left a net surplus equal to the yield of a 

 strip from one to three times as wide as the height of the trees. 



(d) Less extreme cold at night, if the windbreak can serve the pur- 

 pose of checking the evaporation of moisture from objects upon which 

 it falls as rain, sleet, or snow. In such case a benefit to blossoming 

 fruit trees was obtained in 1908 which amounted to as much as 10 

 bushels per tree in the subsequent crop of apples. 



(e) A slight increase in the capacity of the air for moisture, because 

 of its greater warmth in the daytime. This increase is not sufficient 

 to increase evaporation where the wind has been checked. 



The absolute value of any of these influences will increase with the 

 degree of efficiency of the windbreak, and their total amount on any 

 side of the windbreak will depend upon the direction, velocity, and 

 desiccating power of the prevailing winds (see diagram 34). 



