MEASURING FACTORS. 17 



sunlight compared with that at noon. Thus, for example, light at 

 8 a. m. or 5 p. m. may be valued at only 25 per cent. Hence, the 

 shading of a given point at noon means a greater loss than the same 

 shade thrown on the point in the early morning or late afternoon. 

 For the purpose of comparing different species under different con- 

 ditions, the amount of light used by a given windbreak is expressed 

 as a percentage of the total light which would fall during the entire 

 day on a strip of ground parallel to the windbreak and as wide as 

 the height of the trees. 



A row of trees may use 75 per cent of the light which would natu- 

 rally fall upon the strip of ground 50 feet wide contiguous to the 

 trees, and in the 50 feet next outside of this the shadow may repre- 

 sent the use of only 25 per cent of the light. If this lesser shadow 

 were combined with the greater and concentrated on the first strip, 

 it might then be said that the shading is equal to 100 per cent of the 

 light which would fall upon a strip of ground 50 feet wide. If, then, 

 the height of the windbreak is 50 feet, the shading amounts to 100 

 per cent of a strip of ground whose width is 1-ht. If, however, the 

 height of the trees in this case is only 25 feet, the amount of light 

 used amounts to all of that which would fall upon a strip of ground 

 whose width is 2-ht., or 200 per cent of the light on a 1-ht. strip. 

 By this system, in comparing different windbreaks, the element of 

 tree height may be eliminated. It may be seen that this figure 

 which expresses the quantity of the shade produced by the wind- 

 break may be more than 100 per cent. 



SOIL MOIS" UHE. 



The relation of trees to crops in th^ 1 competition for soil moisture 

 is decidedly important. The tree roots reduce soil moisture adja- 

 cent to windbreaks, and this effect is usually apparent in the yield 

 of the crop growing there. A special effort has been made to deter- 

 mine the root extent of the various species which are used for wind- 

 breaks in order to decide which are least likely to damage crops 

 extensively in seasons "when the rainfall is not sufficient for both 

 trees and crops. Just as trees vary in the vigor of their stem and 

 crown growth, they show varying root vigor. Even the so-called 

 taproot species have some lateral roots in the well-aerated soil not 

 far below the surface. These are the ones which compete with the 

 cultivated plants for moisture, and the greater their length, the 

 greater the possible damage to crops. 



To determine the extent of the tree roots of various species, a 

 number of soil samples, taken at regular intervals out from the base 

 of the trees, show to what extent, as expressed in distance and in 

 moisture percentages, the roots have an appreciable effect in depleting 

 the moisture supply of the soil. The samples are taken at depths of 

 92290 Bull. 8611 2 



