76 WINDBREAKS. 



In the following estimates of timber products distinction is made 

 between groves in which close competition between the trees existed 

 and narrow belts or rows in which the trees had free access to light 

 and unlimited root space on at least one side. 



CALCULATION OF AREA CHARGEABLE TO WINDBREAKS. 



In many instances estimates in groves and wide belts were made 

 onlv for the inside rows, because they alone were growing under true 

 forest condition. It is not difficult to calculate the actual area which 

 such trees occupy. With narrow belts and single rows, however, 

 allowance must be made for ail of the space occupied by the branches. 

 As far as possible the -figures in Table 3 have been used for this 

 purpose. 



Since damage to corn is, on the whole, the least marked the calcula- 

 tion of acreage necessarily chargeable to single-row windbreaks is in 

 most cases based on the greatest damage which results to cornfields. 

 This represents only the area shaded by the trees, and not the area in 

 which damage from sapping might be felt in very dry years. 



The area chargeable to a row of trees on account of damage to crops 

 is about proportional to the height of the trees. But the row can 

 not be charged with this area for all of the years the trees have been 

 growing. On an average, approximately one-half of the area has 

 been damaged during the whole life of the trees, though, to be safely 

 conservative, the factor of three-fifths has been used. This estimate 

 is amply liberal, because damage during the first few years was very 

 slight. The debt of the trees was at first small and the interest car- 

 ried through the full period of years was much less than that from 

 damage done in later years. A comparison with equal annuities from 

 field crops is, therefore, not quite fair to the trees. 



The following is the equation for determining the acreage occupied 

 by a row of trees a mile long, when A represents acreage and OF the 

 factor of loss to corn as expressed in percentages of the height of 

 trees, H: 



43560 



When the acreage must be figured from a shading factor, SF, the 

 equation becomes 



.FxiXgX|X528Q 



43560 



If the acreage occupied by a belt is to be calculated, the distance D, 

 between the outside rows must be added. The equation then 

 becomes 



A 



43560 



