SIZE OF EFFICIENT WINDBREAKS. 



91 



with the height of the trees. But in most cases the height growth 

 of the trees is much more rapid in the first half of the period than 

 in the second half. This is especially true of cottonwood. Table 25 

 gives the height at different ages and under varying conditions. 



TABLE 25. Height growth of cottonwood. 



Age. 



Kind of windbreak and qual- 

 ity of situation. 



Grove. 



Class A. Class B. 



Feet. 

 20.0 

 39.0 

 52.4 

 62.0 

 69.3 

 75.5 

 81.1 

 86.2 



Feet. 

 10.8 

 19.5 

 27.5 

 35.5 

 42.7 

 49.4 

 55.2 

 60.5 



Row or 



narrow 



belt. 



Class A. 



Feet. 

 15.0 

 25.6 

 35.7 

 45.7 

 55.8 

 65.9 

 76.0 

 86.1 



This shows that the height growth in groves is very rapid at the 

 outset, but that the rate of growth decreases after the trees begin to 

 crowd each other for root space. In rows and narrow belts there is 

 less incentive to rapid height growth at the start, because the trees 

 may develop more fully their side branches. But the rate of growth 

 is maintained very uniformly up to the fortieth year, at which time 

 the height of the trees in a row will be almost the same as in a grove. 

 Beyond this age, which represents maturity for cottonwood, it is 

 probable that the rate of growth falls off rapidly in both cases. 



If the benefit is calculated for each year from the time of planting, 

 on the basis of the height of the trees, and if these annual incomes are 

 carried with compound interest to the time of maturity of the timber 

 at the end of a 40-year rotation, the protective value of a good cotton- 

 wood windbreak a mile long amounts to $35,585.50. Against this 

 must be balanced the value of the grain crops which might have 

 been grown each year. Using $22.50 per acre, or 45 bushels of corn 

 at 50 cents, the gross grain value taken for calculating benefit, these 

 successive crops at the end of 40 years, with interest to the end of the 

 period, may be valued at $2,138. To this must be added the initial 

 cost of planting an acre, $10, with interest, making $48.01. There- 

 fore the total cost of the windbreak has been $2,186.01 per acre, and 

 its protective value has been sufficient to pay the entire cost of 16.28 

 acres. Even supposing that the windbreak has no timber value, or 

 that it can not be cut without endangering the adjacent crops, then 

 the protective value up to 40 years has paid a full crop rental on all 

 the land occupied by a belt of trees with a width of 134 feet, which is 

 equal to 1.55 times the height of the mature trees. 



