92 WINDBREAKS. 



But on the basis that the timber crop is to be cut at 40 years, and 

 that the value of this, if not burdened with interest, is comparable 

 to the net value of field crops, the annual protective value may be 

 balanced against the accrued interest on the initial cost of planting 

 and on the crop value of the land. Using the same figures and simply 

 deducting the crop value (40 X $22. 50) and initial cost, $10, it is 

 found that the interest amounts to $1,276.01 . With only this interest 

 to pay, the belt may profitably contain 27.88 acres, and may be 230 

 feet wide, or 2.67 times the hejght of the trees. 



Whether the high efficiency of the cottonwood windbreak, with a 

 width of only 150 feet, can be maintained when the trees become 

 very old, depends on the quality of the ground on which it is growing, 

 and upon the feasibility of introducing smaller trees which will form 

 an understory and fill up the gaps left by the loss of the lower branches. 

 On poor ground the aggregate protective value up to 40 years is less 

 than on good soil, and it is therefore obviously unprofitable to plant 

 so large an area. With a decreased width and decreased density of 

 stand upon the poorer situations, it becomes necessary to substitute 

 for cottonwood some species of more dense foliage, or one which by 

 reason of greater root vigor and tolerance will form a more compact 

 windbreak. -Green ash, osage orange, and several of the drought- 

 resistant conifers, such as Austrian pine, red cedar, and jack pine 

 may be recommended. Mixtures of hardwoods and conifers are 

 desirable wherever winter protection is needed. (See diagram 33.) 



In order to make profitable the use of wide windbreaks which have 

 the quality of groves, there must be selected for the main body of the 

 shelter a species which will make rapid height growth at the outset, 

 If necessary, it may be underplanted with a slow-growing, dense- 

 foliaged tree, or the latter may be used along the sides of the grove, 

 and may be planted either at the outset or when the main trees of 

 the grove begin to prune themselves rapidly. Under any circum- 

 stances, the total width of the grove (from outside trunk to outside 

 trunk) should not exceed one and one-half or two times the expected 

 height of the trees at maturity. 



SINGLE-ROW WINDBREAKS. 



In the use of single-row hedges an entirely different and much 

 simpler problem is met. Windbreaks of this class can not, at the 

 outset, be charged with any more space than would be occupied by 

 a fence, and as they increase in height, their beneficial influence will 

 each year increase proportionately. This will at all times far exceed 

 the damage from shading and sapping, so that there will never be 

 any debt to be charged to them. It is only when the hedge is cut 

 back, and entirely robbed of its protective value, that the sapping 

 effect may be seriously felt. With osage orange, this effect may 

 extend over an area three times as wide as the trees were high before 



