14 WINDBREAKS. 



ception of air currents over a much wider zone the windbreak may 

 influence the growth of crops and usually the influence is for good. 

 It reduces the mechanical force of the wind; it lessens evaporation, 

 which is always accelerated by air currents ; it stagnates the air and 

 thus increases the extremes of temperature both in the air and in the 

 soil; and, finally, it changes the distribution of the moisture of the air. 

 The constant aim throughout this study has been to painstakingly 

 investigate the influences of both classes, and to determine to what 

 extent the baneful effect of the trees might be reduced without 

 destroying the ability of the windbreak to protect. The studies of 

 physical factors in the zone near the trees were made to determine 

 the requirements, with respect to soil moisture and sunlight, of the 

 species which have proved successful in windbreak planting and to 

 show how the space occupied by the trees may best be utilized to 

 increase its yield without damage to the crops adjacent. The study 

 of the more distant relations of windbreaks was made to determine 

 under what conditions and to what extent benefit may be expected 

 from the protection afforded; to show whether the aggregate benefits 

 from protection alone will warrant the existence of the windbreak. 



SYSTEM OF MEASURING INFLUENCES. 



The general system of study has been to measure and compare the 

 conditions existing under and near the trees with conditions for the 

 same period at a point far enough removed from the trees to be 

 beyond their influence. It was impossible to study simultaneously 

 the various classes of windbreaks in various localities, but care was 

 taken, in making comparisons between different species and different 

 windbreak formations, to consider all essential points. 



In calculating the effect of a windbreak upon crops a given area 

 must be allotted to the trees, and the study of effects must be carried 

 on outside of this area. In the case of groves the area belonging to 

 the trees themselves has been considered to be that lying between the 

 outside rows, measuring from trunk to trunk. A belt of trees or a 

 grove thus occupies a definite space, and measurements of distances 

 in either direction from this area are made from the trunks of the 

 outside rows. A single row of trees can not be said, in this sense, 

 to occupy any space. Measurements of. distances in both directions 

 are made from a common point on the line of the row. By this sys- 

 tem it is possible to compare directly the influence of the different 

 classes of windbreaks in the zone affected by the roots and branches 

 and in the wider outlying zones protected by the trees without con- 

 sidering the area which belongs strictly to them. 



Since the yield of a field of grain depends so much on the methods 

 of cultivation and local soil conditions, an attempt has been made 

 in measuring the effect of each windbreak to determine a normal, 



