PART II. 



MEASUREMENTS OF PHYSICAL FACTORS PHYSIOLOGICAL 

 EFFECTS UPON PLANTS AND ANIMALS CROP YIELDS. 



METHOD OF MEASURING FACTORS. 



Before describing the influences of windbreaks through the agency 

 of physical factors a brief description of the methods employed in 

 measuring these factors will help toward an understanding of results. 

 The amount of competition for sirhlight and soil moisture has been 

 measured; the * atmospheric conditions which affect crops at some 

 distance from the trees have been studied; and the various influences 

 have been segregated and expressed quantitatively. This work 

 comes within the province of plant physiology, and the methods used 

 are those known to plant ecologists. 



SUNLIGHT. 



Of all the factors in the competition between trees and smaller 

 forms of vegetation, sunlight is of the greatest importance, since the 

 large stem and branches and the immense mass of foliage in a tree 

 crown gives the tree a certain advantage over smaller plants. 



The object in measuring quantities of light in connection with this 

 study of windbreaks is to determine what damage may result to 

 crops from the interception of light rays by the crowns of the trees. 

 Since various trees use different amounts of light and since the sev- 

 eral crop plants require different amounts, much may be gained by 

 a study of the light conditions in the areas occupied by different 

 kinds of windbreaks and by the study of the crops upon which they 

 may have an influence. 



The total shading effect of a row of trees is obtained by measuring 

 the extent and intensity of the shadow for each hour of the day. The 

 amount of direct sunlight which passes through the crowns of the 

 trees at any point and time is measured by exposing a strip of solio 

 paper for a given period, under the given light conditions, and by 

 comparing the shade of color thus produced with the shade obtained 

 by a similar exposure in full sunlight. Such exposures are made at 

 intervals of 10 feet on either side of the windbreak and as far as the 

 shadow extends at the time. The amount of light in the shaded area 

 is expressed as a percentage of the full sunlight for that particular 

 hour, and in calculating the total amount of light which reaches a 

 given point in all of the hours between sunrise and sunset, each per- 

 centage is reduced in the proportion of the value of the hourly direct 

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