46 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



Light Measurements in Relation to Minimum Requirements. 



The tolerance of trees to shade may be determined in any one of 

 four ways: 



1. By preparing empirical scales of tolerance, based on experience 

 and long-continued observation of the relative shade-enduring quali- 

 ties of various species when growing together. This method is obvi- 

 ously very crude, and may be very misleading, since such a condi- 

 tion as soil moisture may determine, as directly as does light, the 

 relative positions of the species in any particular stand. The per- 

 sistence of individual branches, or rate of pruning; the maximum 

 density of stands composed mainly of any particular species: the 

 ability of reproduction to thrive in shade; all these things may be 

 considered in preparing empirical scales. 



2. A second method of determining the tolerance of trees is by 

 study of the structure of the leaves. Having determined the normal 

 relations of the tissues of protective and assimilative characters, 

 leaves may be subjected to different degrees of shading. Those which 

 adapt themselves most completely to a variety of light condition- 

 are naturally those which will survive best if placed under trying 

 conditions as regards lack of light. This method, however scientifi- 

 cally it is executed, can not give us absolute comparisons, since the 

 structures of leaves are so variable even under the same conditions 

 that the exact degree of change of structure can not he determined. 

 In other words, this may give indications, but not comparable statis- 

 tical data. 



3. A third method of determining tolerance is the experimental 

 method, which must, of course, be executed in the laboratory, where 

 all other conditions, as well as the supply of light, may be con- 

 trolled. The primary object is the determination of the minimum 

 amounts of light which will sustain life of the several specie- under 

 consideration when all other conditions (especially heat and soil 

 moisture) are nearly optimum. It will be fairly apparent, however, 

 that high temperatures may reduce the light requirement, and 

 low soil moisture may increase it; and, since variations in all of the 

 other conditions will be encountered in the field, it is very desirable 

 that any experimental test should be so conducted that the influence 

 of these other conditions on tolerance may be at least accurately 

 gauged, if not directly measured. 



It is believed that the best results will be secured if each species 

 to be tested is grown under a variety of light conditions, approaching 

 both the optimum and the minimum, and if the tests are so conducted 

 that the physiological effects of each light intensity may be expressed 

 finally in terms of growth, or weight accretion, rather than if depend- 

 ence is placed solely or largely on observations of fatal effects when 



