236 JOURNAL OF FORESTRY 



scales prepared by Wiesner and others after photometric measure- 

 ments in the forest. In fact the latter tend to confuse, rather than 

 clarify our conceptions of tolerance. The tendency of foresters, -as 

 shown by the writing just cited and by Burns,^ is to refuse to accept 

 photometric results when they do not agree with empiric conclusions. 

 I do not know of a single forester who has worked extensively with 

 this subject who would be willing to substitute a scale of light demands 

 obtained by photometric measurements, for the scale of tolerance 

 which he has developed from experience. This is a rather serious 

 indictment of ecological methods. But they have failed to produce 

 convincing results. 



One view of the matter is this : foresters have measured the light 

 factor without measuring all of the other factors which control the 

 existence and the growth of seedlings in the forest. As early as 1907 

 Zon'^ pointed out the importance of root-competition in creating the 

 appearance of shade-suppression, and admonished foresters to "look 

 down, rather than up" for the explanation of so-called shade tolerance. 

 It has also been pointed out^° that less light is required for trees grow- 

 ing on good soils than for the same trees on poor soils, and elsewhere 

 I have mentioned the possible bearing of soil temperatures on the 

 requirement for light. Considering the wide variation in limiting light 

 intensities which have been shown by different authors and in different 

 localities, one might almost conclude that these other factors were 

 all-important, and that light was really not a limiting factor at all. 

 While this is going to an extreme, we simply must admit that light 

 measurements are of no value alone. 



Another view of the reason for the failure of field measurements 

 to establish the physiological importance of the light factor is that 

 the method has been photo-chemical instead of photo-thermal. Forest- 

 ers have been perhaps the slowest to recognize the deficiency of photo- 

 chemical measurements, and although the Wiesner-Clements method 

 has been subjected to a great deal of criticism beacuse it takes account 

 of only one kind of rays, while the intensities of other rays may or 

 may not vary proportionately in different spectra, nevertheless forest- 



* Bums, Geo. P. "Discontinuous Light in Forests." Bulletin 193, Vermont 

 Agr. Exp. Station, 1916. 



•Zon, R. "A New Explanation of the Tolerance and Intolerance of Trees." 

 Proc. Soc. Am. Foresters, II, 1, 1907. 



i»"Zon R., and Graves, H. S. "Light in Relation to Tree Growth." Bulletin 92, 

 U. S. F. S., 1911. 



