SOME PROBLEMS IN LIGHT AS A FACTOR OF FOREST GROWTH 229 



ing in different geographic regions seemingly requires dift'erent amounts 

 of light for its best development. In one place it can get along with 

 very little light, growing under the shade of other trees ; in other places 

 it needs almost full daylight. 



What is the cause of this difference in light requirements of the 

 same species? 



Among practitioner foresters there is a common opinion that the 

 same species can get along with less light, the richer and more favor- 

 able are the soil conditions and the warmer the climate. They base 

 their cultural operations on this generally accepted theory. Can heat, 

 soil fertility, and soil moisture compensate for light? As for heat, 

 there are good reasons to believe that since heat and light are merely 

 different modifications of the same radiant energy, differing only in 

 the length of the waves. Both of them, or their sum, therefore, can be 

 considered as one factor. Moreover, it is well known that heat in 

 general affects the rapidity and energy of all chemical processes. It 

 can therefore be assumed that it affects also the process of assimila- 

 tion of carbon dioxide (CO2). As a matter of fact, there are observa- 

 tions which clearly show that the greatest production of dry substance 

 can take place in light intensities which may be the weaker the greater 

 is the temperature and the amount of chlorophyl in the leaves of the 

 plant. 



The assumption, however, that soil moisture or soil fertility does 

 compensate for deficiency in light would seem to be open to serious 

 doubt. It is certainly in contradiction with Liebig's law of minimum 

 which, in my judgment, applies to all factors of forest growth. This 

 law may be formulated as follows : 



The life and the development of a plant is controlled by that factor 

 which under given conditions is nearest to the minimum limit abso- 

 lutely necessary for the life of the plant. 



Since without a proper understanding of the factors that modify 

 the effect of light the true value of light as a factor of forest growth 

 cannot be definitely settled, I would like to see this committee suggest a 

 series of simple experiments which might take us a peg further in the 

 solution of this problem. 



1. A Test for Determining the Effect of Soil Fertility upon Shade 

 Endurance of Forest Trees. 



If the capacity of a species to withstand shade increases with the 

 improvement of the soil conditions in which it grows, then it should 



