238 JOURNAL OF FORESTRY 



Thus a body, which would become superheated to the extent of 20 

 degrees in still air, would show possibly only 3 or 4 degrees of excess 

 temperature in a current of air moving at the rate of 200 or 300 feet 

 per minute. 



It is perfectly obvious that this simple physical fact explains in a 

 large degree the ability of plants to exist with very low light intensi- 

 ties. Heavy shade in the forest or elsewhere is almost certain to be 

 accompanied by stagnation of the air. Light of 10 per cent intensity 

 in the densest forest may be equivalent in heating value to full light 

 at the crowns of the taller trees. 



If we apply a measure of the heating efifect of insolation to the 

 light intensity figures mentioned in the early part of this paper, on the 

 basis of the relative wind exposure likely to be met with in the forest 

 types where the measurements were taken, we have figures for the 

 different Rocky Mountain species roughly as follows: 



Heating Factor 



Minimum Light on basis of Net Heating 



Species Sustained Growth Air Movement Restdt 



Yellow pine .250 .20 .050 



Lodgepole .100 .40 .040 



Douglas fir .050 .80 .040 



Spruce and fir 020-.050 1.00 .035_ 



These figures are, of course, the merest approximations, and I 

 have introduced them simply to show that a consideration of light 

 values in connection with their probable physical effects tends strongly 

 to eliminate the discrepancies between species which have been so 

 disconcerting. I am convinced that similar treatment of all photometer 

 results would bring different measurements much closer together and 

 would tend to show that chemically active light was not the controlling 

 factor at all. 



My message to foresters, which may or may not have interest for 

 more advanced ecologists, is simply this: If we are 'to obtain a solution 

 of the problems which are conceived in the terms "tolerance" and 

 ''intolerance," and if we would give quantitative expression to the 

 requirements of the various species of light, we must consider this 

 factor in its full meaning of radiant energy ; we must consider the 

 physical value of radiant energy rather than its chemical value; and 

 finally we must determine its effective physical value in combination 

 with the heat of the air rather than its theoretical physical value. Such 

 determinations will soonest lead to conclusions if carried on where 



