228 JOURNAL OF FORESTRY 



light-needing pine forest, for instance, the light must still have a great 

 amount of red, and particularly of blue and indigo, while in the shade 

 of a shade-enduring spruce forest the light is poor in red. blue, and 

 indigo rays. 



Numerous laboratory observations similarly showed that the sun 

 leaves and the shade leaves of the same species absorb different 

 amounts of different rays of the spectrum. If this is the case, the 

 question of the value of the quality of light in the forest and its effect 

 upon the coming up and development of the young growth can be 

 settled, in my opinion, by a comparatively simple series of experiments. 

 The experiments suggested may be carried out as follows : 



Locate two forest stands of which one should be composed of a 

 light-needing species, such, for instance, as western yellow pine, and 

 the other of Engelmann spruce or Douglas fir. In the central Rockies 

 such stands can be found close to one another. Find some small areas 

 in these two stands where the light intensities as measured with 

 Clements' or Wiesner's photometers at the same time of the day are 

 nearly the same. Locate on those areas a number of young plants of 

 different species — ^shade-enduring and light-needing — of the same 

 age, origin of seed, and treatment; if necessary, in pots or, if the soil 

 is the same, in the two stands, in a nursery bed especially prepared 

 for this purpose, and give the plants the same amount of water and 

 care. Spectro-photometric measurements should be made under the 

 canopy of the two stands. Observations for several years upon the 

 growth and development of these seedlings in the shade of the two 

 stands similar in practically every respect except the quality of the 

 light, should bring out definitely, if even only empirically, the signifi- 

 cance of the effect of different rays of the spectrum upon the develop- 

 ment of young growth in the forest. This, in my judgment, is another 

 problem upon which investigators should concentrate their attention 

 and point out the road to its solution. 



3. Compensating Factors of Light 



The effect of "light" upon forest growth is still little understood. 

 In part it seems to be the luminous and chemical rays that have the 

 greatest effect, in part it appears to be the heat rays ; furthermore, the 

 effect of all these elements of solar radiation appears to be further 

 modified by favorable soil conditions, such as moisture and fertility. 

 Many observers have established the fact that the same species grow- 



