ROLE OF LIGHT IN NATURAL AND ARTIFICIAL REFORESTATION 235 



growth of the seedlings is very high. Clements*' states that the best 

 growth of lodgepole seedlings is secured only in full light, but he 

 found fairly good growth with intensities as low as .140. The writer 

 found abundant and vigorous reproduction of Engelmann spruce and 

 alpine fir in a stand where the measured light intensity was about .450. 

 In Oregon'^ good growth of this species was found with an intensity 

 of .021. 



An exact determination of optimum light ^conditions for seedlings 

 is almost out of the question, because there is no exact standard as to 

 what constitutes good growth. Considering only those measurements 

 which refer to the minimum of light under which life may be sustained, 

 we should have rounded figures for the various species about as fol- 

 lows: yellow pine .250; lodgepole pine .100; Douglas fir .050; Engel- 

 mann spruce and alpine fir from .020 to .050. Yet we have evidence 

 of the inexact determination of even these minima, by the occurrence 

 of seedlings of all species, except possibly yellow pine, where light 

 intensities are not over .010. 



For nursery conditions we have very little data. All of the Rocky 

 Mountain species commonly grown for reforestation purposes succeed 

 admirably with a light intensity of .500. This is the degree of shading 

 usually given for practical reasons, being productive of taller plants 

 than are produced in full light. With western yellow and jack pines, 

 the latter strongly resembling lodgepole, experiments were conducted 

 at the Halsey nursery, where atmospheric and soil temperatures are 

 far above the normal for habitats of both species. Under these condi- 

 tions, only in one year, when the temperature and sunlight were con- 

 siderably below normal, has less than 50 per cent shade appeared to 

 be favorable. With light intensities of .400 and .200, there has been 

 marked diminution of growth, and with the least of these, even con- 

 siderably retarding of germination, but it is safe to say that an in- 

 tensity of .200 is far above the minimum requirement of either species, 

 when other conditions are favorable, 



Zon and Graves^ have pointed out that empiric scales of tolerance 

 have usually proven of just as great final value as the more exact 



* Clements, F. E. "The Life History of Lodgepole Burn Forests." Bulletin 

 79, U. S. F. S., 1910. 



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

 92, U. S. F. S., 1911. 



