260 G. A. PEARSON 



• 



suffered severely while partially shaded plants in the same bed 

 escaped injury. 



It is believed that the whole question resolves itself into one 

 of moisture, although it has not been proven that certain solar 

 rays do not exert an injurious effect upon the young plants by 

 other physiological action than that of increased water loss. It 

 has been established that aspen stands, despite the fact that the 

 trees and the usually luxuriant herbaceous growth underneath 

 them use up a large portion of the moisture in the soil, create 

 conditions more favorable to the establishment of planted Douglas 

 fir than are found in open situations. Undoubtedly this con- 

 clusion has certain limitations. As the young Douglas firs grow 

 older they demand more light, and the densest aspen thickets 

 will have to be thinned to permit the best development of the 

 planted trees. The lack of sunlight results in a spindly form 

 accompanied by pale and poorly developed needles. This, 

 rather than arbitrary rules based upon mathematical expressions 

 of the requirements of the species, must guide the forester in 

 determining when more light is needed. 



Although results in open situations will as a rule be less satis- 

 factory than where the ground is occupied by aspen, reforestation, 

 in the smaller openings at least, is by no means to be despaired of. 

 Possibly the problem can be solved by the use of plants specially 

 developed to meet these conditions. , Yellow pine, because of its 

 lower moisture requirements and greater demands for light will 

 probably prove more suitable than Douglas fir for openings within 

 the natural range of the former. 



In altitudes above the range of Douglas fir and yellow pine, 

 Engelmann spruce is the species to plant in aspen thickets. 

 Engelmann spruce seedlings are even more sensitive to strong 

 sunlight and drought than are Douglas fir seedlings, and therefore 

 the results of the foregoing experiments may be expected to apply 

 in a greater degree to Engelmann spruce than to Douglas fir. 

 Spruce will grow in the densest aspen thickets, but in openings 

 particularly of southerly exposure it establishes itself with diffi- 

 culty. Bristle cone pine {Pinus aristata) grows on such sites, 

 and can doubtless be planted successfully where Engelmann 

 spruce fails. 



