148 ECONOMICS OF FORESTRY. 



light requirements of the various species are based 

 the most important forestal operations. According 

 to relative tolerance of shade, the species can be 

 graded from the most tolerant to the least tolerant, 

 into shade-enduring or light-needing. Those spe- 

 cies which, like the beech or sugar maple, the 

 hemlock or the fir or spruce, form dense crowns 

 evidently need less light than those with lighter 

 foliage, for the interior leaves of these crowns can 

 grow and function in the dense shade. On the 

 other hand, the light-foliaged, open-crowned larch 

 or pine, aspen or poplar, ash or birch, show their 

 extreme sensitiveness to the absence of light by 

 the very openness of their crowns, by losing early 

 the lower branches unless they are fully lighted, 

 and in the forest by the inability of their seedlings 

 and young progeny to endure the shade of neigh- 

 bors or even of their own parent trees. 



To offset this drawback in their constitution, they 

 have usually some advantage in the character of 

 the seed, and are mostly endowed with a rapid 

 height growth in their youth, so that, at least when 

 the competition for light starts with even chances, 

 they may secure their share by growing away from 

 their would-be suppressors. They can keep them- 

 selves in a mixed forest only by keeping ahead and 

 occupying the upper crown level. The tolerant 

 species, on the other hand, able to thrive in the 

 shade of light-foliaged species, usually increase 

 more slowly in height ; but their capacity of 



