34 



All trees ultimately thrive best in full enjoyment of light and then only 

 develop their characteristic form. Uut, just as some species can adapt 

 themselves to excess or deficiency in moisture conditions, so some can sub- 

 sist and even thrive with less light than others, and we can classify and 

 grade the species accordingly into tolerant or shade-enduring and intoler- 

 ant or light-needing. 



The dense spruce and fir forest shows by the number of trees that can 

 occupy an acre the capacity of the species to thrive in the shade of neigh- 

 bors, while the open pine forest gives an indication that the species re- 

 quires larger amounts of light to thrive. 



The densely- foliaged crown of the hemlock, with the branches beset 

 with leaves into the very interior, attests its extreme shade endurance, 

 while the light- foliaged, open-crowned larch or poplar, ash or birch, or 

 even pine, show their extreme sensitiveness to the absence of light by the 

 very openness of their crowns, by losing their lower branches early and by 

 the inability of their seedlings and young progeny to endure the shade of 

 neighbors 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 themselves in a 

 mixed forest only by keeping ahead and occupying the upper crown level, 

 as the White Pine does. The tolerant species, on the other hand, able to 

 thrive in the shade of light- foliaged species, usually increases more slowly 

 in height ; but their capacity of shade endurance assures to them a perma- 

 nent place in the forest. 



Many of them are characterized by a height growth which, though 

 slow, is persistent ; while the light-needing species, by falling behind in 

 their rate of height growth, often lose in the end what they attained in their 

 youth. As a result the shade endurers finally become dominant and the 

 light needers occur in the mixed forest only sporadically, the remnants or 

 single survivors of groups, all the outside members of which have perish- 

 ed ; and only when a windstorm or insect pest creates an opening of suffic- 

 ient size is a chance for their reproduction given. 



Thus the composition and general appearance of the mixed forest is 

 largely influenced by this difference in light requirements of the species 

 present and its numerical make up also depends upon the require- 

 ments by each individual and its capacity to get ahead of its neighbor. 



Just as in the mixed forest the species are distributed according w/ 

 their shade endurance, so in the pure forest of one species, or of species of 

 equal tolerance, will the different-sized or different-aged trees develop side 

 by side according to available light, each crowding the other, the laggards 

 being finally killed by the withdrawal of light. 



