28 FORESTRY 



which decide the contest. Chief of these is the struggle 

 for light. 



A species may secure its foothold and light in one of 

 three ways. It may grow faster than others, in which case 

 with an even start, it is sure to win for the time being. It 

 may get along with less light than others. Or by reason 

 of small windblown seeds, it may be able to distribute its 

 seeds in accidental openings in advance of other species. 

 All these methods have their advantages. The kind which 

 relies on its ability to grow rapidly is not sure of surviv- 

 ing beyond a single generation. The critical time comes 

 in the reproduction and the establishment of seedlings. If 

 the competing species, which needs less light, has come in 

 under the shade of the original stand, or along with it, the 

 seedlings of the first species will be unable to grow in this 

 shade, while those of the other will easily survive. The old 

 trees might die with no survivors, and the site will belong 

 to the competitor. 



Tolerance. — This ability to grow with less light is 

 known as tolerance. Just as in the case of moisture, it is 

 not beneficial to be deprived of light, but the less of it a 

 tree needs, the greater advantage it has in the struggle for 

 existence. Trees needing a great deal of light are intol- 

 erant. 



The tolerance or light requirements of different species 

 are hereditary, and species may be classed according to 

 their relative ability to endure shade. But it is not true 

 that the same species always requires the same amount of 

 light for vigorous growth. All other factors which in- 

 crease the vigor of a tree tend to decrease the need of light. 

 For this reason we find that young trees are more tolerant 

 than old, the need for light steadily increasing as the tree 

 grows older. The life vigor of a plant is largest when it 

 is a seedling — it will do more on less food and less light 

 than at any later time. This conforms to the general laws 

 of life development of all animals and plants. The chance 



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