184 STUDIES OF TREES 



growing on the same area, all demanding the same things 

 out of a limited supply. The struggle for existence, there- 

 fore, becomes keen, many falling behind and but few sur- 

 viving. 



This struggle begins with the seed. At first there are 

 thousands of seeds cast upon a given area by the neighboring 

 trees or by the birds and the winds. Of these, only a few 

 germinate; animals feed on some of them, frost nips some 

 and excessive moisture and unfavorable soil conditions 

 prevent others from starting. The few successful ones 

 soon sprout into a number of young trees that grow thriftily 

 until their crowns begin to meet. When the trees have 

 thus met, the struggle is at its height. The side branches 

 encroach upon each other (Fig, 123), shut out the light 

 without which the branches cannot live, and finally kill 

 each other off. The upper branches vie with one another 

 for light, grow unusually fast, and the trees increase in 

 height with special rapidity. This is nature's method of 

 producing clear, straight trunks which are so desirable for 

 poles and large timljer. In this struggle for dominance, 

 some survive and tower above the others, but many become 

 stunted and fail to grow, while the majority become entirely 

 overtopped and succumb in the struggle; see Fig. 139. 



But in this strife there is also mutual aid. Each tree 

 helps to protect its neighbors against the danger of being 

 uprooted by the wind, and against the sun, which is liable 

 to dry up the rich soil around the roots. This soil is different 

 from the soil on the open lawn. It consists of an accumula- 

 tion of decayed leaves mixed with inorganic matter, forming, 

 together, a rich composition known as humus. The trees 

 also aid each other in forming a close canopy that prevents 

 the rapid evaporation of water from the ground. 



The intensity of these conditions will vary a great deal 



