THE LIFE HISTORY OF TREES IN THE WOODLOT 39 



intense. It is a struggle for existence. Not all the trees that 

 started as seedlings can survive. As the trees continue to 

 grow larger, a smaller and smaller number of them can occupy 

 the space which all occupied before. Many of the weaker 

 ones must die. Those that fall behind in the struggle are 

 crowded out of the association. Not being able to secure the 

 sunlight on which their life depends, they are unable to manu- 

 facture food and so die. Only those that win out in the 

 struggle, owing to their faster growth, survive. It is a sur- 

 vival of the fittest. Those that live on are the survivors of 

 hundreds, sometimes of thousands, of competitors. They win 

 out in the struggle because they are better adapted to their 

 surroundings than their neighbors. The struggle is a con- 

 tinuous one. Even the dominant trees soon come into compe- 

 tition with other dominant trees as they continue to grow up 

 and spread out their crowns, and the struggle begins over 

 again between these trees with even greater energy. 



Natural Pruning. So we see that at the time the trees are 

 exerting a beneficial effect on each other, they are carrying 

 on a deadly struggle. But it is to this very struggle upward 

 for light and space that we owe the long, straight, slender 

 trunks of the trees in the woodlot. While the trees are grow- 

 ing rapidly in height the lower branches are dying because of 

 lack of sunlight. In this way the living crowns become con- 

 fined to the tops of the trees. On a growing tree in the open 

 side branches are able to expand in the sunlight and remain 

 alive and often clothe a tree to the ground. But in the forest, 

 as we have already seen, as soon as a branch is shaded from 

 the sun it dies because it can no longer manufacture food. 

 Not only do the side branches die, but the tree finally throws 

 them off. This the tree accomplishes by compressing the dead 

 branch next the trunk by each succeeding annual layer of wood, 

 until finally it becomes so weakened at that point that it is 

 broken off by the wind or snow or falls off of its own weight. 

 This process is known as natural pruning. In this way a 



