IMPROVEMENT THINNINGS IN 



THEORY OF THINNING. 



In order that one may understand the principles which underlie 

 the process of thinning, he must know something of the physio- 

 logical growth of single trees, and of that collection of trees known 

 as a forest. 



Plants are made up of tissues composed of numberless small cells. 

 These cells are largely composed of carbon derived from the carbon 

 dioxide of the air and water. The carbon dioxide enters into the 

 leaves through minute pores (stomata), and by the action of the 

 sunlight on the green chlorophyll grains, the process of assimila- 

 tion, which in plants answers to digestion in animals, takes place. 

 The carbon is combined with the water and a small amount of 

 mineral matter taken from the soil by the roots to form the grow- 

 ing material of the plant, while the oxygen is returned to the air. 

 Therefore any crowding or shading which deprives the tree of these 

 necessary agents, foliage and sunlight, checks its volume growth 

 in proportion. 



To take a simple example, let us suppose a plantation set out 

 with seedlings 6 by 6 feet apart ; then there will be 1,210 trees on 

 an acre, and each tree will have 36 square feet in which to spread 

 its branches. When the side branches meet we have what is called 

 a closed stand, and a struggle commences. It is characteristic of 

 trees to take all the room they can get; and not having any more 

 at the sides, they seek the sunlight by growing upwards at a rapid 

 rate. Trees even of the same species differ in their rate of growth, 

 so that some get ahead of the others; and when they do, they 

 spread out their side branches and so overtop their weaker neigh- 

 bors. Unless these overtopped trees happen to be in the class called 

 by foresters tolerant, i.e., shade-bearing, they will soon sicken and 

 die. By the time our plantation is fifty years old, only 300 trees 

 will remain of the original 1,210. 



In the early life of the forest, say the first fifteen to twenty 

 years, a sharp conflict of this kind is very useful, for it produces 

 all tall, straight trees ; in the second place, on account of the dense 

 crowding the side branches are killed off when young, and the tree 

 is free from knots; and in the third place, the ground is kept 

 shaded and the moisture retained in the soil. After twenty years 

 growth, however, these objects have been accomplished, and then 

 the forester steps in and opens up the stand so as to allow the 

 crowns to spread. Larger crowns mean, of course, a more rapid 

 increase in volume growth. Furthermore, the slower-growing trees 



