27 



not be difficult. Thinning, however, is much more complicated, and 

 should not be undertaken without a clear idea of its objects and of 

 the methods by which it can best be done. 



The quantity of wood formed by a tree depends upon the area of 

 leaf surface which is exposed to sunlight. A small leaf surface pro- 

 duces a small amount of wood and a large leaf surface produces a 

 large amount of wood. A healthy tree is constantly endeavoring to 

 increase its leaf surface, and it is by taking advantage of this inherent 

 tendency that the forester is enabled to control the amount and qual- 

 ity of the wood produced. 



Trees increase their size in two ways — growth in height and growth 

 in diameter. Trees which are crowded while young grow rapidly 

 in height, through the effort to get the crowns into the sunlight. 

 Crowding produces long, slim stems, which serve as a basis upon 

 which to form the valuable timber of later years. When the stem 

 is of a sufficient height, thinning gives room for an increased crown 

 development and, in consequence, a larger leaf surface in the trees 

 remaining. In other words, the desired height and straightness of 

 stem having been obtained, the new wood tissue which is added 

 goes to increase the tree's diameter, and this begins the period of 

 most profitable development. Thus the natural tendency of trees 

 toward rapid height growth' when young is strengthened by crowding, 

 and the later tendency to slower height growth and more rapid growth 

 in bulk is encouraged by thinning. 



With regard to their ability to bear shade, trees are divided into 

 two classes — tolerant and intolerant. Tolerant trees are those which 

 will bear more or less heavy shade in youth and which will, in con- 

 sequence, develop fairly well even when overtopped by other trees. 

 Intolerant trees are those which make poor growth or even die if sun- 

 light is cut off. Examples of tolerant trees are spruce, sugar maple, 

 and hemlock; of intolerant trees, larch, black walnut, and yellow 

 poplar. Tolerant trees will grow in very dense stand, and since their 

 branches persist even when shaded it is necessary to crowd such 

 trees at first in order to form good stems. When the desired form 

 of stem has been produced the stand should be heavily thinned. 

 Intolerant trees will grow in dense stands only when comparatively 

 young, and thinning is less necessary with them than with tolerant 

 trees. 



A plantation composed of rapid-growing species will, when from 

 10 to 20 years old, contain three distinct classes of trees: (1) Sup- 

 pressed trees, those which have been outgrown by competitors and 

 whose tops are completely overshadowed; (2) intermediate trees, 

 those whose tops are more or less exposed to the sunlight but the 



[Cir. 81] 



