27 



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. The rate of increase, however, is never greatest in both 

 ways at the same time. Trees which are crowded while young grow 

 rapidly in height, through the effort to get the crowns into the sun- 

 light, but the diameter growth is correspondingly slow. 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. Height 

 growth then becomes less rapid and diameter growth more rapid. 

 In other words, the desired height and straightness of stem having 

 been obtained, the new wood tissue which is added now goes to in- 

 crease the tree's diameter, and this begins the period of most profit- 

 able 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 diameter 

 growth 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 consequence, 

 develop fairly well even when overtopped by other trees. Intolerant 

 trees are those which make poor growth or even die if sunlight is cut 

 off. Kxamples of tolerant trees are spruce, sugar maple, and hem- 

 lock; 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] 



