THE FOREST CANOPY 41 
is often differentiation. This is usually in the form 
of spring and summer wood—spring wood on the 
inner side and summer wood on the outer edge. 
This is very distinct in our Southern pines. In 
many instances growth is so slow that the annual 
ring is hardly visible, while in other cases it may 
be an inch or more in width. The annual produc- 
tion of a ring or layer of wood is so uniform through- 
out the great wood-producing regions of the globe 
that foresters base their calculations upon it. The 
volume of wood which is added to the trunk each 
year is called the annual increment of the tree. The 
sum of the annual increments of all the trees on an 
acre gives the annual acre increment. This is difficult 
to ealeulate on an acre of very irregular mixed woods. 
At the same time a knowledge of the rate of growth 
is a very helpful guide, so that one may not inad- 
vertently cut the wood faster than it really grows. 
The average annual increment is the amount which 
one may cut each year without injury to the forest. 
The increment is the interest; the growing stock, the 
capital. As the forest becomes more and more uni- 
form in nature, and as the conditions of growth are 
improved, the increment, or yield, increases until the 
forest approaches a normal state, and the yield or in- 
crement equals or approaches the possibility of the 
forest. Thereafter calculations are more uniform, 
