YIELD OF ASSOCIATED SPECIES 95 
On page 80 twenty-five years is shown to be the 
length of time which must elapse after cutting the first 
crop of Spruce down to twelve inches before the same 
amount can again be obtained. It is desirable to know 
what the condition of the hardwoods will be at the end 
of that time if the merchantable timber is cut now. It 
was not the purpose of this investigation to make an 
exhaustive study of the growth of the hardwoods, but in 
order to gain some slight knowledge of their develop- 
ment a number of measurements were taken by means 
of Pressler’s Zuwachsborer. This is a short hollow 
auger, fitted with a fine wedge, by which a narrow cylin- 
der about an inch long may be bored from the trunk of 
a tree, to show its present rate of growth in diameter. 
With the help of this instrument a number of measure- 
ments were taken of the growth of trees commonly 
found in mixture with the Spruce. The number of 
trees measured, their average diameters, and the num- 
ber of years required to grow one inch in diameter are 
given in the table below. It was found that the growth 
for the different diameters varied so greatly that an av- 
erage was taken of the trees of all diameters. It will 
be seen that the rate of growth of the hardwoods is 
very slow. Thus, the time required for Birch to grow 
one inch in diameter is twenty years, and for Hard 
Maple sixteen. 
