24 
sired crop. This is primarily a matter of the proper control of 
the density throughout the life of the stand. There was an 
abundance of material for study below twenty years and 
above sixty years, but there was rather a scarcity of material 
between these ages. It was, therefore, necessary to follow in 
part some other line of study in order that the deficiency 
might be overcome. It was decided to make a study of crown 
development both from living material and from existing 
yield tables. If the area of the ideal crown for any age be 
known, then the corresponding number of trees per acre 
could be found, and thus a criterion established for periodic 
thinnings. 
YIELD TABLES 
A. EXPANSION OF CROWN 
It has been noticed in Tables VI and VIII that stem diam- 
eter is a function of crown radius. In general, the larger the 
crown the larger the diameter of the stem. Growth of crown 
is essentially different from either height growth or diameter 
growth. We have already seen that within at least certain 
wide limits growth in height is not affected by the density of 
the stand. For the variations in density in ordinary forest 
practice, height growth may be said to be beyond the control 
of the forester. Diameter, however, being a function of the 
crown radius, may be varied by varying the crown radius, or, 
in other words, by varying the density of the stand. This 
latter is secured by various degrees of thinning. 
With open grown trees height growth, diameter growth 
and crown growth follow much the same curves of growth. 
If we consider an open grown oak or pine, the longest 
branches are very low down and may even rest on the ground. 
Pine branches are easy to study and these show that the ex- 
pansion of the crown is at first very slow, then very rapid for 
a few years, and then after this rapid growth there is a very 
gradual decline. This is precisely the same type of growth as 
for height and diameter. 
