Apr 1, 1925 
617 
The Construction of Taper Curves 
The sheets carrying the original data 
gathered in the field are sorted in the 
usual way into diameter and height 
classes. The diameters are then aver¬ 
aged and taper curves are drawn for 
each height class, based on diameters 
inside of the bark (fig. 4). Inside 
diameters are almost never taken at 
breast height. Accordingly, the curve 
is simply brought down from the first 
measurement to the base line of the 
graph by eye. The stump diameters 
should not be plotted, and no stump 
flare or basal swelling be allowed for, 
as it will tend to cause later difficulties. 
It is indeed much better to leave out 
the actual diameter inside of the bark 
at breast height, even if the figure is 
available, as on large trees it may be 
affected by stump swelling and intro¬ 
duce error into the curves at this time. 
of neighboring size classes, is open to 
a certain amount of scientific criticism. 
In Europe, form is generally held to 
vary with the density of the stand and 
to vary almost as widely in a single 
size class *as in a whole stand. It 
therefore becomes most proper to 
assign a single form quotient to the 
whole stand on the basis of density. 
Possibly we would do well enough by 
simply throwing all size classes to¬ 
gether and taking a single average 
form quotient. In the work done by 
the writer, however, there is a fairly 
regular trend of form-quotient values 
according to size, the quotients falling 
as diameter increases (in Douglas fir) 
and increasing with height. Possibly 
this is due in part to a rough correla¬ 
tion of size with density in many of our 
unmanaged forests; but, whatever the 
Fig. 5.—Hundred-foot class Douglas fir, showing method of smoothing irregular form-quotient values 
Numbers at points indicate number of trees as basis 
Later it will be easy to take care of 
any peculiarities in d. b. h. measure¬ 
ments due to any cause. It is not at 
all necessary that the average diameter 
in any group should fall at the even- 
inch class or that the average height 
should fall upon the mean height of 
the class. After the rough diameter- 
inside-bark taper curve is drawn, the 
diameter at a point one-half way be¬ 
tween breast height and the tip of the 
tree is found and then divided by 
d. b. h. inside bark as read from the 
graph. This gives the assumed abso¬ 
lute form quotient. After form quo¬ 
tients for all the size classes have been 
computed the values are evened off by 
curves. 
The propriety cf ass : gning to each 
size class its appropriate form quotient 
which shall be in harmony with that 
cause may be, the result is quite 
apparent, and consequently the form 
quotients of the various size classes 
have been harmonized by curves. 
The values for each height class may 
first be conveniently smoothed sepa¬ 
rately, as the form quotient tends to 
change very slowly within the same 
height class with the species studied by 
the author. (See fig. 5 for the 100-foot 
class, South Idaho Douglas fir.) There 
are also many diameters in each height 
class, which gives much longer curves 
than would be obtained if an attempt 
were made to draw curves for each 
diameter class separately. All of these 
curves should then be plotted upon a 
single sheet so that the relations be¬ 
tween height classes may be judged, and 
any necessary adjustments made, for it 
is obvious that values should not run 
