June 21, 1924 
A Study of Douglas Fir Reproduction 
1239 
Fir seedlings wilt with a difference of 1.16 inches in root length over humus 
depth, while the difference between vigorous seedling roots and humus depth is 
3.13 inches. This would indicate a maximum depth of-humus of between 1.50 
and 1.75 inches for vigorous fir seedlings under protection, and 1 to 1.25 inches in 
the open. Spruce seedlings wilt with a difference of approximately 1.04 inches 
in root length, the difference for vigorous seedlings being 1.57 inches. Thus the 
maximum humus depth for vigorous seedlings will be about 1.4 inches. Without 
protection this figure will be about 1.1 inches. 
The average humus depth in inches over the plots in 1919 was: 
Slash 
scattered 
Slash piled 
and burned 
Average 
Clear-cut__ 
1.90 
1.20 
1. 55 
Original forest_ 
1.18 
Shelterwood_ 
1.30 
.75 
1.02 
Selection.. 
1. 25 
.77 
1.01 
Except upon the slash-covered area under clear cutting, in no instance was 
the humus too deep for fir, and the same may be said for spruce where offered a 
modicum of protection, although the margin in most instances was smaller. 
Clear cutting in general is not desirable, considered from the standpoint of repro¬ 
duction. Under the original stand, although the margin for healthy spruce 
seedling development is rather small, the long-term history of the stand indi¬ 
cates that spruce reproduction is actually favored over fir. With the great 
number of spruce seed germinating in virgin stands, it is quite possible that 
enough of these find favorable humus conditions to provide replacement, and that 
other conditions, such as lack of immediate protection and lack of moisture, act 
unfavorably upon the fir. 
HEIGHT AND PROTECTION CORRELATED 
When all species were grouped as protected and unprotected, it was found that 
protected seedlings are uniformly taller than those unprotected, this being more 
true of fir than of spruce. The average difference for fir is 0.27 inch, while for 
the spruce it is only 0.02 inch; but this latter figure is misleading due to an 
apparent reversal of form of the selection seedlings, which were 0.45 inch taller 
when unprotected. In every instance, the humus is from 0.3 to 0.6 inch deeper 
for protected fir and spruce seedlings. Although no measurements of root 
lengths were made, it is hardly likely that humus depth has any influence upon 
survival after the third year. However, assuming that the humus has increased 
in depth only a little since the advent of the seedlings, the favorable effect of 
protection combined with relatively shallow humus is evident. 
It is not known at what rate humus has accumulated upon the plots since the 
original measurements were made, although it is reasonably certain that the 
heavy stand of aspen upon the clear-cut plot is having the effect of building up 
the humus layer and making it more difficult for seedlings to get started. This 
may also be the case upon the scattered-slash areas of the selection and shelter- 
wood plots, but the young stand which has already established itself here makes 
it unnecessary to give any more consideration to the character of the humus. 
An effort has been made in figures 1, 2, and 3 to illustrate the trend of height 
growths for Douglas fir, Engelmann spruce, and limber pine seedlings beginning 
with the fourth year, under the various methods of cutting and under the original 
stand. Insufficient data were at hand to construct a curve for limber pine on the 
shelterwood area, but the curves that are shown emphasize the superior per- 
