Reprinte(l from JOURNAL OF FOHES'l'RY, VOL. 54, No. 9, September 1D50
PUIf.;:i;)sr:d by the FOr€;s\
Service for official use
George R. Staebler
Effect of Controlled Release on Growth of
Individual Douglas-Fir Trees
Puget Sound Research Center,
Pacific Northwest Forest and Range
Experiment Station, Forest Service,
U. S. Department of AgricultUl'e
'1'HIS paper presents preliminary
results of a "spot-thinning" study
in a 41-year-old, well-stocked, Site
IV, Douglas-fir stand 011 the Wind
River Experimental Forest in
southwest Washingtoll. The ex­
periment, installed in the spring of
1952, is designed to evaluate the
effect of different degrees of release
on dominant, codominant, and in­
tenrediate trees.
intermediate, dominant, codomi­
nant, etc.
Diameters of the study trees
were measured at time of treatment
and again in September 1955 after
3 growing seasons. Total heights
were measured in 1952, but not in
1955. More complete measure­
ments, including height, will be
made after 5 years' growth record
is available.
Methods
Results
A one-chain grid was laid out
in approximately 14 acres of the
stand. At each grid-point, the
nearest dominant, codominant, or
intermediate tree was chosen that
had at least three competitors. '1'0
be a competitor, a tree had to be a
dominant, codominant, or inter­
mediate, and be nearer to the sub­
ject tree than D+2 feet where D
equals the d.b.h. in inches of the
competitor.
Four treatments of three crown
classes are involved: 0, 1, 2, or 3
competitors cut around 10 each
elominant, codomillant, and inter­
lllc(liate trees. Thus the experiment
includes 120 study trees. The num­
ber of competitors to be cut was
chosen at random and trees cut
were "chief" competitors, deter­
mined by observation. Orown
classes of study trees were selected
in rotation at consecutive grid
points, i.e. dominant, codominant,
In the 3 years following' treat­
ment, dominants grew more than
codominants and codominants more
than intermediates as mig'ht be ex­
]1ected (Table 1). More important
is the change in gTowth with treat­
ment, the data indicating a pro­
gressive increase in growth from
o to 3 competitors cut. The differ­
ence is greatest between no cutting'
and 1 competitor cut, and least be­
tween 2 and 3 competitors cut.
,Vithin crown classes other than
dominant, growth does not follow
such a neat pattern, although in
every crown class the growth is
least where no trees were cut.
Analysis of variance shows that
differences among crown classes
and among' numbers of competitors
cut are highly significant.l The in­
teraction between crown classes
and number of" competitors cut
(i.e., the tendency for release ef­
fects to change in going' from one
crown class to another) was not
significant.2
In a supplementary analysis, re­
gression of diameter growth over
d.b.h, was calculated for each of
the four treatments, pooling' the
crown class data (Fig. 1). The re­
grcssion for 1 competitor cut is
substantially above the control
(0 competitors cut) and 2 compet­
'One percent level test. 'Five percent level test. TABLE I.-AVERAGE 3-YEAR DIAMJilTER GROWTH BY CROWN CLASS AND NUMBE.R OF
COMPETITORS CUT
CompetitOl's
cut
Dominant
Number
0
1
2
3
Avemge1
Crown class
Codominant
Intermediate
Average'
Inches
.69
.43
.19
.44
.72
.29
.55
,94
1.05
.56
.48
.33
.48
.31
.60
.62
.85
.48
.28
.55
lAverages weighted by the number of trees rcpl'esented in each crmyn anel treat­
men t clnsR.
JOURNAL
568
1.6
1.4
(J)
W
:c
u
2
1.2
I
1.0
:i
a:)
d
Competitors Cut
,8
Z
:c
r-
3:
0
,6
.4
(!)
.2
.0
4
0
O.B. H.
FIG.
8
12
16
PRIOR TO TREATMENT - INCHES
1.-'l'hree J'0ar diameter growth by d.b.h. fOl' each of t.he treatment.s.
itors (!ut is well above 1 compet­
itor, but the 3 competitors cut re­
gression is practically identical
with that for 2.
When measurements were made
in 1955, nine trees either had died
or had suffered broken tops and
were omitted from the calculation.3
These losses are almost certainly a
result of thinning since they were
confined to codominant and inter­
mediate trees that had been re­
leased. No further attempt at anal­
Tsis was made for this preliminary
report.
Growth is strongly related to
d.b.h. within the crown class and
3Anal 'sis of variance was made using'
(lisproportionate
subclass
fl'eqnencie. .
The nn 11 hypothesis becomes: « Crown
class and rclease do not affect the g'l'owth
of t.rees tllat survive the measurement
pl'I'ioil."
treatment classifications. Some of
the differences in growth within a
crown class, apparent from Table
1, are due to larger or smaller trees
in one treatment than in another.
This fact is not accounted for by
the analysis of variance technique.
More refined analysis has not been
made because of the short growth
period involved and the prelimi­
nary nature of this report.
Discussion
This experiment is designed to
use individual trees for observa­
tion, rather than some unit area as
in more usual thinning trialR. It
insures close control of the thin­
ning treatment applied and per­
mits evaluation of the treatment
on trees occupying different posi­
tions in the stand. Consequentl T,
OF FORES'rRY
the results have more than usual
meaning in understanding the re­
actions of trees to thinning. Con­
tinued observation should add con­
siderably to the type of informa­
tion required for the formulation
of scientific marking rules.
Although these preliminary re­
sults are based on only three years'
growth following treatment, they
already reveal interesting and sig­
nificant trends.
Dominant trees, which often are
thought to grow at the maximum
rate possible for the site, have re­
sponded well to release in this well­
stocked, Site IV stand. In fact,
Figure 1 shows that the greatest
response, in terms of inches of in­
creased grovvth, occurred on the
largest trees-that is, toward the
dominant end of the scale. Appar­
ently the better developed crown
and root system of large (domi­
nant) trees permits them to make
better, or at least more immediate,
use of the increased room from
thinning.
It must be kept in mind, how­
ever, that in dense stands such as
the one in which this study is lo­
cated, dominant trees are not so
well differentiated from the lower
crown classes as in stands less well
stocked nor, perhaps, as in stands
on higher sites. That dominant
trees had at least three competitors
is evidence that even they were
crowded.
The point of diminishing returns
is reached quickly as release is in­
creased. Cutting 1 competitor in­
creases growth substantially. Cut­
ting' 2 increases it still more, but
not twice as much. Little added
growth results from cutting 3,
rather than 2, competitors. How­
ever, the effects of heavier release
may be more lasting so that where
the interval between thinnings is
long, cutting 3 competitors may
prove superior to lighter treat­
ments.
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