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Experimental Thinnings in Young Dottglas-Pir
NORMAN P. WORTHINGTON AND LEO A. ISAAC Pacific Northwest Forest and Range Experiment Station (Reprinted from NORTIiWEST SCIENCE, Volume XXVI, No.1, February 1952 )
Experimental Thinnings in Y O1tng Dottglas-Fir NORMAN P. WORTHINGTON
AND
L EO A. ISAAC Pacific Northwest Forest and Range Experiment Station
D
OUGLAS-FIR OCCURS over a wide range in western United States,
but reaches its maximum development in those parts of Washington and Ore­
gon west of the summit: of the Cascade Range. This region is regarded as one
of the finest timber-growing areas in America. Most of the land is recognized
as chiefly valuable· for. forest purposes. When white settlement started, this
region was clothed with dense coniferous stands, mostly virgin Douglas-fir.
Here and there were patches of young growth that had sprung up in openings
caused by fires or severe winds. Timber harvesting and land clearing were
accelerated as the settlements grew. The acreage of virgin forests rapidly dimin­
ished and for the most part was replaced by young-growth stands. By 194 5
the forest survey showed a tOtal area of 9 million acres of virgin saw timber
and 11 million acres of young growth; another 5 million acres was nonstocked
or in various stages of restocking. The acreage of old growth is still diminish­
ing, and the acreage of young growth increasing.
Almost without our realizing it, cutting operations in Douglas-fir have
shifted from old Limber to young. Each year about half of the tOtal acreage cut
is in young-growth stands that should provide the forests of tomorrow. State
reports for Washington in 1949 show that 52 per cent of the acreage cut was
in young growth and the percentage for Oregon is probably almost as great.
Early-day cutting in these young stands was, for the most part, made on private
holdings easily accessible to markets. Most cuts were made on a liquidation
basis, taking out all material that was merchantable and leaving behind the
smaller unmerchantable trees. Usually the cuts were so heavy that only
scattered and broken stems were . left. These soon fell like jackstraws inco a
tangled mass, and the stand was completely destroyed. The destructiveness of
such cutting is recognized by public and industrial foresters alike. As a result,
studies are under way to determine methods of cutting the young forests that
thrifty reserve stand of high quality for future growth.
Young-growth forests of the Douglas-fir region have certain silvical char­
will preserve
acteristics that mote or less determine the manner in which they should be cut.
Regeneration, usually established as an even-aged stand, makes its most vigor­
[1] 2
EXPERIMENTAL THINNINGS IN YOUNG DOUGLAS-FIR
ous growth in full top light. There are many more stems per acre than will be maintained until harvest time, but this is necessary in order that the lower limbs may be shaded off to provide clean boles and clear lumber. Fast-growing individuals in the stand assert their dominance early and crowd Out the weaker trees. If a stand is allowed to groyv to ,maturity before harvest, a vast number , of these stems 'are crowded out and thus a portion of the growth power of the acre is wasted. These characteristics favor intermediate harvest cuttings, or thin­
nings, and our problem is to learn how to remove the surplus for marketing, yet preserve the remainder of the stand for future growth and later harvest.
Intermediate cuttings in young Douglas-fir, while practiced for almost a .
century in the plantations of Western Europe, are a new practice in the natural
stands of the Pacific Northwest. Many stands have now had their first thinning
on both private and public lanc;ls, but to .our knowledge no young stands have
had two thinnings on a commercial scale in which careful records were kept
of the growth rate per acre before cutting and between cuts. Two experimental
cuttings, however, do provide such records. They were made in young stands
in the Olympic National Forest of Washington, one at Mt. Walker, the other
at Kugel Creek. The cuttings were planned by E. J. Hanzlik and established by
the National Forest personnel in 1934 and 1937. Since then the Pacific North­
west Forest and Range Experiment Station has continued the experiment. A
second thinning was made on these areas in 1949. During the 12 year period
between thinnings, three measurements of growth have been completed. The
records for these experiments show the value of intermediate cuttings. For the
purpose of comparison, similar records are given for a 56-year-old plantation
in Denmark, except that thinnings started very much earlier in that instance.
lWt
.
Walker Area
A series of four plots covering 3.1 acres was established during 1934-1937
in a 60-year-old, well stocked stand at the base of Mt. Walker, near Quilcene.
They consisted of two small plots in a IS-acre area to be thinned, and two
unthinned check plots, all in a stand with a site index of 110 feet. The soil
was a shallow, loamy sand underlain by gravel and larger fragments from the
older bas ltic rocks: Slope varied from 1 5-50 per cent; drainage was excessive.
A medium to heavy thinning was made in 1937. The trees cut were chiefly
intermediate and suppressed ones, but some of the rougher dominants and co­
dominants were removed when this would release trees of better' form. The
intensity of these cuts ranged from 29 to 3 5 per cent of the cubic-foot volume.
An average of 27 cords per acre was removed (Table 1). When cut four years
after establishment, the original stand on plots to be thinned was less dense
than the stand on the check plots. This lower density would produce the effect'
of an earlier thinning on growth.
NORTHWEST SCIENCE
_____________
--'--_.-.J
_____________________ _______
NORivIAN P. lVORTHINGTON and LEO A. ISAAC
3
•
,'.
During the spring of 1949, the thinned plots were cut for the second time,
removing 8 per cent or 6 cords per acre. This time a few intermediate trees,
but chiefly codominants, were taken out to give greater release to trees of
better form and vigor.
In the 12 years, net annual periodic increment, expressed in square feet of
basal area for all trees 2 inches diameter breast height and larger, was 3.3
square feet on the thinned plots. 'This was 22 per cent greater than on the
checks, although average growing stock on the thinned plots was 27 per cent
less. Net annual periodic increment by cubic-foot volume for all trees 6 inches
d.b.h. and larger, was 144 or 9 cubic feet greater on the thinned plots, and
their increment rate per unit of growing stock was more than a third greater
than on the unthinned checks. Board-foot periodic increment on all trees 12
inches d.b.h. and larger was much alike on all plots, averaging 1002 board
feet on the thinned as against 1009 on the checks. The rate of increment in'
board feet per unit of reserved volume on the thinned stands, however, was
one-fourth higher since the growth occurred on less growing stock. Part of the
difference in net growth rate is caused by mortality in the check plots but there
was a greater increase in the net growth of individual trees on the thinned
plots. Diameter growth adjusted to a 10-year interval (Fig. 1) further illus­
trates the substantial increase in growth of the individual trees on the thinned
plots. Through a reduction in the number of stems per acre (436 trees on the
unthinned plots as against 2 52 on the thinned), the growth energy of the site
has been utilized by fewer and larger stems. Of greater importance than the
increase in growth is the fact th,at growth was of better quality and was ob­
tained in spite of the fact that part of the stand was harvested.
Kugel Creek Area
At Kugel Creek, 40 miles west of POrt Angeles, Washington, 3 thinned
plots in a 10-acre thinned area, and 1 unthinned check, covering in all 3.5
acres, were installed during 1937. This area was in a 38-year-old, even-aged,
well stocked stand, with a site index of 146 feet. The soil was a deep sandy
loam on a level situation with moderate drainage. Here again (as on Mt.
Walker) the stand was more dense on the check area than on the area to be
thinned; this gives the effect of an earlier thinning. The thinning treatment
was substantially the same as at Mt. Walker, except that one plot was CUt
lightly, favoring well-formed dominants and codominants. From 16 to 39 per
g
cent of the cubic volume was removed, amounting to an avera e of 25 cords
per acre. A second thinning in 1949 removed 14
to
19 per cent; this cut was
'
similar to the second at Mt. Walker except that more rough dominants were
t ken. These Kugel Creek plots, of a younger age class and with a better site, .
VOLUME XXVI, FEBRUARY 1952
4
EXPERIlHENTAL THINNINGS IN YOUNG DOUGLAS-FIR
;o
"
---r,
m�------'
---,
..-----,---r-
; A. TOTAL STAND
Mt WALKER
I
'JI-----'-----'----- -I- ------"I
,
:001----1..-.--1>1--; -
THINNED
r:zz::z::::J Ne1' Increment
MO;'tQllly
nllnnlnqs
;:
--
lool---tl-.
.... -t---l
;:
1}:1r::
v�======�==�
,J.V
I
KUGEL
I
__-THINNED
... __ __ ( fCK
REEK
,
" ,,--+I -----i
---'-------/- - --r--//
///
IOf--
/:
rI
-- ---
J
.,.
08 H - INCHES
a DISTRIBUTION OF TREES
IN DIAMETER CLASSES
!
:
I
THINNED
IO r------;:;r:r---,
,I -
;0
Fig. i.-Comparison of diameter growth by
d.b.h. classes, adjusted to a lO-year basis.
eM.
6
TH.
CH
I
1H
CH " rn
,,/1 TH.
IS"
14
44 INCH OIAMeTeR CLAsSeS
"
;1-4
2Z'
TM
Fig. 2.-Growth in basal area on Kugel Creek,
thinned and check plots, lO-year basis.
had responded more rapidly to the first release and were in greater need of a
second cut.
Here again, net periodic annual increment over the period of record was
greater on the thinned plots. Basal-area increment was 5.3 square feet per acre
(Table 1), or 1. 5 square feet more than on the check plot. Because the thinned
plots had far less growing stock, their increment rate per unit of growing stock
was more than twice that of the check. Net periodic annual cubic foot growth
on the thinned plots averaged 274 cubic feet, or 2 cubic feet greater per acre
than that on the check; increment rate percentage per unit of growing stock
was about two-fifths more for the thinned plots. Net periodic annual board­
foot increment per acre per year was 1917 feet, or 276 more than the check,
and an increment rate of about double that of t.l;1e check. Although some dif­
ference in growth rate arises from mortality on the check plot, a comparison of
diameter growth as shown in Figure 1 emphasizes the much larger average
increment per tree made by the thinnd stems over the unthinned. This effect
is substantially greater than at Mt. Walker because the Kugel Creek stand is
younger and on a better site.
The effect that thinning has had on growing stock, increment, distribution
by diameter classes, and mortality, expressed in basal area, is illustrated in Fig­
ure 2. This chart makes more obvious the relation between net increment,
mortality, and total growing stock. The gross increment-that is, net incre­
ment plus mortality-was somewhat similar for both the thinned and checkNORTHWEST SCIENCE
NORMAN P. WORTHINGTON and LEO A. ISAAC
5
plots. The greater net increment of the thinned plots was obtained through
I
reduction of natural mortality and through the greater growth per tree in the
residual stand. Figure 2A emphasizes that the greater increment in thinned
stands was obtained on substantially less growing stock. Figure 2B shows the
distribution by 4-inch diameter classes of the thinnings removed, also for both
thinned and check plots, the. net increment, and mortality.
Table I.-History of two thinning experiments in Douglas-fir region
(all dara per acre) '*
A verage
Item
Age
1r!.
Trees
flO.
d.b.h.
in.
Ave.
Basal
hr.
area.
It .
Jq. ft.
Volume
cu.
ft.
board It.
Mr. Walker-Site Index 110
Average for check plots (2)
Original stand
Stand at
____
_______
60
522
8.7
74
210
6,124
17,798
65
492
9.1
82
220
6,880
22,685
Stand at
______
70
456
9.7
85
233
7,348
27,459
Stand at
_____
75
436
10.3
87
250
8,148
32,934
0.11
Periodic annual increment
0.9
2.7
135
1,009'
229
6,946
21,085 80
2,218
Average for thinned p loes ( 2 )
Original stand
____
First
__
thi ning
at
9.7
183
8.9
10.2
so
149
4,728
15,293
5,792 64
265
___________
70
253
11.0
86
168
5,674
21,097
75
252
11.6
89
186
6,310
26,305
__
30
9.7
16
496
657
__
222
11.9
170
5,814
25,648
144
1,002
Before thinning
Second
80
448
___
After thinning
Stand
64
____
thinning
After thinning
0.18
Periodic annual increment
89
0.8
3.3
Kugel Creek-Site Index 146
Check Plot (1)
Original stand
38
603
8.3
73
228
6,225
13,859
Stand at'
_____
45
482
9.9
85
255
8,315
24.538
Stand at
____
50
440
10.7
93
9,489
33,556
272
1,641
211
6,071
12,963
75
2,059
4,429
0.20
Periodic annual increment
1.8
3.8
Average for thinned plots (3)
I
Original stand
__
First thinning
__
After thinning
__
______
Stand at
Before thinning
_
Second thinning
After thinning
464
9.1
197
8.3
38
267
9.7
75
136
4,012
8,534
45
256
ILl
89
173
5,774
19,750
50
252
12.1
98
199
7,296
31,536
46
11.6
34
1,211
4,840
206
12.1
164
6,085
26,696
274
1 ,917
38
_
__
50
Periodic annual increment
025
75
98
1.9
5.3
• B sal
uea for ill crees 2 in. diameter breast height and larger. Cubic volume for aU crees 6 in. d.b.h. and
larger [0 a 4-in. minimum top. Board-fooe volume for crees l2 in. d.b.h. and larger co an 8-in. minimum tOP •
International Y<t-in. kerf. which approximaees lumber recovery value for reasonably efficient milling units.
VOLUME XXVI, FEBRUARY 1952
6
EXPERIMENTAL THINNINGS IN YOUNG DOUGLAS-FIR
Discussion of Mt. Walker and
Kugel Creek Cuttings
The results of these experiments in young-growth Douglas-fir have shown
'that except for board-foot growth at Mt. Walker, net periodic annual incre­
ment per acre on a basal-area, cubic-foot, and board-foot basis has, been in­
creased by th.innings; also growth has been of better quality, and material has
been harvested that otherwise would be lost.
The tests extended over a 12-year period for stands which were 38 and 60
years old at time of first thinning. The original cut was too severe on probably
all but one plot and the period between cutS too long, as Danish experimentS
that follow will show. Even so, volume growth has been greater and of better
quality as a result of the cut: The practical justification for thinning in these
two cases is that mortality was salvaged in merchantable form; furthermore,
the removal of 37 per cent of the growing stock actually increased the net
growth of the stand and put this growth on large stems. The optimum growing
stock and the upper limit of what can be safely removed have yet to be deter­
mined accurately for Douglas-fir of different age classes. Further experimenta­
tion is needed to determine these points.
A 56-Year-Old Douglas-Fir Plantation
in Denmark
Plantations in Denmark show the possibilities of light and frequent thin­
nings. The life history of one such Douglas-fir plantation is shown in Table 2.
More than 4000 trees per acre were planted in 1884. When the first regular
thinning was made 24 years later, there were more than 1500 trees to che
acre. A third of these stems were removed in che first cutting, taking out trees
in che smaller diameter classes that were dying. From 1908 on, thinnings or
improvement cuttings 'were made at 2- to 6-year intervals. During a 30-year
·
period, a total of more chan 9000 cubic feet was removed from this stand. It
was then 56 years old, had a total of 86 trees to the acre, with an average diam­
eter of nearly 17 inches, an average height of 95 feet, a basal area of 13 1
square feet, and a volume of more than 5000 cubic feet to the acre, This makes
a total growth of over 248 square feet of basal area and 14,000 cubic feet per
acre in a 56-year period. Mean annual increment was 7.8 square feet of basal
area and 250 cubic feet per acre, which is far above the volume recovery in
56-year-old natural stands of equal s te'class in the Pacific Northwest.
Besides providing. an early income, this Danish stand has put more growth
and growth of a better quality on larger stems than occur in our .natural site
III stands. It constitutes living evidence of what can be accomplished wich
young-growth Douglas-fir stands under careful management, provided topogNORTHWEST SCIENCE
Table 2.-Cutring hisrory of Douglas-fir planrarion in Denmark; '*'
si ce inJex 130 fet:c (all data per acre)
Srand age,
24 yrs.
spring
1908
8canJ scaciscics
28 yes.
aucumn
1910
30 yrs.
au Cllll1n
1912
33 yrs.
autumn
1915
season,
and year
37 yrs.
autumn
1919
of
measurtmenc
41 yrs.
spring
1924
44 yrs.
spring
1927
47 yrs.
autumn
1929
50 yrs.
aurumn
1932
56 yes.
autumn
1939
-------------- _ . .
Before thinning
No.
of stems
D.h.h. (in.)
Height (ft.)
Basal are-J. (sq. fr.)
Volume (cu. fr.)
Removed
__.. ___. ___ ._______. __.
1,556
4.6
4-1.3
179.6
4,4ii7.5
1,027
5.5
50.8
167.5
4,487.5
772
6.1
54.:1
159.0
4,644.6
571
7.2
60.7
160.2
4,987.7
389
8.9
69.2
167.'1
5,716.6
267
10.5
76.8
160.0
5,645.0
207
11.7
81.7
15·i.l
5,67.3.7
164
12.8
86.3
149.9
5,673.7
134
14.3
90.9
150.9
5,816.6
116
16.4
96.1
170.6
6,888.4
529
3.7
39.0
39.5
828.9
255
4.7
45.6
30.7
643.1
20t
5.0
48.9
27.-1
757.4
U:>2
5.6
53.8
31.1
857.5
122
7.4
65.9
1,229.1
60
9.3
73.8
2H.3
928.9
43
10.8
80.7
27A
971.8
30
10.7
84.6
IB.7
700.3
18
13.5
90.9
17.9
714.6
30
15.6
98.4
39.H
1,700.7
1,027
5.0
45.9
140.1
3,658.6
772
5.7
, 52.5
136.8
3,844.4
571
6.5
57.1
131.6
3,887.2
389'
7.8
63.0
129.1
4,130.2
267
9.5
70.9
131.4
4,487.5
207
10.8
77.7
131.7
4,716.1
164
11.9
82.3
126.7
4,701.8
134
13.4
86.3
131.2
4,973.4
116
14.5
90.2
133.0
5,102.0
in rhinning
No. of Cems
D.b.h. (in.)
Height (fr.) ..
Basal area (sq. fr.)
Volume (cu. ft.)
. __. __._ ._ ......_ ....__.
36.-1
()
?;j
()
?;j
I-i
E5
4\
I-i
()
Afrer rhinning
No. of stems
D.h.h. (in.)
Height (ft;)
Basal area (sq. ft.)
Volume (cu. fc.)
_._._. __________....___.
8
5
__...____._.
......
D.b.h. (ill.)
Height · (ft.)
Basal area (sq. fc)
Volume (cu. fc.)
Mean annual yield
......
\D
VI
N
l
t-<
h1
()
Periodic annual incremenc
X
X
<:
>-<:
_________. ___.
86
16.7
95.1
130.8
5,187.7·
__ . ....
_ . . ___.
(cu. h.)
___._
0.17
1.6
9.1
276.3
241.2
0.20
0.9
ILl
400.1
245.0
0.13
1.2
9.2
366.8
255.3
0.27
1.6
9.6
396.6
265.5
J:l
0.25
1.5
9.5
289.4
279.6
• From
u!lj>ubli hed yield. ables for Douglas·fir plan( cions in
enmark.
asic data supplied
_
la",<1 CO English unlls by PacifIC NOHhwes( Fuct.:s( and Range Experunent Sta(lOn .
0.30
1.3
7.5
319.2
280.6
by O.
0.30
1.3
7.7
324.0
283.2
0.30
1.5
6.2
2B1.1
285.B
0.32
1.0
6.3
297.7
285.5
Cl
286.B
1Jars(rand Jorgensen, Langespe, Denmark, (rans·
-..J
8
EXPERIMENTAL THINNINGS IN YOUNG DOUGLAS-FIR
raphy is such that partial cuts can be made and that a market exists for the
material that can be removed.
The Kugel Creek stands were 38 years old when first cut, and hence may
be compared with the Danish plantation from 19 19 to 1932. Differences in
treatment were that the Danish stand had received 4 thinnings before 19 19,
and received 4 thinnings instead of 2 during the 13-year period.
The periodic net annual increment obtained was 7.2 square feet of basal area
for the Danish plots as against 5.3 square feet at Kugel Creek. The Danish plots
made an annual periodic increment in volume of 302 cubic feet as against 274
cubic feet at Kugel Creek. The basal area and the cubic-foot volume of the
growing stock at Kugel Creek, however, were 17 per cent and 8 per cent higher,
respectively, than in Denmark. Therefore, we may conclude that the greater
yields obtained by the Danes, both in increment and increment rate, occurred
because of earlier, lighter, and more frequent thinnings that made better use of
the inherent growth potential of the site. It is interesting to note that the site
index at Kugel Creek is 17 feet higher than in Denmark. It is believed that if
the same thinning practices had been used here as in Denmark somewhat larger
yields would have resulted.
Conclusions
The foregoing case studies are examples of how young Douglas-fir stands,
in their native habitat and in Europe, react to thinning. Naturally, possibilities
will vary with stands and localities and markers, but thinnings do offer prom­
ise. Because of improved transportation facilities, the depletion of old-growth
stands, and greatly improved markets for small-sized material, the possibility
of making early improvement cuttings is rapidly developing in the Douglas-fir
region. Where topography and markers are favorable, thinnings may now begin
when the stand reaches 30 years of age, instead of waiting for a final harvest
at 75 to 100 years. The intermediate cuts should be light and frequent to over­
come any tendency toward windthrow, which might result from opening the
crown canopy too rapidly.
It seems likely that a considerable portion of the volurne of these young­
growth stands can be harvested in such a way that full growth 'wiU be realized
on the area and a high quality forest be maintained for the final cut. The gains
that can be made from successive light cuts are largely by:
a) Salvaging much of the volume now lost through mortality and decay
in natural stands
b) Prolonging the rotation, i.e., the time until finiJ harvest, through the
period when growth is most rapid and the highest quality material is produced
NORTHWEST SCIENCE
NORlvIAN P. l'V'ORTHINGTON and LEO A. ISAAC
9
c) Increasing the toral net volume growth per acre and putting more of
it on fewer, larger, and cleaner stems
d) Providing early returns from the stand for the purpose of retiring early
initial COStS and carrying charges
These findings show that better forest practice can be good business in
young Douglas-fir on both public and private forest land.
,VOLUME XXVI, FEBRUARY 1952