mACIFIC u,s, DEPARTMENT OF AGRICULTURE
•
PORTLAND, OREGON
PNW-.3.5
RESPONSE TQ THINNING
by Norman
60- YEAR" OLD
P.
DOUGLAS F R'
...
M:(rthington
Thir,ty years of growth after t e; first. thinning Jn a 60--year-old
Douglas-fir stand· on site ·rv· show·'that heavy thirining· substantially
depressed gross increment in ensuing years. However, a moderate thin­
ning reduced gross increment only slightly. Growth was well above
normal on both moderately thinned and unthinned stands, but was a
little less than normal on heavily .thinned areas, Records singl out
the increased increment obtainable on actual stands as compared with
normal yield table estimates. Main advantages in the thinnings were:
(1) salvage of mortality; (2) reallocation of stand growth potential to fewer, larger, and higher quality trees; and (3) realization of earlier returns through thinning. Presumably, the stand was too old for heavy thinning to speed up residual tree growth sufficiently to compensate for loss of increment on trees c t. Four of six areas, loc'ated at the base of Mount \\Talker, Olympic l
National Forest, near Quilc.en·e, _Wash , were. thinned in 1934 and 1 9 ;37 .:­/
Basal areas removed were 31-:and '37 percent on moderately thinned plots
9 and 10; 44 and 50 percent• on heavily thin
_ ned' plots 6 and 7 (figs. 1 .:· and 2)
Thinning, chiefly t,rorii bel,ow r m ved mqstly suppresse.d and intermediate trees, but als.6 include_d;some dominants· and ·codominants '
to eliminate "wolf" trees and·imp!'ove..sp·adng. '. ,?lots 5 and 8.were
. left untouched as checks. ·A.s.ec·onc1.Jighf thinning in 1949. arid a
third in 1958, on three plqt .' \ im:p·;roved:,spacing; reliloved 'poorer grade·
trees of lower crown class ;"·and si;l.lvaged::d,eg:d, or dying trees. The
original stand on plot 7 w9-s:2
. 0·p rcent
. 'les tn basa
. l.are.k and_c bic
volume than an average for the fi\re other plots' .
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l/ Worthington, Norman :!;':,
. .and Isaac, Leo A. ·Experimental thin­
nings in young Douglas-fir. Northwest sc:L 26: 1-9, illus. 19S2 • Figure 1.--Maunt Walker plot 10, first thinned moderately ( 31 percent basal area ) in 1937. Photo was
taken in 1949, during a second thinning. Grass cubic volume increment after 30 years was 91 percent
of that in an unthinned area visible in right background.
Figure 2 ••• Mount Walker plot 7 as it
appeared after heavy thinning ( 50 per·
cent basal area ) in 1934; shows open
condition of residual stand.
Gross 30year increment in cubic volume was 75
percent of that in unthinned stand, with
greatest growth loss occurring during
first 5 years after thinning.
Gross increment on both moderately thinned stands was within
9
percent of the unthinned stands in terms of cubic volume and practically
identical in terms of Scribner board feet.
The heavily thinned plots
25 percent below in cubic volume growth, or 20 percent in Scribner
Net increment was generally highest on moderately thin­
scale (table 1).
fell
ned plots,
lower on unthinned plots,
and lowest on heavily thinned plots.
Mortality was plainiy lower on all thinned areas,
than one-half that on unthinned plots.
all mortality on thinned stands was,
Further,
or could be,
averaging less
roughly one-half of
salvaged.
Unsalvaged
losses averaged 8 cubic feet, or 19 board feet Scribner, per acre per
year for all thinned areas.
Irrevocably lost were 38 cubic feet, or 47
board feet, on unthinned areas.
Still, losses cannot be considered
serious in either case.
Growth percent rates show greater efficiency of growing stock among
Average annual gross rates were 2,2 percent for
3.2 percent for board feet, after thinning, but only
all thinned stands.
cubic volume,
or
1.8 and 3.1 percent, respectively, in unth nned stands.
The better rates
for the thinned stands are,
numbers.
of course,
directly related to reduced tree
Average stand diameter
growth was
23 percent greater on thinned
areas
growth
(3.48 inches versus 2.65 inches),
There was an actual increase in
although part of the difference is the effect of eliminating most
suppressed trees in initial thinning.
Restricting plot computations to only the
100 largest trees per acre
confirms the greater efficiency of both volume and gross increment on
thinned as opposed to unthinned stands:
Percent of stand represented by
the
Unthinned
plots
1964 stand:
5 and 8
100 largest trees
Lightly thinned
plots
9 and 10
Heavily thinned
plots
6 and 7
59
63
75
78
97
98
68
81
99
83
89
100
Cubic volume
58
59
61
68
83
85
Board-foot volume
(International)
64
72
84
69
71
88
Basal area
Cubic volume
Board-foot volume
(International)
Board-foot volume
(Scribner)
Gross increment
(1934-64):
Basal area
Board-foot volume
(Scribner)
3-
Table 1.- -Thirty-year increment and
Plot
number
5 (unthinned)
(heavily
thinned)
7 (heavily
thinned)
I
.p..
I
8 (unthinned)
9 (moderately
thinned)
10 (moderately
thinned)
Item
mortality record, Mount Walker thinning plots (1934 -64)1/
Site
index,
1964
Basal
area
cut
Feet
Percent
Beginning stand
Gross increment
Mortality
Net increment
111
--
Stand after first thinning
Gross increment
Mortality
Net increment
Thinnings, 1949 and 1958
Salvaged mortality2/
114
---
Stand after first thinning
Gross increment
Mortality
Net increment
113
--
Beginning stand
Gross in-crement
Mortality
Net increment
103
Stand after first thinning
Gross increment
Mortality
Net increment
Thinnings, 1949 and 1958
Salvaged mortality
Salvable mortality, 196
101
Stand after thinning
Gross increment
Mortality
Net increment
Thinnings, 1949 and 1958
Salvable mortality, 1964
111
---
--
-
--
--
--
--
--
--
---
--
-----
--
--
--
--
Stems
Basal
area
2
Volume 1
Cubic
Cu. ft.
Number I
International
I
Scribner
-------------- Bd. ft. ------------
0
---
365
--
205.3
102-.<i
29.8
72.S
6,9{)3
4,595
883
3,712
38,690
33,379
. 4,136
29,243
16,668
24,760
1,348
23,412
44
------
144
116.9
8L2
17.8
63.4
20.3
5.3
4,337
3,425
652
2,773
801
210
27,184
26,646
6,780
19,866
5,111
1,402
13,321
19,499
1,942
17,557
2,934
951
50
----
126
--
86.4
78.6
4.3
74.3
3,025
3,416
143
3,273
18,190
24,092
76{)
23,332
9,632
17,552
400
17,152
678
216.3
103.6
52.0
51.6
6,615
4,548
1,420
3,128
32,463
30,480
4,680
25,800
10,773•
21,388
1,471
19,917
114.6
102.9
18.5
84.4
28.7
1.8.
4,608
4,164
636
3,528
968
71
24,650
29,491
3,7ti3
25,728
5,354
459
7,581
22,490
2,298
20,192
934
286
157.8
99.5
11.4
88.1
27.9
7.2
5,764
4,202
400
3,802
979
278
35,343
29,105
2,491
26,614
5,056
1,900
17,295
23,515
1,517
21,998
1,435
1,315
-
0
---37
------
-
31
------
105
--
--
29
--
21
3
14
--
--
332
-312
--
38
--
58
2
6
217
--
14
--
47
3
9.1
ll Plots 9 and 10, thinned in 1937, are adjusted to a 30-year basis.
352
2,342
1,568
/ Cubic volume is for entire stem, International rule for trees 6.6 inches d.b.h. and larger to a 5-inch top, and Scribner rule for trees 11.6 inches
d.b.h. and larger to an 8-inch top.
]_/ Salvaged mortality is included in both "mortality" and 11thinnings."
!±./ Dead trees included in "mortality" which would have been salvaged had another thinning been made.
With wider spacing,
the increases in d.b.h. increment most strik ­
ingly illustrate the effects of thinning
( fig. 3).
A significant rise
is evident, beginning with a very close similarity in rates for both un­
thinned plots,
to the greatest increase for plot
the fewest trees and smallest average diameter.
7, which was thinned to
No significance should
be attached to the differences in slope of individual plot lines,
possibly for plot
except
9 where most of the 100 largest trees were concentrated
toward the small end of the diameter range.
7.0
-
6.0
t3
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5.0
UJ
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3.0
/
/
"'
-- _
HEAVILY THINNED
--MODERATELY THINNED
-- UNTHINNED
(INCHES)
D. B. H. 1934
Figure 3••• D. b. h. increase, 100 largest trees.
Mount Walker thinning plots, 1934-64.
It can be reasonably inferred that the moderate thinnings on plots
9 and 10 were better suited to the low-site condition than were the
heavier cuttings on plots 6 and 7. Moderate thinnings also compared
much more favorably with unthinned stands in gross yield.
Gross cubic­
foot increment after moderate thinning, though only 82 percent of the
unthinned standard during the first 15 years, was 103 percent in the
last 15 years, whereas increment after heavy thinning improved from 6 6
percent to within 82 percent during the latter period. Such recovery
reaffirms the tardy response elsewhere noted among older,
stands .1/
l/
Worthington, Norman P.,
and Staebler, George R.
thinning of Douglas-fir in the Pacific Northwest.
Bull.
1230, 124 pp., illus.
1961.
( See
pp.
young-growth
Commercial
U.S. Dep. Agr. Tech,
25-26.)
-5­
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