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Cro'wn Expansion and Stem Radial Growth
Of Douglas-Fir as Influenced by Release
BY
DONALD L. REUKEMA
Abstract. A study of 32 felled and dissected codominant trees (29 released and 3 unre­
leased) in a 50-year-old Pseudotsuga lIIe11ziesii stand in western vVashington indicated
that release tends to reduce rather than accelerate crown expansion during the first few
growing seasons following thinning. This effect was most pronounced in the upper part
of the crown. Moreover, when released and unreleased sides of a given tree were com­
pared, no differences in branch elongation were apparent. Stem radial growth was
increased by release, progressively improving lower on the stem. Thus, apparently crown
buildup was not a major contributing factor in the stem-growth response of released
trees.
DOUGLAS-FIR (Pseudotsuga meJ7(Mirb.) Franco) timber crop comes
under increasingly intensive manage­
ment, foresters must better understand
how trees grow and why they react as
they do to various cultural treatments,
such as thinning. It is known that in­
tensity, frequency, and type of thinning
can vary considerably without appreci­
ably altering stand increment since the
growth capacity of the site is redistri­
buted to fewer trees. However, the most
efficient l'edistribution of increment could
be achieved only if the trees retained are
those capable of responding best to
release.
Response to release has no simple expla­
nation. \Vhen a tree is released, most
subsequent improvement in stem growth
is probably the sum result of (1) improved
aerial environment, (2) reduction in root
competition, and (3) the resultant increase
in crown surface. This paper reports on a
study of the effects of release on Cl'own
expansion and its relation to stem growth.
As
THE
ziesii
The Study
Trees studied were on the Voight Creek
Experimental Forest in weste1'11 vVashing­
ton. This is a typical 50-year-old DouglasReprintecZ from
fir forest, growing on soils derived from
glaciated materials and in a moderate
climate. The entire area is devoted prima­
rily to an experiment designed to test
various thinning regimes.
Data for the study came from 29 re­
leased and 3 unreleased trees. Of the
released trees, 18 were in a stand (A)
thinned during the 1950 growing season;
11 were in a stand (B) thinned following
the 1950 growing season. Thinnings
removed most large, limby dominants and
some codominant trees. Data were collec­
ted in 1958 and 1959, respectively, just
prior to the second thinnings. The three
unreleased trees were in a nearby un­
thinned stand (C).
Subject trees were all codominant at
the time of release, showed no apparent
damage other than "minor" freeze dam­
age, and had had at least three compet­
itors no less than 2 inches smaller in
The author is on the stafl' of the Pacific North­
west Forest and Range Expt. Sta., Forest Ser­
vice, U.S. Dept. Agric., Portla.nd, Ore. Paper
based on a Ph.D. thesis, Univ. of Washington,
1962. The study area, the Voight Creek Experi­
mental Forest, is maintained by the Pacific
Northwest Forest and Range Expt. Sta. in
cooperation with the St. Regis Paper Co. Manu­
script received June 28, 1963.
FOREST SCIENOE, Volume 10, Number 2, June, 1964 Purchased by the U.S. Forest Service fOl' official use, diameter than the subject tree and within
l.5D feet.l Many competitors were larger.
Released trees had been partially freed by
the cutting of at least one of these com­
petitors.
Trees were felled, and branches and
stem cross sections were removed, tagged,
and collected. Measurements made were
age, total height, height to live crown,
annual stem elongation, number of
branches, branch and branchlet elonga­
tion, and stem radial increment at various
heights.2 Diameters and positions of all
trees over 5 inches dbh within 40 feet of
each subject tree were recorded and
mapped.
The subject trees averaged 45 years in
age, 3 12.4 inches in dbh, and 100 feet in
height, with a mean crown length of 39
feet composed of 21 branch whorls. Mean
basal area for all trees within 1.75D feet4
of each subject tree before thinning was 13
square feet. Cut competitors (averaging
3 trees, 19 inches dbh, and 17 feet distant
from the subject tree) made up about
half the total basal area competition
before thinning. In general, trees were
released on two sides, but degree of re­
lease (number, size, and proximity of cut
competitors) varied considerably (Table
1).
Results and Discussion
Throughout the period of observation,
all patterns of stem radial growth and
stem and branch elongation exhibited
fluctuations. Most fluctuations were apI"D" is the diameter breast high in inches of
the competitor. Thus, for example, to be within
1.5D feet of a subject tree, a 12-inch competitor
would be within 18 feet.
20n the first 18 trees, periodic radial increment
only (9-year prerelease and 9-year postrelease)
was measured. Annual radial increment was
measured on the remainder.
3At stump height, 1 foot above mean ground
line.
4Three trees within 1.5D feet was a criterion for
selection of subject tree&, but all trees within
1.75D feet were considered competitors.
parently related to annual val'latlOn in
precipitation and temperature. On the
other hand, an unprecedented freeze in
November 1955 (Duffield 1956) was
responsible for sharp growth rate de­
creases in 1956 and erratic recovery in
subsequent years. Because of the damag­
ing effects of this freeze, release effects
have been evaluated primarily for the 5­
year period followihg the time of release,
1951-55.
Stem Radial Growth
In general, stem radial growth attained a
maximum within the crown; below this
point it decreased progressively down­
ward, except for an increase near the base
of the stem (Fig. 1). Ring widths at a
given internode tended to decrease as the
tree grew older. Both of these patterns
are consistent with findings of other inves­
tigators (Onaka 1950, Duff and Nolan
1953). However, there were many fluc­
tuations in the annual pattern.
Release improved stem radial growth.
Growth rates tended to be about the
same on released and unreleased trees
neal' the base of the crown, but were bet­
ter on released trees at points progressive­
ly lower on the stem (Fig.l). On released
trees, the average ratio of radial growth 5
years after release to that 5 years before
release varied from about 75 percent at
the crown base to 125 percent at breast
height; on unreleased trees, the ratio
remained about 70 percent throughout
the length of the stem. This pattern, like­
wise, is consistent with the observations
of others (Meyer 1931, Schumacher 1932,
Behre 1932, Bickerstaff 1946, Yerkes
1960, Forward and Nolan 1961a). Al­
though magnitude varied somewhat, in­
dividual trees generally followed the indi­
cated trends (Table 1).
Yearly fluctuations in stem radial
growth varied in direction and magni­
tude not only between trees but at differ­
ent heights on a given tree. Patterns also
differed between released and unreleased
trees (Fig. 2). At breast height, radial
growth rates on all but one released tree
volume 10, number
2,1964 / 193
T/lBLE 1. Summary oj individual tree characteristics and development.
Co;:>.petitortrees
Description of subject trees (1958)
Stand
,,'
Stump
!lo,
Dbh
'Iotal
he.ight
Cro'llIl
length
'hllorls
in
Basal area
leased
sides
}lefore
thin
ning
ReM
J:loved
l'er
Growth before and after ,thinning
Trees cut in thinning
N=her
Average
dhh.
AVerage
diatanci!
from sub..
ject tree
Periodic ste.m
r
g
at
t
t
: !:
9 years
before
9 years
after
a
Annual stem
t
a
l
r
o!-
Z
1949M
50
1951
55
Annual atec,
elongation
Annual branch
o
O
l/
:
:i
1949M 1951M
,
O
55
1949- 195150
55
t
Stand A:
II
43
12.3
97
36
18
14
49
20
16
0.82
100
1.8
83
0.65
53
12.6
104
32
23
17
44
18
19
.SO
70
1.0
100
.65
76
52
13.0
107
)0
2Z
16
64
18
17
.64
128
.9
74
.52
10",
99
105
52
14.1
111
47
26
19
57
"
25
,64
137
1.3
77
.58
47
12.6
111
47
26
13
61
19
15
,68
120
1.1
76
.59
8L
48
12.5
109
J6
2Z
14
.90
78
1.7
74
.60
97
"
13.4
90
38
19
15
53
"
26
.94
83
2,0
70
.13
115
45
12,8
98
41
"
20
"
20
15
.78
77
2.0
58
.82
69
46
13.7
102
47
24
24
41
2l
18
.84
90
1.'
44
12.7
110
40
19
24
70
24
18
,98
106
2.0
72
,86
90
40
12.2
95
43
24
47
13
15
1.10
69
1.5
,37
.76
62
20
.72
119
1.9
79
.72
81
.96
114
2.5
78
,79
104
"
.67
12
44
12.8
103
48
2l
14
54
21
13
39
12.2
97
49
21
11
23
15
30
15
12
1.0
102
2.7
77
.87
83
60
18
"
1.10
J3
1.6
124
.67
104
90,
14
46
14.0
104
51
15
46
12.4
"
"
18
16
44
12.0
96
50
23
"
20
18
77
2.0
69
,74
17
44
11.6
97
38
19
13
17
11
.74
114
2.1
55
.78
85
18
44
13.4
103
41
18
55
20
12
1.22
66
2.4
81
LOS
72
12
SUndE:
19
45
14.2
110
52
24
12
68
2Z
28
1.18
116
0.15
109
2.1
84
.59
114
20
45
11.0
98
22
17
16
56
25
2Z
.52.
1-46
.05
164
1.4
73
.61
70
21
44
14.8
98
42
24
13
B2
25
23
1.20
.13
128
2.0
2Z
43
12.6
108
37
18
17
72
18
2Z
1.04
80
.12
82
2.8
71
.72
95
23
42.
13.7
102
33
18
24
1.18
79
.11
105
1.6
90
.56
90
24
45
11.3
97
35
23
15
16
.68
90
.07
109
1.8
47
,54
85
25
42
11.0
91
23
20
27
,$IS
72
.09
11t
2.0
.71
78
154
20
15
.75
26
45
11.4
94
34
23
49
17
.73
120
,10
106
.8
167
.48
27
45
9.5
94
30
17
42
25
14
,59
nO
,07
120
2.2
73
.78
83
28
42
B.4
82
33
23
3l
14
10
.30
137
,02
250
.9
169
.46
143
29
41
13.2
99
"
2Z
1,06
112.
.11
125
1.6
58
.84
71
30
51
12.3
98
36
18
10
.79
66
.07
91
1.7
100
.62
113
"
46
11.8
98
29
16
12
.93
53
78
1.3
12?
.74
107
3l
45
10.6
96
38
18
.B7
57
91
2.4
83
.77
107
Average
45
12.3
100
39
2l
.88
"
119
1.8
84
'.70
94
10
"
18
Standel
12
50
19
17
.09
}j Internodes counted fr= the tip of the tree as of 1958; internode 40 is approximately breaat height.
1/
]./
Whorl 5 refera to the fifth worl fro", the tip of the tree in any given year (one,l.'horl higher each year).
Percent representa ratio of gro'olth after thinning to that before thinning.
were greater in 1954-55 than in 1949-50.
At the base of the crown, 5 of the 11 re­
leased trees showed increases. At both
levels, all three unreleased trees showed
decreases.
Crown Expansion
Height growth. Average tree heights at
various ages closely approximated "nor­
194
/
Forest Science
mal" for site III (McArdle et al. 1949).
Heights and height growth of individual
trees varied considerably from the aver­
age, however (Table 1).
Contrary to the general rule that thin­
ning has no effect on height growth of the
main canopy trees (Smith 1962), release
apparently tended to slightly reduce
height growth. The 1950 rate of height
growth was never regained on the average
released tree, whereas it was exceeded in
1952 and 1954 on the average unreleased
tree (Fig. 2). On the other hand, three
released trees showed improved growth
and one unreleased tree showed reduced
growth (Table 1).
Previous investiga­
tions of response of branch elongation to
release have been very limited. However,
the general opinion is that a major factor
in improved stem growth following re­
lease is the larger photosynthetic surface
of the crown resulting from the increased
rate of crown expansion into openings
created by thinning (Kramer and Koz­
lowski 1960, Smith 1962). Results of the
present study indicate that such was not
the case, as there was no general improve­
ment in branch elongation due to release.
In fact, on most released trees branches
elongated more slowly relative to pre­
release rates than those on unreleased
trees.
Three patterns of branch elongation
were evident: (1) in a given year each
branch tended to elongate less than the
one above it; (2) in each successive year a
branch tended to elongate less than in the
previous year; and (3) elongation at a
given branch internode (measuring from
the stem outward) tended to become less
on successively higher branches. This is
consistent with findings of other investi­
gators (Friesner and Jones 1952, Ahmed,5
Forward and Nolan 1961b).
Elongation decreased quite uniformly
from younger to older branches on all
trees, although relative growth rates
varied considerably between years as
well as between trees. Linear regressions
of 9-year elongation over branch age were
all significant at the 5-percent level or
higher. Average elongation at various
levels in the crown as of 1958 and ranges
around these averages as determined
Branch
elongation.
5Ahmed, S. S. 1956. The effect of application of
artificial fertilizer on height, radial, and branch
growth of 35-year-old Douglas-fir. Unpublished
M.F. thesis, Univ. Wash.
10
'"
"'
u.
o
w
w
'"
....
15
AP PROXIMA T E BASE
OF LIVE CROWN
"
,
,
20
u.
o
0..
....
,
I
25
UNRELEASED,'
(3 TRE ES)I
V
'"
o
'"
u.
w
Q
o
Z
'"
w
....
z
,
,
I
I
30
I
I
I
I
I
I
I
I
I
I
I
I
----\-­
,
\
,
\
35
u.
o
40
0.3
\
0.4
AYERAG E
0.5
0.6
0.7
0.8
0.9
1.0
5-YE AR RA DIA L GR O WTH
(INCHE S)
1951.55
FIGURE 1. Vertical distribution of 5-year radial
stem growtll on released and unreleased frees.
from the regression analyses were as
follows:
lVllOr!
No.
10
15
20
Average
(Feet)
5.45
3.89
2.34
Range (Feet) 3.55-7.40
2.91-5.10
1.40-3.60
Regression analyses conducted for each
individual year for 3 years prior to re­
lease and 9 years following release revealed
that no two trees followed similar patterns
throughout the entire period of compari­
son. DeviMions from these regressions
showed no consistent tendency for
branches on released sides of a tree to
elongate more rapidly than those on
volume 10, number
2,1964 /
195
tree were derived from these regression
analyses. Results for branches five whorls
down from the tip of the tree (success­
ively one whorl higher each year) are used
here as an example (Fig. 2). The patterns
further substantiate the lack of improve­
ment in branch elongation due to re­
lease.
other sides. Where exceptions occurred,
the position in the crown where improved
elongation occurred varied from one tree
to another.
Elongation patterns in successive years
for any given branch and for branches in
a given position relative to the tip of the
0.20 Inches
STEM
RADIAL
GROWTH
( INTERNODE 40)
(APPROXIMATE B.H. )
0.15
0.10
,
-
1
....
.... ___
............ UNREL EASED"
(3 TREES)
2.5 Feel
w
w
tL
STEM
-, - -- -
EL ONGATION
2.0
,. .....
....
.... ...,
,.
"
,." ,
UNRELEASED
,(3 TREES)
,
1.5
RELEASED
( 29 TREES) 1.0
1.0 Feel
BR.ANCH
ELONGATION
( WHORL
5
FROM TIP)
-
0.8
,.
0.6
lb
I ... ....
.
... '" /
..J
<C
::l
Z
Z
<C 1
..... .... ....
.. .. _
°
0
W
et:
U
Z
-
w
;;:..J
-w
1-",
tL
0.05
IZ
w
....
w
0.4
I
1948
JI
.... .... "
"
",
,. "
... --",::,:,
":-:---RELEASED
(29 TREES)
l
-... .
UNRELEASED
\( 3 TREES)
\\
,
,A
,
.... ...
/ "" .....
;;:..J
_w
1-'"
tL
°1
1950
I.
1952
1954
1956
1958
FIGURE 2. Comparison of patterns of annual stem radial growtll, stem elongation, and brancli elongation
on released and unreleased trees. Trees were released following 1950 growing season.
196
/
Forest Science
Average annual elongation for the 5­
year period following release, 1951-55,
exceeded the 1949-50 average on only 9
of the 29 released trees, the range being
from 52 to 154 percent; whereas on the 3
unreleased trees, the range was from 107
to 113 percent (Table 1). The greatest
reductions on released trees occurred in
1951 and 1952. Elongation in 1951 on
released tl'ees ranged from 62 to 123 per­
cent and exceeded 96 percent of the 1949­
50 average on only three trees, whereas
the range on unreleased trees was from
99 to 108 percent. Averages were 82 and
103 percent for released and unreleased
trees. In 1952, the average for released
trees increased only to 85 percent, where­
as that for ul11'eleased trees increased to
115 percent. Similar differences existed in
successive annual elongations of given
comparative branches on released and
unreleased trees.
This response was somewhat contrary
to observations of others. Forward and
Nolan (1961b) found that a 30-year-old
red pine tree undergoing an increase in
growing space from 6- to 24-foot spacing
definitely responded to release, branches
in the 5th to 10th whorls showing the
greatest response. No improvement took
place in the uppermost or very lowermost
branches. Based on less detailed studies,
favorable response of branches to release
has also been observed on Douglas-fir
(Ahmed,6 Becking and Laar 1958) and
Japanese larch (Lehtpere 1960). This is
perhaps due in part to differences in
degree of release.
Although branch elongation decreased
on most released trees in the present
study, it has been noted that it did in­
crease on some. Relative to their prere­
lease rates, branches on trees having slow
branch elongation before release elongated
better following release than those on
trees having had more rapid branch
elongation.
Lateral branchlets, due to their relative
abundance, may contribute greatly to
the functional crown surface. Other in­
6See footnote 5.
vestigators have found more twigs and
greater needle weight per tree in thinned
stands (Moller 1947, Savina 1960). De­
velopment of lateral branchlets encom­
passes two factors: (1) Sprouting of new
laterals from dormant buds, and (2)
elongation of laterals. There were numer­
ous sprouts from older portions of bran­
ches, but it is uncertain how much of this
sprouting was due to crown release, as
amount of sprouting varied considerably
among both released and unreleased
trees. Elongation of lateral branchlets
was about equal to or greater than that
of the lowermost main branches. Regres­
sion analyses showed no significant diff­
erences in elongation at various positions
in the crown and no consistent differences
between released and ul11'eleased sides of
a tree could be seen.
Much emphasis has been given to the
increased crown length following thin­
ning that results from longer retention of
lower branches. However, the retention of
these branches without a substantial in­
crease in rate of elongation would appear
to be of very limited value to the economy
of the tree. These branches are growing
much more slowly than those higher in
the crown, and foliage on them is only a
small fraction of the total for the tree.
Relationships Between Crown Expansion
and Stem Radial Growth
Other investigators have observed rela­
tionships between crown diameter and
dbh; between current growth rate and (1)
ratio of crown diameter to dbh, (2) crown
ratio, (3) crown surface area, and (4)
increment in crown surface area; and
between stem growth within the crown
and number and size of branches (A1'110Id,7
Ilvessalo 1950, Campbell,8 Lehtpere
1960).
7 Arnold, Dale L. 1948. Growing space ratio as
related to form and development of western
white pine. Unpublished Master's thesis, Univ.
Idaho. (Abstract in J. For. 47:370.)
8Campbell, Robert K. 1958. Variations in
crown form attributes of Douglas-fir (Pseudo/suga
menziesii). Ph.D. thesis, Univ. Wash.
volume 10, number
2,1964 / 197
•
TABLE 2. Relative changes in radial growth and stem and branch elongation for the period
1951-55 compared with 1949-50.
Radial growth
(Breast height)
Stem elongation
Branch elongation
(Whorl 5)
Change
Released
Unreleased
Released
------------
Increase
Decrease
None
10
1
o
0
3
0
3
22
1
In the current study no consistent
relationships could be seen between rates
of stem radial growth and any of the
above variables. There was a significant
pretreatment relationship between radial
growth and rate of branch elongation,
but the relationship did not continue
following release.
Some trees with increased branch
elongation, including all three unreleased
trees, showed decreased stem radial
growth. On the other hand, several trees
having decreased branch elongation had
increased radial growth. Yet others
showed increases or decreases in both
growth responses.
In most years, radial growth and elon­
gation increased or decreased together
(Fig. 2). However, increases in radial
growth in 1950 and 1954 were associated
with irregular changes in elongation. In
1955 and 1958, the two growth responses
tended to be opposite- one increasing
while the other decreased.
During the 5-year period following
release, stem radial growth tended to be
greater on released than on unreleased
trees, whereas stem and branch elongation
tended to be less (Table 2). Within these
overall differences in growth response of
released and unreleased trees, there were
variations in annual patterns, the major
ones occurring in 1951 and 1953. In 1951,
most released trees showed sharp decrea­
ses in radial growth and stem and branch
elongation; unreleased trees showed lesser
decreases in radial growth and stem elon­
gation and a slight increase in branch
198
/
Forest Science
Unreleased
m
Released
Unreleased
mu ------------
2
1
0
8
19
0
3
0
0
elongation. In 1953, on the other hand,
released trees generally showed increases
in all three, and unreleased trees showed
decreases. Apparently these are differen­
tial responses to climatic stimuli.
Within the confines of this study, de­
gree of release had little apparent effect
on response of the tree. However, this
may be due in part to the difficulty of
defining competition and release. Since
relative radial growth at various heights
on the stem and branch elongation in
different parts of the crown varied from
year to year, the effect of release at
another time might have been different.
Apparently, since branch elongation and
the resulting increase in crown surface
were greater on unreleased trees, im­
proved stem radial growth on released
trees was more dependent upon such
factors as increased availability of light,
moisture, and nutrients than upon crown
buildup.
Literature Cited
BECKING, J. H., and A. VAN LAAR. 1958. De
dunning van Douglas. [Thinning Douglas fir.]
(Abstract.) For. Abstr. 19:4190. (Published in
Dutch in Ned. Bos(ch)B.-Tijdschr. 30(4):
107-117. 1958.)
BEHRE, C. E. 1932. Change in form of red spruce
after logging and of northern white pine after
thinning J. For. 30:805-810.
BICKERSTAFF, A. 1946. Effect of thinning and
pruning upon the form of red pine. Dom. For.
Servo Canada Dept. Mines &Resourcesj Lands,
Parks & Forests Branch. Silv. Res. Note 81.
26 pp .
DUFF, G. H., and N. J. Nolan. 1953. Growth and
morphogenesis in the Canadian forest species.
I. The controls of cambial and apical activity
in Pinus I'csinosa Ait. Canad. J. Bot. 31:
471-513.
DUFFIELD, J. W. 1956. Damage to western
Washington forests from November 1955 cold
wave. Pacif. Nthwest. For. Range Exp. Sta.
Res. Note 129. 5 pp.
FORWARD, D. F., and N. J. NOLAN. 1961a.
Growth and morphogenesis in the Canadian
forest species. IV. Radial growth in branches
and main axis of Pinus I'csinosa Ait. under con­
ditions of open growth, suppression, and
release. Canad. J. Bot. 39:385-409.
and N. J. NOLAN. 1961b. Growth and
morphogenesis in the Canadian forest species.
V. Further studies of wood growth in branches
and main axis of Pinus I'csinosa Ait. under
conditions of open growth, suppression, and
release. Canad. J. Bot. 39:411-436.
FRIESNER, B. c., and J. J. JONES. 1952. Corre­
lation of elongation in primary and secondary
branches of Pinus I'csinosa. Butler Univ. bot.
Stud. 10:119-128.
lLVESSALO, Y. 1950. On the correlation between
the crown diameter and the stem of trees.
Metsatieteellisen Tutkimuslaitoksen Julkaisu.
38(2):1-32.
KRAMER, P. J., and T. T. Kozlowski. 1960.
Physiology of trees. McGraw-Hill Book Co.,
--�, Inc., New York. 642 pp.
LEHTPERE, R. 1960. The crown expansion of
Japanese larch after heavy thinning. Forestry
33:140-148.
McARDLE, R. L., 'V. H. MEYER, and D. BRUCE.
1949. The yield of Douglas fir in the Pacific
Northwest. U.S. Dept. Agric. Tech. Bull. 201.
74 pp.
MEYER, W. H. 1931. Effect of release upon the
form and volume of western yellow pine. J.
For. 29:1127-1133.
MOLLER, C. MAR:. 1947. The effect of thinning,
age, and site on foliage, increment, and loss of
dry matter. J. For. 45:393-404.
ONAKA, F. 1950. The longitudinal distribution of
radial increments in trees. Kyoto Univ. For.
Bull. 18:1-53.
SAVINA, A. V. 1960. [The physiological justifi­
cation for the thinning of forests. ] U.S. Off.
Tech. Servo OTS 60-51130, 91 pp. [Transl.
from Russian. ]
SCHUMACHER, F. R. 1932. Effect of partial cutting
in the virgin stand upon the growth and taper
of western yellow pine. Calif. agric. Exp. Sta.
Bull. 540. 32 pp.
SMITH, D. M. 1962. The practice of silviculture.
Ed. 7. John 'Viley & Sons, Inc., New York.
578 pp.
YERKES, V. P. 1960. Effect of thinning on form of
young-growth Douglas-fir trees. Pacif. Nth­
west.For. Range Exp. Sta. Res. Note194. 5 pp.
volume 10, number
2,1964 / 199