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Spacing Tests in a Douglas-Fir Plantation hI
di
to
pI
BY
s
KENNETH R. EVERSOLE
T
w
2:
1.
to
w:
st
WHAT is the best spacing for Douglas-fir
in plantations? How does spacing affect
stand development and the growth, form,
and' quality of individual trees? These im­
portant questions have been the subject of
study since 1925 at the Wind River Ex­
perimental Forest near Carson, Washing­
ton. Here, plantations were established to
test spacings ranging from 4 x 4 feet up to
12 x 12 feet. Results from these various
spacing tests are discussed in this report.
Some 137,895 acres of plantations have
already been established in the Pacific
Northwest (Oregon and Washington) on
the national forests alone. About 14,000
acres were planted in 1954.
A sizable
investment is repJ:1esented since planting
costs now average about $23 per acre on
the- national forests. Whether the standard
8 x 8 foot spacing gives the best return on
the investment has been the subject of
debate among foresters in the Douglas-fir
region for many years. One faction argues
for closer spacing. This group says close
spacing should be used to shade out lower
branches and ground cover. The defenders
of wide spacing say that close spacing entails
prohibitive planting costs and results in snow
break and reduced growth of the crop trees.
These arguments are of practical impor­
tance. The forest manager needs informa­
tion from actual spacing tests, such as those
at Wind River, in reaching a decision.
14 j. Forest Science
The Experiment
In April 1925 the Pacific Northwest Fores
and Range Experiment Station establishe
a 14.5-acre plantation on a twice-burne
Site IV (site index 1 10 feet) area at a
elevation of 1,350 feet. The plantatio
was divided into six blocks to test spacin
from 4 x 4 up to 12 x 12 feet. The 1-1
planting stock was grown .in the Wind
River Nursery from seed collected near
Roy, Washington. During the first 3 year
survival checks were made annually, and
all dead seedlings were replaced. The
plantations were remeasured at approxi
mately 5-year intervals thereafter.
The fall 195 1 measuremen ts were take
on one 0 A-acre plot in the 12 x 12 fa
spacing and on three 0.25-acre plots i
each of the other spacings. Diameters of a
trees over 1.5 inches were measured with
diameter tape Heights of 40 trees in eac
plot were measured with Abney level a
chain. A height-diameter curve was co '
structed. fot; each spacing to use in estimat
The author is Forester on the Gifford Pinch
National Forest. When he made this study,
was with, the Pacific Northwest Forest an
Range Experiment Station, Forest Servi
U. S. Dept. Agnc. He wishes to acknowled
the assistance of Leo A. Isaac, who was
charge of the experiment from its beginnin
trt
fOI
pr j
till
4
to
th;
Tt
Spa
(fe
­
5
6
8
10
12
Nal
Ta
Spa(
(fel
­
5
6
8
10
12
natic
ing volumes. On each plot the diameter of
the tree with average basal area (dominant
and codominant stand) was calculated. The
height of this tree was read from the height­
diameter curve for each spacing and used
to estimate site' index. The time since
planting was used as the age.
n
most volume of large products (Fig. 1)
The total-stem cubic volume in the close
spacings is distributed on many small stems.
Many of these will soon die. Statistics from
a nearby natural stand are similar to the
8 x 8 spacing.
•
Growth and mortality.
Periodic annual
growth and mortality (Table 2) show how
the plantation has changed during the most
recent remeasurement period. The variation
in total-stem cubic volume growth does not
appear to be associated with spacing at this
' a ge. Judging by evidence on the ground,
the variation is apparently associated with
slight differences in site quality. The trend'
of board-foot volume growth is exaggerated
by the heavier ingrowth in the wide spac­
ings. Even so, the wide spacings are well
ahead in board-foot production. Natural
thinning is beginning in the 4 x 4-foot
spacing, but the death of suppressed trees
will probably do little toward releasing the
crop trees.
Stand Development
Several trends associated
The live stand.
with spacing are evident in statistics for the
27-year-old plantation (Table 1). In trees
1.5 inches d.b.h. and larger, basal area and
. total-stem cubic volume decreased with
wider spacing. On the other hand, in the
stand 6.6 inches d.b.h. and larger, this
trend was reversed in basal area and board­
foot volume. Indeed, the 6,070 board feet
produced by the 12 x 12 spacing is many
times greater than the 268 board feet in the
­ x 4 spacing. Ingrowth soon will tend
. to level off this difference, but it is evident
that the wide spacings have produced the
st Fore
tablishe
-burne
a at a
[antatio
spacm
Table ·1. 1951 statistics of the live stand for each spacing) acre basis
rhe I­
e Win
Spacing
(feet)
Stand 1.5 inches and over
Number.
of trees
Basal area
(sq. ft.)
Volume
(cu.ft.)
­ X 45 X 5
1,830
132.0
1,4-80
112.8
2 320
)
1,890
6
1,030
103.8
1,870
580
78.6
405
91.2
.287
80.5
75.5
X 6
8 x 8
10 X 10
12 X 12
Natura12
598
Number
of trees
Stand 6.6 inches and over
Volume
(eu.ft.)
Basal area
(sq. ft.)
Volume!
(bd. ft.)
8
2.4-
61
6
1.5
34
137
8.6
191
753
'1,470
30
67
19.8
425
1,950
166
57.7
1,282
5,500
1,740
176
65.4-
1,4-60
6,070
1,44-0
98
35.7
778
3,190
268
1,690.
lInternational liS-inch rule.
2Data from a nearby natural stand of the same age.
T able 2. Periodic annual growth and mortality) 1945-1951) acre basis
Spacing
(feet)
-
Pinch
udy, h'
'est an::l
Service
owledge
.
.
was l
rinning:
)
'
t
Basal area
(sq. ft.)
Annual growth
(cu. ft.)
Volume!
(bd. ft.)
Number
of trees
Annual mortality
Basal area
(sq. ft.)
Volume!
(cu. ft.)
4- X 4
5 X 5
6.15
6.26
176.8
45
24-.7
0.84
156.4-
23
5.7
0.15
1.6
6
5:93
157.1
126
5.0
0.17
2.2
x 6
8 X 8
10
12
11.6
5.4-0
138.4-
198
1.0
0.02
0.2
X 10
6.87
193.6
820
0.7
0.03
0.4-
X 12
5.85
167.7
853
1.0
0.13
.2.3
---
lCubie volume for trees 1.5 inches and over i
nation al 1 IS-inch rule.
board-foot volume for trees 6.6 inches and over,
vol.
1,
no.
by Inter­
1, 1955 / 15
Height
domina?
dence t:
th e ave
domina:
w as as
In ge
greater
nant a:
spacin&
spacin§
Fig. 1. Effect at spacing on tree size in a 27-year-old plantation at Douglas-fir. Trees are
same age and have grown under approximately the same conditions in both spacings. (Left) 4 x
foot spacing where trees average 3.6 inches in diamCiter and 33.7 fu't in height. (Right) 12 x 1
foot spacmg, trees average 7.2 1;'11Jches in diameter and 46.2 feet in height.
Normality.
The effect of spacing on tree
height has affected site determination sig­
nificantly. Therefore, in computing stand
normality for the various spacings, a site
index of 1 10 feet, from the nearby natural
stand, was used. Normality for the differ­
ent spacings computed for the conventional
measures was as tabulated below.
Number
of trees
Spacing
Cubic
volume
Percent Percent
4-
5
6
8
10
12
X
X
X
X
X
X
45
6
8
10
12
9476
53
30
21
15
125
102
101
80
106
9­
Board - foot volume
(Interna'tionaZ
1/8-inch rule)
Percent
16
8
4-5
100
327
361
Even though the wide spacings were far
below normal in number of stems at age 27
years, they were near normal in total-stem
cubic volume and, far above normal in
board-foot volume.
16
/
ForelSt Science
The Average Tree
H eight and diameter.
erage tree increased with wider sp
(Fig. 2). The average tree in the 12­
12-foot spacing was twice the diameter
the average tree in the ­ x ­ spacing and'
was 12.5 feet taller'­ When the plantation .
was only 5 years old, the trees in the ­ x
..
and 5 x 5 spacings were taller than
in the wider spacings.1 By the time the were 10 years old this trend was and the difference in height has
steadily ever since. Average diameter
followed a similar trend. During the
recent growth period, average diameter
the 4- x 4- spacing increased only 0.6
During the same period, average
in the 12 x 12 spacing increased 1.9
HE\(
( feE
4
1Isaac, Leo A. 1937. 1 0 years' growth
Douglas-fir spacing-test plantations. Pacif.
west For. Range Exp. Sta. Res. Note No. 23,
OJ
(incl
'way for these two plantings, the difference
in predicted yields in a laO-year r<?tation is
about 19,000 board feet, Scribner volume.
Height of the average dominant and co­
dominant. The spacing test provides evi­
dence that density has a marked effect on
the average height of dominant and co­
dominant trees. After 27 years the trend
was as follows:
Height o f t1fVerage
dominant and codominrmt
(feet)
Spacing
(feet)
4- X 4-
38. 1
5 X 5
35.2 6 X 6
38.7 8 X 8
39.9
10 X
10
4-6.3 12 X
12
4-8.4-
In general, wider spacing resulted in greater height growth. The average domi­
nant and codominant in the 12- x 12-foot spacing was 10 feet taller than in the 4 x 4 spacing. If site is calculated in the accepted This wide difference in indicated pro­
ductivity does not seem reasonable for a
planting area that is fairly uniform.· The
analysis casts doubt on the use of the height
of the average dominant and codominant
tree as a true index of site quality in
young-growth stands.
The codominants,
at least, are strongly affected by spacing.
There is an almost step-like break in the
codominant crown level from the 4 x 4 spacing into the lO x 10 spacing. This break is very apparent when looking from an elevated vantage point in the 4 x 4 plantation. Would the heights of the tallest domi­
nants alone be a better measure of site
46.2 447 35.0
10 D.B.H.
(inches)
SPACING
(feet)
o.
2
4x4
5x5
6x6
8x8
IOx lO
12xl2
Natural
Fig. 2. Height and diameter of the average tree in each spacing at age 27 years.
vot.
1,
no.
1, 1955 /
17 quality? That they may be is indicated by
calculating site index2 from the average
heights of the 6 tallest dominants on adja­
cent plots in the 4 x 4 spacing and the
10 x 10 spacing:
Spacing
(feet)
4 x 4
10 x 10
Site index using
height of average
domincmt and co­
dominant in 1951
(feet)
Site index using
average height of
6 tallest dominants
in 1953
(feet)
100
120
125
137
Site index calCulated from the average
height of the 6 tallest dominants on a 0.25­
acre plot is apparently less affected by
spacing than site index measured in the
usual way.
Limb size and height to live crown. The
effect of spacing on limb size is often
stressed.3 In 195 1 40 trees in each spac­
ing were measured for limb size and height
to live crown. Limb size was taken as the
average of the 2 largest limbs in the whorl
above and the whorl below d.b.h., mea­
sured 1 inch from the trunk. A tabulation
of results follows: Spacing
(feet)
4
5
6
8
10
12
X
X
X
X
X
X
4
5
6
8
10
12
Limb size
(inch )
0.36
.43
,48 .54
.73
.73
H eight
to live crown
(feet)
19.5
13.7 10.7
8.9
8,4
6.7
An increase in limb size with wider spac­
ing is apparent, but even the large limbs
in the 12- x 12-foot spacing averaged less
than three-fourths inch. These limbs are
dead and will not make fur-ther growth.
2Staebler, George R. 1948. Use of dominant
tree heights in determining site index for
Douglas-fir. Paci£. Nthwest For. Range Exp.
Sta. Res. Note No. 44. 3M unger, T. T. 1946. The spacing in plan­
tations. Pacific Nthwest For. Range Exp. Sta.
Res. Note No. 34.
18 / Forcut Sc ence
The limbs in the close spacings are sma
but even in the 4 x 4 spacing little de
wood will be produced in a 100-year rot
tion without artificial pruning.4
Conclusions
Records from the 27-year-old spacin
test plantation of Douglas-fir indicate tha
1. Wide spacmg m plantations h
little effect on total-stem cubic volume i
the first 27 years, but increases board-fo
volume greatly.
2. Average diameter is greatly increa se
by wide spacing.
3. Wide spacing produces a taller dam
inant and codominant stand (and a high
site index when measured in the conve
tional way).
4. Average limb size is increased
wide spacing.
The findings should prove useful to t
forest manager in deciding what spacing
use under a given set of conditions and o.
jectives. His problem is to balance plantin
costs against estimated future retur
Planting costs per tree will be nearly t
same regardless of spacing. Therefore, t
wider the spacing (up to 12 x 12 feet 0
Site IV land), the lower the initial cost a
the sooner material of sawlog size will
produced. Since, even in close spacin
pruning will be necessary to produce cle
lumber in 80 - 100-year rotations, it may
best .to keep planting costs low by spacin
trees as far apart as 10 feet, with the e
pectation of pruning at an early age. If
12-foot spacing is used, careful plantin
and early replacements may be necessary
avoid large openings. In contrast, whe
very early thinnings for small pole-si
material are anticipated, closer spacin
would be desirable.
4Kachin, Theodore. 1940. Natural pruni
in second-growth Douglas-fir. Paci£. Nthwe
For. Range Exp. Sta. Res. Note No. 31.
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