Crown Development and Site Estimates in a
Douglas-Fir Plantation Spacing Test
ROBERT O. CURTIS
DONALD L. REUKEMA
Abstract. Relationships among stem and crown dimensions of Douglas-fir were examined 43
years after planting on site IV land at initial spacings of 4 X 4 through 12 X 12 feet. Average
dbh, height, and crown dimensions of the largest trees (largest 20 percent or 100 trees per
acre by dbh) and of comparable crown classes all increased consistently with increase in initial
spacing. Trees of similar dbh or total height were currently quite similar in crown dimensions,
although they had arrived at this condition by somewhat different routes. Striking differences
among spacings in apparent site indices are attributed mainly to restriction of height growth by
competition rather than to real site differences. Average heights of several stand components
were compared as bases for site index estimates; heights of a fixed number of the largest
diam eter trees were most nearly consistent among spacings, although no procedure eliminated
spacing effects. Comparisons suggest possible usefulness of live-crown length as one criterion
for acceptable site trees in stands of abnormal density. High initial density in low-site stands
can lead to serious underestimates of potential productivity. Forest Sci. 16:287-301.
Additional key words. Pseudotsuga menziesii, site index.
IN 1925 a Douglas-fir plantation spacing test was
established on site IV land on the Wind River
Experimental Forest near Carson, Washington.
Following stand closure, diameter and height
growth and volume growth per acre have all
increased from close to wide initial spacings
(Isaac 1937; Munger 1946; Eversole 1955;
Reukema 1959; Reukemal). Differences have
increased with advancing age. Unlike many
spacing tests in other species, initial spacing
appeared to strongly affect height growth of
dominant and codominant trees. The area has
been considered quite uniform in site quality,
and increasing differences in heights and in
estimated site indices based on these heights
have been viewed as effects of spacing. Today,
these plantations of equal age and believed
equivalent site present striking contrasts in tree
size, apparent vigor, and crown development.
Crown and stem dimensions were measured in 1967 on a sample of trees in each
1
Reukema, Donald L. Forty-year development of Douglasfir stands planted at various spacings. (In preparation for
publication, Pacific Northwest Forest & Range Exp. Sta.,
Portland, Oregon.)
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2
As used in this paper, “mean tree” is the tree of quadratic
mean diameter of a specified stand component. Quadratic mean
diameter = diameter of tree of mean basal area (Curtis 1968).
The authors are, respectively, Principal Mensurationist and
Silviculturist, Pacific Northwest Forest and Range Exp. Sta.,
Forest Service, U. S. Dep. of Agr., Portland, Oreg. Manuscript
received Sept. 22, 1969.
volume 16, number 3, 1970 /
Reprinted from F OREST SCIENCE, Volume 16, Number 3, September, 1970
Purchased by the USDA Forest Service for official use.
287
of the experiment, volume production
comparisons, and a discussion of stand
development will be given in a subsequent
paper (see footnote 1).
The Study
The Study Area. The spacing test plantations
occupy a site IV alluvial flat at an elevation of
about 1,350 ft. The soil is a loose sandy loam
with sporadic admixture of basaltic gravel
and cobble. This juvenile soil is developing
on pumiceous alluvium, generally 4 to 7
ft deep and underlain by lightly fractured,
basaltic rock.3 Felled timber on the area was
accidentally burned in 1920, following which
all usable material was salvaged. In 1924 the
area was reburned by a very intense fire that
destroyed all reproduction and burned all
duff and debris down to mineral soil.
In the spring of 1925, 1–1 stock was
planted at spacings from 4 X 4 through 12 X
12 ft, with different spacings contiguous but
without replication. Subsequently, three ¼acre plots were established in each 2.8-acre
plantation, excepting the smaller 12 X 12
plantation which contains a single 0.4-acre
plot. Recent measurements were confined to
these plots.
Basic Data. Stand tables were available
from the 1965 remeasurement. Height and
crown dimensions were measured in 1967
for a sample of 214 undamaged trees—
approximately 36 trees per spacing, equally
distributed among plots, and across the range
of diameters present on each plot. Values
measured or calculated included:
1. Dbh outside bark (D).
2. Total height (H
H).
H).
3. Height to base of live crown (HLC),
defined as the lowest whorl with live
branches in at least three quadrants,
exclusive of epicormic branches and whorls
not continuous with the main crown. For a
few trees with very lopsided crowns, heights
to each half of the crown were measured and
averaged.
3
Personal communication with Richard E. Miller, Soil Scientist,
Pacific Northwest Forest and Range Exp. Sta., Olympia, Wash.
288
/ Forest Science
4. Average crown width (C
CW),
W), defined
defined
as twice the mean length of the longest
live branch in each of eight consecutive
45° sectors. Length was determined as the
measured distance from center of stem to
vertical projection of branch tip.
5. Crown length (CL), calculated as the
difference, H — HLC.
5. Volume of crown, considered as a
paraboloid.
7. Surface area of crown, considered as a
paraboloid.
Measurements of height and crown dimensions in 1945, based on a different
sample, were also available, as well as some
data on height and crown length taken in
1951.
Analysis. The general plan of analysis was:
1. Calculate regressions of total height on
dbh and of crown dimensions on dbh and on
total height, for spacings and for individual
plots.
2. Compare regressions for possible
differences among
differences
among spacings.
spacings.
3. Use the regressions for each spacing
to calculate stem and crown dimensions of
a number of possible mean trees, including
(a) average of all trees, (b) average of each
crown class, (c) average of dominants and
codominants, (d) average of the 100 largest
diameter- trees per acre, and (e) average of
the largest 20 percent by diameter.
4. Compare heights of mean trees specified by alternative site tree selection rules for
consistency among spacings and apparent
reasonableness as bases for estimation of
site index.
Comparisons of regressions were based
on both inspection of curves and on analysis
of covariance (Table 1). In this analysis,
the “among spacings ” F-value tests the
hypothesis of no difference in regressions
among locations occupied by plantations
of different initial spacings. Since the
experiment lacked replication, spacing is
confounded with location. Interpretation
of a “significant difference” and associated
trend in the curves as “caused by spacing ”
is a subjective judgment, dependent on
the untestable but apparently reasonable
assumption of homogeneity of site across
the range of spacings. T h e a r e a h a d
b e e n chosen for its apparent homogeneity,
and subsequent examinations of soil and
topography have given no indication of
any gradient across spacings, although
there is some indication of minor local site
differences irregularly distributed within the
area.
Regression models used for tree stem and
crown dimension relationships were based on
inspection of scatter diagrams and arbitrary
choice of functions which appeared to fit
the data. Subsequent plotting and trial of a
number of alternative equations indicated
that most of these relationships could have
been satisfactorily represented by the wellknown “allometric” equation, y = axb, often
observed to hold for the dimensions of
growing organisms.
Results and Discussion
Average tree dimensions and stand
productivity differ among plots within spacings; presumably these differences result
from some combination of localized variations in site quality and in past injuries
influencing tree and stand development.
Heavy mortality following initial planting,
amounting to 36 percent in the first season,
may have contributed to variation among
plots. Dead trees were replaced each year
for 5 years after initial planting, but location
of replacements is unknown. Numbers of
trees have been substantially reduced by
subsequent mortality.
Relationships which differ significantly
(Table 2) within spacings have evidently
been influenced by such location effects.
Relationships which differ among but not
within spacings almost certainly reflect
true spacing effects. Differences both
with and among spacings indicate probable confounding of spacing and location
effects.
Height Growth and Height-Diameter Curves
When first measured, 5 years after establishment, average heights of all trees were
greatest in the 4 X 4 and 5 X 5 spacings (Isaac
1937). At the 10-year remeasurement, this
difference had disappeared; and since then,
average heights (both all trees and largest 100
(by dbh) per acre) have been greater at the
wider spacings, with differences consistent
and steadily increasing with age.
Differences among spacings in the heightdiameter curves have always been rather
slight, indicating that the influence of stand
density on height growth has been roughly
proportional to its influence on diameter
growth. Despite this similarity, there has
been a clear trend of change over time.
In 1945, when crowns were just closing
in the wider spacings, differences in heightdiameter curves were small except for the 4
X 4 spacing (Fig. 1). The greater elevation
of the curves for close spacings probably
reflects initial restriction of diameter growth
following stand closure, which occurred first
in the 4 X 4. Contrary to the belief that height
growth is less affected by competition than
is diameter growth, the subsequent upward
displacement of these curves was greatest at
the wide spacings. By 1967, a quite different
and fairly consistent pattern with respect to
spacing had developed, with heights for a
given dbh increasing slightly with spacing
(Fig.2).
When height-diameter curves for plots
within each spacing are compared, the
volume 16, number 3, 1970 / 289
higher curves are associated with greater
stand diameter and greater height of
dominants and codominants, suggesting
that position of the curves is affected by
minor site differences and possibly by
mortality differences as well as by initial
spacing. Average curves for the 5 X 5 and
8 X 8 spacings have always been slightly
lower and those for the 4 X 4 and 10 X 10
slightly higher than might be expected
by comparison with the others, and these
differences are consistent with differences
in stand diameter and other measures.
290
/ Forest Science
Crown-Stem Relationships
Regressions of 1967 crown dimensions on
tree diameter and height show that trees of
given dbh or total height but in different
spacings are quite similar in other stem and
crown dimensions. Although crown widths
for trees of given dbh increased with spacing,
no clear and consistent trends of the curves
of crown length, live-crown ratio, or height
to live crown are evident.
Crown Width. Since stand density expressions are frequently based on the crown
volume 16, number 3, 1970 /
291
width-diameter relationship, the finding
that among spacing differences in crown
width-diameter regressions are significant
292
/ Forest Science
while those within spacings are not,
isexpected. Figure 3A compares spacing
regressions calculated under the assumption
of common slope, since differences in slope
were not significant but those in elevation
were. Differences in crown width for trees
of given dbh are not large, but the increase in
crown width with spacing is consistent and
undoubtedly a real effect of spacing.
In the crown width-total height regressions, the 12 X 12 had considerably
greater crown widths than other spacings,
probably reflecting more recent stand closure.
Otherwise, there was little indication of any
trend with spacing.
Height to Live Crown. The 1967 regressions
of height to live crown on dbh differed
significantly within spacings but not among
spacings. Curves for individual plots within
each spacing were quite variable and, like
the height-diameter curves, those of higher
relative position appeared to be associated
with greater stand average diameters and
heights. Heights to live crown of stand mean
trees were slightly greater in wider spacings,
due to greater mean tree diameters.
Although data do not permit calculation of
comparable curves for earlier measurements,
relationships among spacings have differed
substantially in the past. In 1945 live crowns
were just beginning to recede in the closer
spacings, and by 1952 average heights to
live crown ranged from 6.7 ft in the 12 X
12 spacing to 19.5 ft in the 4 X 4. Following
stand closure, live crowns receded rapidly in
the wider spacings also.
Crown Length. Although corresponding
differences in crown lengths among spac-
ings existed during the period of stand closure,
differences among spacings in crown lengths
of trees of similar dbh or total height had
largely disappeared by 1967, except in the 12
X 12. Differences in regression slopes were
not significant, and significance of differences
in elevation is due almost entirely to the 12
X 12 spacing (Fig. 3B) and reflects its more
recent stand closure.
Differences among spacing regressions of
crown length and of live crown ratio on total
height were not significant, although nearly
so; and there was no indication of any trend
with spacing aside from the greater elevation
of the 12 X 12 curve.
Crown Volume and Surface Area. Regressions
of these values on dbh differed significantly
among spacings but not among plots within
spacings. The corresponding curves showed
clear trends, with estimated values for a given
dbh increasing with spacing in a manner
consistent with previously discussed trends in
crown widths and crown lengths. Stand totals
of crown volume and crown surface area were
obtained by applying these regressions to
the 1965 stand tables (Table 3). Total crown
volume increased as spacing increased from 4
X 4 through 12 X 12 ft, consistent with trends
in volume growth noted earlier. If suppressed
trees are excluded from the comparison, a
similar trend also exists with crown surface
area.
Comparisons of Crown Classes Among Spacings
Average trees of a given crown class but
in different spacings differ widely in both
volume 16, number 3, 1970 /
293
294
/ Forest Science
volume 16, number 3, 1970 /
295
crown development and height (Fig. 4).
Trees of crown development similar to
dominants in the closest spacings are
296
/ Forest Science
assigned to progressively lower crown
classes with increasing spacing but are
quite similar in dbh and total height.
This is consistent with Baskerville’s (1965)
observation that in balsam fir stands of equal
age, widely varying density, and assumed
equivalent site, dominant trees of given
dbh in dense stands appeared identical
in other dimensions to intermediate and
suppressed trees of the same dbh in low
density stands.
“Crown class” has been defined (Soc.
Amer. Foresters 1958) as “A designation
of trees in a forest with crowns of similar
development and occupying similar povolume 16, number 3, 1970 /
297
sitions in the crown cover.” But, in these
stands of widely differing density, trees of
similar crown development do not occupy
similar positions in the crown cover. Since
classification is on a relative scale peculiar
to the individual stand, trees of the same
crown class but growing under different
stand conditions do not in general represent
equivalent conditions of past or present
competition.
Comparisons of Stand Mean Trees by Spacings
Stands are often compared in terms of some
“mean tree,” most commonly the average
(quadratic mean dbh) of all trees above a
fixed minimum diameter (1.5+ inches in
this study). Other frequently used mean trees
are average of dominants and codominants,
average of the 100 largest diameter trees per
acre, and average of the largest 20 percent
by diameter.
In these plantations, all the above mean
trees show pronounced differences among
spacings in both stem dimensions and crown
development (Fig. 5).
298
/ Forest Science
Site Index Estimates and Competition Effects
Site index estimation procedures are commonly based on the belief that average height
of the larger trees in a stand is little affected
by differences in density and associated
competition, although exceptions have been
observed (Lynch 1958, Holmes and Tackle
1962, Collins 1967). Such an assumption is
clearly untrue in these plantations.
Site estimates for Douglas-fir have long
been based on average height of dominants
and codominants (McArdle et al. 1961). In
these plantations, estimates of site index by
this procedure range from about 80 for the
4 X 4 and 5 X 5 spacings to 120 for the 10
X 10 and 12 X 12 (Table 4). Neither early
height measurements nor topography nor
any known soil characteristics suggest the
existence of real site differences of anywhere
near this magnitude. Differences in apparent
site index among spacings are attributed
principally to effects of differing intensity
of competition on height growth of trees
classed as dominants and codominants.
Site index estimates can also be based on
average heights of other stand components,
including dominants only, the largest
20 percent by dbh (King 1966), and a
fixed number of the largest trees per acre.
Although the optimum procedure cannot be
determined from comparisons for a single
site condition, sensitivity to stand density
effects is clearly an undesirable attribute.
A satisfactory procedure should give
approximately equal average heights and
corresponding site index estimates for each
spacing in these plantations of equal age,
widely differing density, and approximately
equal site. From this standpoint, alternative
procedures can be ranked on the basis of
relative variability of site estimates among
spacings, as expressed by the coefficient of
variation of mean site tree heights.
Such a computation gave for the 1967
data:
Mean tree
Average of all trees 1.5+
inches
Average of largest 20 percent
Average of dominants and
codominants
Average of dominants
Average of largest 100/acre
bbyy ddbh
bh
Average of largest 40/acre1
by dbh
Coefficient
of
variation
0.24
.20
.19
.18
.16
.16
.16
“Top height” as currently used in Great Britain (Bradley et al. 1966) and
elsewhere in Europe.
1
Although none of the above rules eliminates the trend of average site tree height
with spacing, a fixed number4 of the
largest trees per acre gives the most nearly
consistent estimates.
For the averages of trees selected by each
of these rules, there is a pronounced trend
in relation to spacing of not only total
height but also dbh and both absolute and
relative crown dimensions (Fig. 5). Both
the average of dominants and co-dominants
and the average dominant tree in the 4 X 4
spacing are markedly inferior in both stem
dimensions and crown development to the
same crown classes in the 12 X 12 (Fig. 4).
Differences are even greater for the largest
20 percent, which consists of 285 trees per
acre in the 4 X 4 spacing, including many
codominants, compared with 53 trees in the
12 X 12 consisting of the larger dominants
only (Table 5). The basic difficulty is that
these rules classify trees on scales which
vary with stand density and are relative to
the existing stand rather than to any standard
condition. Clearly, these rules do not choose
trees of comparable development in these
stands of widely differing density, and they
cannot give comparable estimates of site
index under the conditions existing in these
plantations.
Averages used in this study were based on the N
largest trees per acre on each plot, without regard to
location. An average based on the single largest tree
on each of a series of 1/
1/N
/N acre subplots would be
somewhat more representative. Although a comparison
by Reukema (see footnote 1) indicated little difference
in corresponding diameters for these plots, differences
between the two methods could be important when
applied to larger and more variable areas.
4
volume 16, number 3, 1970 /
299
Fully consistent site estimates under
conditions comparable to these plantations
would require either (1) a site tree selection
rule choosing trees subject to equivalent
cumulative competition effects in stands
of different density (if such trees exist) or
(2) some means of adjusting conventional
site index estimates for density effects.
MacKinney et al. (1937), Lynch (1958),
and Alexander et al. (1967) have made such
density adjustments in other species, and if
such effects are indeed widespread in young
Douglas-Fir, similar procedures might be
applicable.
In stands of comparable density, such as
those used as the basis for McArdle et al,’s
site curves, crown development and relative
position of trees within the stand (crown class)
are closely associated. This is not the case for
trees of similar relative position growing in
stands of widely differing density. In these
plantations, crown development of trees
of similar relative position but in different
stands, like development in diameter and in
height, has been progressively reduced with
increasing density, reflecting the cumulative
effects of differences in competition.
Competition effects on crown development and on stem development have been
associated since crown closure. At present,
the regression estimates of heights corresponding to either a dbh or crown length
equal to that of the largest trees present in
the closest spacing are quite similar for all
spacings, though not identical.
Thissuggeststhatcriteriaforselectionofsite
trees including some standard of acceptable
crown development in addition to relative
position in the stand might reduce errors in
site index estimates arising from differences
in stand density. Or, that abnormal height
development due to cumulative competition
effects should be associated with deviations
fromthenormalpattern of crown development,
and that the latter might be used in a manner
analogous to Lynch’s or Alexander’s
procedures for adjusting site estimates for
differences in stand density. In either case,
ease of measurement suggests expression
of crown development in terms of crown
300
/ Forest Science
length and total height rather than the crown
area-diameter relationship often used as
the basis for stand density measures. These
possibilities emphasize the desirability of
systematic crown measurements in spacing
and thinning studies in young stands.
These plantations have both (1) relatively
poor site and (2) uniformity of age and
spacing characteristic of plantations. Stagnation is often thought to be associated
with poor site, and crown differentiation
is slower and less pronounced in uniform
plantations than in many natural stands,
which may accentuate this tendency. It is
not definitely known whether similar effects
of competition on average heights and on
site index estimates exist over the range
of sites present in the Douglas-fir region
nor whether manipulation of stand density
after initial stand establishment has similar
effects. It does seem clear that competition
effects on height growth and on site index
estimates are important in poor site stands of
high initial density, and could lead to serious
underestimates of the potential productivity
of such lands.
Literature Cited
ALEXANDER, R
R.. R
R.,
., D
D.. TACKLE, and W. G. DAHMS. 1967.
Site indexes for lodgepole pine, with corrections
for stand density; methodology. USDA Forest
Serv. Res Pap RM-29, 18 p. Rocky Mountain
Forest Range Exp Sta, Ft. Collins, Colo.
BASKERVILLE, G. L. 1965. Dry-matter production in
immature balsam fir stands. Forest Sci Monogr 9,
42 p.
BRADLEY, R. T., J. M. CHRISTIE, and D. R. JOHNSTON.
1966. Forest management tables. Great Brit Forest
Comm Booklet 16, 219 p.
COLLINS, A. B., III. 1967. Density and height growth
in natural slash pine. USDA Forest Sery Res Pap
SE-27, 8 p. Southeast Forest Exp Sta, Asheville,
N.C.
CURTIS, R. O. 1968. Which average diameter? J
Forest 66:570.
EVERSOLE, K. R. 1955. Spacing tests in a Douglas-fir
plantation. Forest Sci 1:14-18.
HOLMES, JJ.. R
R.. B
B.,
., aand
nd D
D.. TACKLE. 1962. Height growth
of lodgepole pine in Montana related to soil and
stand factors. Mont State Univ Bull 21, 12 p.
ISAAC, L. A. 1937. 10 years’ growth of Douglas-fir
spacing-test plantations. USDA Forest Serv Pacific
Northwest Forest Range Exp Sta Forest Res Notes
23:6. Portland, Oreg.
KING, J. E. 1966. Site index curves for Douglas-fir in
the Pacific Northwest. Weyerhaeuser Forest Pap 8, 49
p.
LYNCH, D. W. 1958. Effects of stocking on site
measurement and yield of second-growth ponderosa
pine in the Inland Empire. USDA Forest Serv
Intermountain Forest Range Exp Sta Res Pap 56, 36
p. Ogden, Utah.
MCARDLE, R. E., W. H. MEYER, and D. BRUCE. 1961.
The yield of Douglas-fir in the Pacific Northwest. U S
Dep Agr Tech Bull 201 (rev.), 74 p.
MACKINNEY, A. L., F. X. SCHUMACHER, and L. E.
CHAIKEN. 1937. Construction of yield tables for
nonnormal loblolly pine stands. J Agr Res 54:531545.
MUNGER, T. T. 1946. The spacing in plantations. USDA
Forest Serv Pacific Northwest Forest Range Exp Sta
Forest Res Notes 34:3-4. Portland, Oreg.
REUKEMA, D. L. 1959. Some recent developments in
the Wind River Douglas-fir plantation spacing test.
USDA Forest Serv Pacific Northwest Forest Range
Exp Sta Res Note 167, 7 p. Portland, Oreg.
SOCIETY OF AMERICAN FORESTERS. 1958. Forestry
terminology. Ed 3, 97 p.
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301