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Rotation-Age Results from a Loblolly Pine Spacing
Trial
Ralph L. Amateis and Harold E. Burkhart
This study reports cubic-foot volume yields for particular product definitions from a 25-year-old loblolly pine spacing trial and shows how closely, in the absence
of thinning, total and merchantable wood production are linked to initial spacing. Results at the close of the study indicate that (1) high-density plantations
can be managed on short rotations for woody biomass production; (2) pulpwood yields can be maximized at a planting density in the neighborhood of 680
trees/ac; (3) the production of solidwood products, without imposing thinning, requires lower establishment densities, with as few as 300 trees/ac planted
resulting in a substantial proportion of the total yield recovered as large sawtimber; and (4) a ratio of between-row to within-row planting distances of at least
3:1 does not substantially affect yield production. Considered together, the results of this study suggest that no single planting density is optimal for the wide
array of product objectives for which loblolly pine is managed in the South. Rather, managers must select an appropriate planting density in view of the products
anticipated at harvest.
Keywords: growth and yield, density, spacing, Pinus taeda L.
Few decisions have a greater impact on the growth and devel-
opment of loblolly pine plantations than how many trees are
planted per acre. Managers know that planting density will
affect the quantity and quality of wood harvested at rotation, as well
as the type and timing of intermediate silvicultural treatments.
Given the importance of initial spacing on the growth and de-
velopment of forest stands, spacing trials have been established for
many tree species. Evert (1971) published a comprehensive review
of many spacing studies established where plantation forestry is
practiced. He noted that results from many of these studies were
limited because of inadequacies in the definition of study objectives,
the experimental design, the longevity of the study, or the measure-
ments collected. For loblolly pine, two of the better known studies
with at least 25 years of history are the Hawaii spacing trial on the
island of Maui and the Calhoun Experimental Forest trial in South
Carolina (Harms et al. 1994).
In an effort to increase understanding of how loblolly pine plan-
tations grow in the southern United States, a set of loblolly pine
spacing trials was established at four sites in Virginia and North
Carolina in the spring of 1983. The primary goals for the study were
to (1) evaluate the effects of spacing and density on the growth,
development, and survival of loblolly pine trees; (2) provide data for
modeling growth and yield relationships; and (3) determine the
optimal (in a biological or growth and yield sense) planting densities
for particular product objectives. This report presents results related
to goal 3 of the study. Yield in relation to four definitions of stand
volume was analyzed, namely stand volume and volume of all trees
above a specified threshold diameter limit for pulpwood, chip-and-
saw, and sawtimber utilization.
The Study
Design and Field Procedures
The experimental design for the study was the nonsystematic
design presented by Lin and Morse (1975) in which plots of differ-
ent sizes and shapes containing equal numbers of trees fit together to
form a compact block (Figure 1). Applying this design, a spacing
factor (F) of 4 ft was chosen, and four levels of that factor (1F, 1.5F,
2F, and 3F) were selected and randomly assigned to row and column
positions on a two-dimensional grid. The intersection of the row
and column factors defined 16 plots, each with a specific spacing and
density. The factorial arrangement of 16 plots, each with seven rows
and seven trees within each row, made up a compact block of about
2.5 ac, including guard trees (Figure 1). Each block contained 4
square plots (4 ⫻4,6⫻6,8⫻8, and 12 ⫻12 ft) and 12
rectangular plots (4 ⫻6, 4 ⫻8, 4 ⫻12, 6 ⫻4, 6 ⫻8, 6 ⫻12, 8 ⫻
4,8⫻6,8⫻12, 12 ⫻4, 12 ⫻6, and 12 ⫻8 ft). Thus, each
rectangular plot had a companion plot that was the same spacing
and density but shifted 90 degrees with regard to the row and col-
umn spacing (e.g., 4 ⫻12 ft and 12 ⫻4 ft have the same spacing
and density, but the row direction of one is the column direction of
the other). Additional details of the experimental design as applied
to this study can be found in Amateis et al. (1988); Burkhart (2002)
provides an overview of design options for spacing trials.
Four sites were selected, two in the Piedmont and two in the
Coastal Plain (Table 1). All sites were cutover areas that had received
mechanical site preparation and burning treatments following har-
vest. Three blocks were established at each site. In most cases, blocks
at a site were contiguous, or nearly so. The planting stock used was
genetically improved 1-0 loblolly pine bareroot seedlings. The two
Coastal Plain sites were planted with material from Coastal Plain
Manuscript received July 29, 2010; accepted January 13, 2011. http://dx.doi.org/10.5849/sjaf.10-038.
Ralph L. Amateis (ralph@vt.edu), Department of Forest Resources and Environmental Conservation, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
Harold E. Burkhart, Department of Forest Resources and Environmental Conservation, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Support from the
Forest Modeling Research Cooperative at Virginia Polytechnic Institute and State University is gratefully acknowledged.
Copyright © 2012 by the Society of American Foresters.
SOUTH.J.APPL.FOR. 36(1) 2012 11
ABSTRACT
seed sources, and the two Piedmont sites were planted with material
from Piedmont seed sources. For the first 2 years following estab-
lishment, both herbaceous and woody competing vegetation were
controlled with herbicides. Otherwise, no management treatments
were applied in this study.
Measurements included groundline diameter at ages 1–5 and
dbh annually from age 5. Total height was measured annually
through age 10 and biennially thereafter. A damage assessment code
was collected annually. At age 25, crown class information was
collected on all trees. No assessment of stem quality was made dur-
ing the life of the study. Therefore, Table 2 presents average stand
characteristics for all trees by spacing at ages 5, 10, 15, 20, and 25.
Unique features of this study include blocks established at two
sites within each of two major physiographic regions, an extreme
range of planting densities from 2,722 to 302 trees/ac, ratios of
within-row to between-row distances varying from 1:1 to 3:1, and
annual (or nearly annual) measurements from establishment to con-
clusion of the study (age 25).
History
Periodically over the life of the study, analyses were completed to
examine the growth and development of loblolly pine under differ-
ent establishment densities. Zhang et al. (1996) used data through
age 10 to study the effect of spacing and density on juvenile loblolly
pine plantation development. The individual tree models developed
from that work can be used to predict juvenile growth of loblolly
Figure 1. Example block of the loblolly pine spacing trials using the nonsystematic design of Lin and Morse (1975): 16 treatment plots
with three-row buffer areas separating treatment plots. Treatment plots of different sizes each contained 49 measurement trees.
Table 1. Sites for the loblolly pine spacing trials.
Physiographic
region State County
Latitude
(N)
Longitude
(W)
Piedmont VA Buckingham 37°26⬘78°37⬘
Piedmont VA Halifax 36°45⬘78°43⬘
Coastal plain NC Halifax 36°11⬘77°29⬘
Coastal plain VA King and Queen 37°31⬘76°43⬘
12 SOUTH.J.APPL.FOR. 36(1) 2012
pine plantations covering a wide range of planting densities. Using
annual measurements through age 8, Liu and Burkhart (1993) de-
termined that the distribution of tree diameters, total heights, and
crown heights were significantly correlated with stand age and num-
bers of trees per unit area. Liu and Burkhart (1994) applied trend
surface analysis of spatial characteristics of tree diameters and total
heights in an effort to separate systematic microsite variation from
variation incurred by intertree competition. Their results showed
that in the seedling period of stand development, the systematic
environmental gradients had a dominant impact on the spatial pat-
tern of dbh and total height; however, the effect of environmental
gradients diminished as stands developed. Bullock and Burkhart
(2003) applied a simultaneous autoregressive model to evaluate the
extent of spatial influence that stems in juvenile loblolly pine stands
have on one another. Analysis of diameter measurement data indi-
cated significant spatial dependency in 23.2% of the spacing trial
plots. Diameter distributions of juvenile loblolly pine stands were
characterized using the two-parameter Weibull function to gain
insight into the effects of stand density and to aid in characterizing
diameter distributions in juvenile loblolly pine stands (Bullock and
Burkhart 2005).
Sterba and Amateis (1998) used the 10-year data to examine the
relationship between basal area increment and crown efficiency and
the ratio of crown surface area to crown projection area. Radtke and
Table 2. Mean stand characteristics by spacing at 5, 10, 15, 20, and 25 years across all locations and replications for the loblolly pine
spacing trials.
Stand variable
Planted spacing (ft)
4⫻4
4⫻6,
6⫻4
4⫻8,
8⫻4
4⫻12,
12 ⫻46⫻6
6⫻8,
8⫻6
6⫻12,
12 ⫻68⫻8
8⫻12,
12 ⫻812⫻12
Planted (trees/ac) 2,272 1,814 1,361 908 1,210 908 605 681 454 303
Age 5
Trees/ac 2,648 1,773 1,318 875 1,175 873 583 664 445 296
Height (ft) 12.2 12.4 12.4 12.9 12.3 12.5 12.6 12.6 12.6 12.7
Basal area (ft
2
/ac) 58 46 37 29 35 29 21 24 17 12
dbh (in) 1.9 2.1 2.2 2.4 2.2 2.3 2.4 2.5 2.5 2.6
Crown ratio (%) 75 79 82 85 83 85 87 87 88 88
Total volume (ft
3
/ac)
a
860 624 482 354 439 351 243 283 194 133
Age 10
Trees/ac 2,491 1,718 1,309 870 1,154 865 580 659 441 293
Height (ft) 27.5 28.6 29.4 30.7 29.3 30.3 31.0 31.0 31.3 31.3
Basal area (ft
2
/ac) 164 146 132 115 127 116 98 107 87 71
dbh (in) 3.4 3.8 4.2 4.8 4.4 4.9 5.5 5.4 5.9 6.6
Crown ratio (%) 42 46 50 58 50 54 62 58 65 71
Total volume (ft
3
/ac)
a
2,638 2,324 2,099 1,835 1,985 1,826 1,549 1,700 1,371 1,107
Pulpwood (ft
3
/ac)
b
246 516 761 1,042 844 1,053 1,107 1,189 1,075 935
Chip-and-saw (ft
3
/ac)
c
0 0 0 0 0 9 26 18 64 221
Age 15
Trees/ac 1,884 1,509 1,177 806 1,073 817 559 625 426 283
Height (ft) 37.6 38.3 39.7 41.6 39.8 41.5 43.0 42.6 43.5 44.2
Basal area (ft
2
/ac) 190 189 179 163 172 165 149 156 137 119
dbh (in) 4.2 4.7 5.1 5.9 5.3 6.0 6.9 6.7 7.6 8.7
Crown ratio (%) 29 31 33 38 34 36 42 38 44 50
Total volume (ft
3
/ac)
a
3,793 3,743 3,616 3,357 3,459 3,402 3,114 3,249 2,871 2,504
Pulpwood (ft
3
/ac)
b
1,421 1,993 2,339 2,628 2,386 2,659 2,685 2,753 2,581 2,337
Chip-and-saw(ft
3
/ac)
c
0 0 111 479 76 381 1,017 836 1,456 1,790
Sawtimber (ft
3
/ac)
d
000000 008178
Age 20
Trees/ac 1,148 1,058 878 650 733 663 500 565 395 265
Height (ft) 46.6 48.6 50.3 52.3 49.9 52.7 54.4 54.5 56.1 57.7
Basal area (ft
2
/ac) 159 177 176 174 158 175 172 179 163 145
dbh (in) 4.9 5.4 5.9 6.8 6.1 6.8 7.8 7.5 8.6 9.9
Crown ratio (%) 27 27 29 32 30 30 34 32 36 42
Total volume (ft
3
/ac)
a
3,716 4,286 4,381 4,456 3,874 4,505 4,521 4,714 4,373 3,967
Pulpwood (ft
3
/ac)
b
2,234 2,995 3,390 3,839 3,111 3,867 4,104 4,222 4,075 3,791
Chip-and-saw (ft
3
/ac)
c
156 395 745 1,675 834 1,566 2,557 2,357 3,043 3,297
Sawtimber (ft
3
/ac)
d
0 0 0 109 0 90 190 120 436 1,359
Age 25
Trees/ac 556 640 567 462 487 455 371 433 331 237
Dominant height (ft) 59.1 61.7 64.3 67.0 62.9 65.4 67.5 67.3 68.6 70.1
Height (ft) 56.3 59.2 60.3 63.4 60.5 63.0 65.5 65.1 66.9 68.9
Basal area (ft
2
/ac) 128 157 163 172 148 166 168 177 171 162
dbh (in) 6.3 6.6 7.1 8.1 7.3 8.0 8.9 8.5 9.6 11.0
Crown ratio (%) 26 27 27 30 28 29 31 29 31 36
Total volume (ft
3
/ac)
a
3,536 4,515 4,800 5,250 4,330 5,009 5,224 5,514 5,394 5,233
Pulpwood (ft
3
/ac)
b
2,912 3,779 4,215 4,820 3,847 4,587 4,909 5,131 5,131 5,066
Chip-and-saw (ft
3
/ac)
c
1,214 1,270 2,196 3,253 2,092 3,038 3,794 3,694 4,276 4,593
Sawtimber (ft
3
/ac)
d
0 100 207 673 167 571 1,249 891 1,690 3,065
a
Outside bark volume, all trees.
b
Outside bark volume 5-in. dbh class and above to 4-in. top diameter outside bark.
c
Outside bark volume 8-in. dbh class and above to 6-in. top diameter outside bark.
d
Outside bark volume 11-in. dbh class and above to 8-in. top diameter outside bark.
SOUTH.J.APPL.FOR. 36(1) 2012 13
Burkhart (1999) used early data from the study to examine relation-
ships between the inflection age of cumulative basal area growth,
crown closure, and crown competition factor. Radtke et al. (2003)
investigated the relationship between competition and age of inflec-
tion of individual-tree basal area curves. Results from these latter
two studies provide insights into how basal area development should
be modeled over the early years for different planting densities.
MacFarlane et al. (2000) examined the assumption that height
growth of dominant trees is independent of initial planting density
using data on the average height of the seven tallest trees at age 16.
Their analysis showed a highly significant negative correlation be-
tween dominant height and initial planting density.
Long-term field studies are subject to catastrophic events. In the
11th year of the trials (winter of 1994), a severe ice storm badly
damaged several plots on the northernmost Piedmont site. Using
stem quality assessment data collected after the storm, Amateis and
Burkhart (1996) developed prediction equations for estimating the
probability of five levels of stem bending and top breakage based on
a proportional odds model.
At age 20, most of two blocks at the northernmost Piedmont site
were attacked by southern pine beetles, and a decision was made to
discontinue measurements on these blocks. Following abandon-
ment, 34 sample trees from 10 spacing treatments were felled, and
data were collected on whorl height aboveground, status of whorl
(live, sound, decayed, or knot), and branch diameter. Dissection of
whorls in the laboratory yielded information on knot size and shape
and branch diameter growth. These data from the field and labora-
tory were used to construct models of branch diameter growth and
knot formation along the boles of loblolly pine trees (Trincado and
Burkhart 2008, 2009).
The southernmost Piedmont location was abandoned at year 20
because of a land sale and subsequent thinning operation. Thus,
from age 21 to the end of the study, 7 of the original 12 blocks were
still being measured: 6 in the Coastal Plain and 1 in the Piedmont.
Data from this study have also been used to evaluate the impact
of rectangularity on growth and development of loblolly pine. Sur-
vival and the development of height, diameter, volume yield, and
basal area were not affected by rectangularity out to a 3:1 ratio of
between-row to within-row tree distance (Sharma et al. 2002a,
2002b). Bole condition and stem asymmetry were not affected by
rectangularity. Although extreme rectangularity has significantly af-
fected maximum branch diameter, the mean branch diameter has
not been found to be large enough to degrade the butt log for the
12 ⫻4 rectangular spacing of this study (Amateis et al. 2004).
In recent years, the trials have yielded important information
about height, dbh, and basal area growth relationships. Amateis et
Figure 2. Total cubic-foot volume outside bark for the 4 ⴛ4-ft, 6 ⴛ6-ft, 8 ⴛ8-ft, and 12 ⴛ12-ft square spacing treatments for ages
5–25 of the loblolly pine spacing trials.
14 SOUTH.J.APPL.FOR. 36(1) 2012
al. (2009) found no correlation between row orientation, spacing
treatment, and height and dbh growth at any age of plantation
development. The mean height and dbh data presented in Table 2
suggest that both are affected by planting density. Work by Anto´n-
Ferna´ndez et al. (in press, a) has quantified and modeled the rela-
tionship between height development and establishment density.
Using the full range of data over the life of the study, they found that
density begins to affect dominant height development by age 6 and
continues to influence height development through age 25. This
confirms the earlier work of MacFarlane et al. (2000) and suggests
that site index determination can be affected by initial density. Ad-
ditional work by Anto´n-Ferna´ndez et al. (in press, b) evaluated the
effect of planting density on basal area development over the life of
the study. Their work quantified the downturn in basal area ob-
served for dense plantings and the asymptotic behavior seen in less
dense plantings. A combined exponential and power function was
used to model both developmental patterns.
These spacing trials, because of the wide range of initial densities
and long-term measurements, have provided important new infor-
mation on maximum size-density relationships and self-thinning.
VanderSchaaf and Burkhart (2007) compared methods for estimat-
ing Reineke’s maximum size-density boundary line slope using
the spacing trial data. The data were also used (VanderSchaaf and
Burkhart 2008) to construct regressions relating stages (density-
independent, density-dependent) and phases (curved approach, lin-
ear, divergence within the self-thinning phase) of the maximum
size-density relationship to planting spacing.
Clearly, results yielded by the trials over the years have met goals
1 and 2 by shedding light on a number of aspects of loblolly pine
plantation growth and development. The purpose of this analysis is
to consider goal 3 of the study: How does planting density affect
yield for particular product definitions?
Methods
Because heights were measured biennially after age 10, linear
interpolation was used to estimate total height for the years lacking
an observed height. At age 25, the crown class (dominant or
codominant, intermediate or suppressed) of each tree was recorded,
and total height was measured. Average height of the dominant and
codominant trees was computed to obtain an exhibited site index for
each treatment plot (Table 2).
Diameter and heights for each live tree were used to estimate total
outside bark volume of the main stem using the individual tree total
volume equation of Tasissa et al. (1997). The Tasissa et al. (1997)
merchantable volume equations were used to compute three esti-
mates of merchantable yield. For trees in the 5-in. dbh class (dbh at
least 4.6 in.) and above, an estimate of merchantable pulpwood
volume to a 4-in. top diameter outside bark was computed. Simi-
larly, trees in the 8-in. dbh class and above to a 6-in. top diameter
outside bark were used to compute an estimate of chip-and-saw
Figure 3. Cubic-foot pulpwood volume outside bark (5-in. dbh class and above to a 4-in. top diameter outside bark) for the 4 ⴛ4-ft,
6ⴛ6-ft, 8 ⴛ8-ft, and 12 ⴛ12-ft square spacing treatments for ages 5 through 25 of the loblolly pine spacing trials.
SOUTH.J.APPL.FOR. 36(1) 2012 15
volume, and trees in the 11-in. dbh class and above were used to
compute an estimate of sawtimber volume to an 8-in. top diameter
outside bark. Per-acre summaries of total and merchantable volumes
were computed for each treatment plot and averaged across all sur-
viving blocks and locations of the study at each age (Table 2 and
Figures 2–5). Recognizing that specific products, merchantability
standards, and product metrics, such as volume or weight, will vary
with local markets, it should be possible to relate the cubic-foot
volume per acre results presented here to other standards and prod-
uct specifications.
Results and Discussion
Figure 2 summarizes total volume outside bark yield production
for the square spacing treatments over the life of the study. Plots
established at high densities and close spacings outproduced plots
established at low densities and wide spacings through the early years
of the study. The 4 ⫻4-ft treatment plots produced more total
volume yield through age 13 than any other treatment plot. By age
15, however, increased mortality in the 4 ⫻4-ft plots reduced yield
production below plots established at lower densities and wider
spacings. Thus, management strategies geared toward total biomass
production suggest that high initial establishment densities and
short rotations will be optimal.
Figure 3 summarizes pulpwood production for the square spac-
ing treatments. No treatment produced more pulpwood volume
than the 8 ⫻8-ft spacing treatment. Where pulpwood production is
the management objective, plantation establishment densities in the
neighborhood of 680 trees/ac should be optimal for rotations of 25
years or less and site qualities similar to those included in these trials.
With regard to pulpwood production, results from this study are
consistent with those of Harms and Lloyd (1981).
Figures 4 and 5 present the development of chip-and-saw and
sawtimber production, respectively, for the square spacing treat-
ments. Management strategies that are optimal for solidwood prod-
ucts will include wider spacings and fewer trees planted. In this
study, the 8 ⫻12-ft and 12 ⫻12-ft spacing treatments produced
more chip-and-saw than other treatments for all ages. The 12 ⫻
12-ft (302 trees/ac) planting density produced more sawtimber
across all ages, by a wide margin, than any other spacing treatment.
By age 25, the 12 ⫻12-ft spacing had almost twice as much saw-
timber as the 8 ⫻12-ft spacing (454 trees/ac planted) (Table 2).
When sawtimber production is the goal and thinning is not part of
an overall stand management plan, 300 trees/ac planted does not
appear to underuse the site.
An important finding from this study is that rectangularity was
not an important factor affecting yield production. By age 25, dif-
ferences between yields of the 4 ⫻12-ft spacing treatments and the
6⫻8-ft spacing treatments (48 ft
2
of growing space per tree) were
not statistically significant. This suggests that where site prepara-
tion and planting costs can be reduced for a given number of trees
planted by establishing fewer rows, spacings with increased be-
tween-row planting distances and decreased within-row distances
can be implemented.
In summary, the major conclusions from this study are as
follows:
1. The particular planting density selected has a far greater effect
on yield and the products obtained at harvest than the degree of
rectangularity. In other words, the shape of the growing space
per tree is not nearly as important as the amount of growing
space per tree.
Figure 4. Cubic-foot chip-and-saw volume outside bark (8-in. dbh class and above to a 6-in. top diameter outside bark) for the 4 ⴛ4-ft,
6ⴛ6-ft, 8 ⴛ8-ft, and 12 ⴛ12-ft square spacing treatments for ages 10–25 of the loblolly pine spacing trials.
16 SOUTH.J.APPL.FOR. 36(1) 2012
2. In the absence of thinning, there is an inverse relationship
between planting density and size of products realized at har-
vest. Product objectives at harvest that include large sawtimber
trees will require fewer trees per acre planted. Assuming no
thinning, a planting regime of 300 trees/ac appears reasonable
for growing sawtimber over a 25-year rotation on sites similar
to those included in this study. Conversely, total biomass pro-
duction goals can best be met by establishing high-density
plantations managed on short rotations.
3. Management objectives focused on realizing pulpwood yields
can be achieved by planting about 680 trees/ac on lands exhib-
iting site quality similar to that of these plantings.
Considered together, the results of this study suggest that no
single planting density will be optimal for all management objec-
tives. Rather, managers will need to consider product objectives
desired at final harvest and whether opportunities for thinning and
other silvicultural interventions will be present during midrotation
when selecting an initial planting density.
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Figure 5. Cubic-foot sawtimber volume outside bark (11-in. dbh class and above to an 8-in. top diameter outside bark) for the 4 ⴛ4-ft,
6ⴛ6-ft, 8 ⴛ8-ft, and 12ft x 12 ft square spacing treatments for ages 10–25 of the loblolly pine spacing trials.
SOUTH.J.APPL.FOR. 36(1) 2012 17
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