From:
Klass, D.L. (editor). 1991.
from Biomass and Wastes XV.
of Gas Technology,
Chicago,
WOODGRASS AND WIDER SPACED SHORT-ROTATION SYSTEMS FOR POPLAR PRODUCI'ION: BIOMASS YIELDS OVER FIVE YEARS Dean S. DeBell, Ph.D. Chief Research Silviculturist Gary W. Clendenen, M.S. Mensurationist USDA Forest Service Pacific Northwest Research Station Forestry Sciences Laboratory Olympia, Washington 98502, U.S.A. ABSTRACT
A comprehensive test of effects of spacing on growth and yield of Populus was
established in western Washington in spring 1986. Two hybrid clones (D-Ol and H-11)
were planted at two woodgrass spacings (0.18 and 0.30-m) and three wider spacings
(0.50-, 1.00 , and 2.00-m). All treatments were replicated three times in a randomized
block design; all plots were fertilized, irrigated, and weeded uniformly.
Mean annual production in the woodgrass treatments did not differ between the two
clones (D-01 and H-11) or two spacings (0.18- and 0.30-m), averaging 6.4 to 7.0
tonnes per hectare over the 5-year period. The highest yield was produced in the second
year (first year of coppice), and it declined thereafter. Cumulative growth in the wider
spacings was substantially better than in the two woodgrass spacings. Per hectare yields
of clone H-11 in the wider spacings at age 5 were two to three times greater than those
obtained with woodgrass; yields of clone D.O 1 in the widest spacing (2.0-m) averaged
17% greater than those of woodgrass, and yields of clone 0.01 in the 0.5- and 1.0-m
spacings were about 60% greater than woodgrass yields. Woodgrua shows little
promise as a viable system for growing Populus for biomass. On the other hand, yields
in the wider spacings, especially for clone H-11, were substantially higher dum pre­
viously expect ed Thus, possibilities for application of the wide-space regimes are
considerably brighter than for woodgrass.
.
411
Energy
Institute
IL
WOODGRASS AND WIDER SPACED SHORT- ROTATION SYSTEMS FOR POPLAR PRODUCTION: BIOMASS YIELDS OVER FIVE YEARS INTRODUCTION Short-rotation woody biomass farms have their origin
in the "silage sycamore" concept proposed 25 years ago in
the southeastern United States (10,9).
The concept
establishing rapid-growing trees at dense
incl uded:
spacings, applying intensive cultural practices, harvesting
on short cycles, regenerating subsequent crops via sprouts
or coppice arising from stumps, and using a high degree of
Experimental plantings were established in
mechanization.
other regions (4,6,7) with emphasis on Populus species and
hybrids.
During the energy crisis of the early 1970's,
this short-rotation approach was suggested as a means to
produce wood for energy (12).
Subsequently, considerable
research has been done on biomass plantations for fiber and
energy, most of it funded by the Short Rotation Woody Crops
In general, such
Program of the U. S. Department of Energy.
work has indicated that production will be greater and
costs will be lower if spacings are wider and harvest
cycles are longer than those evaluated in many of the early
trial s.
Stand densities of 2500 to 4000 trees per hectare
( i. e. , square spacings of 1.6 to 2. 0 m) and rotations of 5
to 8 years are now perceived by many as optimum for
bioenergy crops (11).
Other on-going research suggests
that even wider spacing (up to 3.0 m) and longer rotations
tup to 12 years) may be equal ly productive and perhaps
preferable for some objectives.
In the late 197 0' s and earl y 1980's, Joseph Dula, a
nurseryman in Oregon proposed a radical departure from the
above trends in woody biomass farming (5).
His system
entail ed establishing a Populus hybrid at densities of 100
thousand to 600 thousand rootstocks per hectare with annual
harvests of coppice.
Biomass yields were purported to
axceed 100 tonnes per hectare annually.
Economic analyses
were performed by coupling such yield data with estimated
costs provided by Dula. These analyses suggested that the
system--dubbed "•;�oodgrass"--compared favorably with other
short-rotation density regimes (13), and considerable
interest devel oped in the energy conversion community.
Some forest biologists, however, remained skeptical.
Many
peopl e wondered:
Was this skepticism well-founded or was
it associated with reluctance to accept innovation arising
outside the scientific establishment?
Managers of the Short Rotation Woody Crops Program
decided that a scientific evaluation of the woodgrass
412
concept was needed, and the Olympia Forestry Sciences
Laboratory of the u.s. Forest Service was selected to conduct the investigation.
The study compared two Populus
hybrids at f ive spacings--ranging f rom two woodgrass spacings (0. 18- and 0. 30-m) to one approaching a
conventional pulpwood spacing (2. 0-m). This paper
describes woody biomass production of the plantings over a
5-year period.
METHODS
The experimental site is 12 km east of Olympia,
Washington.
Climate is mild with an average growing sea on
of 190 f rost-f ree days and a mean July temperature of 16
Precipitation averages more than 1000 mm per year,
c.
falling mostly as rain f rom October through May; summers
are periodically dry. The land was previously farmed f or
strawberry and hay crops.
Topography is relatively level,
and the soil is Nisqually sandy loam. The land was
prepared f or planting by plowing and disking in winter
1985-86.
The study was established as a f actorial design with
two Populus clones and f ive spacing treatments, replicated
One clone, D-01, is a Populus hybrid
in three blocks.
(taxonomic identity unknown) developed originally at
University of Idaho and subsequently selected f rom a
Canadian planting by Dula's Nursery of Canby, Oregon (5).
The other clone, H-11, is a
· trichocarpa x · deltoides
hybrid developed and tested by University of Washington and
Square spacings (m by m)
Washington State University 18).
are 0. 18-, 0. 30-, 0.50-, 1. 00-, and 2. 00-m.
Equivalent
trees per hectare are about 310, 000, 110, 000, 40, 000,
10, 000, and 2, 500.
The first two spacings (0.18- and
0. 30-m) are woodgrass treatments recommended to us by Dula
(i.e. , three and one plants per square f oot). Size of treatment plots varies with spacing, but all plots are suf f iciently large to provide at least 100 trees in the interior measurement pl ot (400 trees for woodgrass harvests), and a border around each measurement plot at least one-half as wide as the projected height of trees at harvest. Both clones were planted by hand as unrooted, hardwood
cuttings in late April 1986.
All cuttings were 30 em long
and had a minimum upper diameter of 1 em: they were planted
20 em deep with at least two healthy axillary buds
remaining above ground.
Supplemental nutrients and water were provided
unif ormly in plots of all treatments.
A pre-planting
application of fertilizer ( 16-16-16) provided the
equivalent of 100 kg per hectare each of nitrogen,
413
phosphorus, and potassium.
Additional nitrogen fertilizer,
ammonium nitrate, was applied at 100 kg N per hectare in
May 1988.
Plots are irrigated throughout each summer by a
drip system.
All plots were kept free of weeds by tilling
and hoeing the first year and by herbicides and hoeing the
second year and third year.
Little such work, however, was
needed after the second year.
At the end of the first
year, all positions occupied by dead trees were replanted
with unrooted cuttings: also, any secondary shoots on
plants in the wider spacings (0.5-, 1.0-, and 2. 0-m) were
removed, resulting in stands composed solely of
single-stemmed trees.
Survival, height, and basal diameter were recorded
each year on all plots. Number of living and dead sprouts
per rootstock were also tallied after the second and
subsequent growing seasons in woodgrass plots.
Yield data
for the woodgrass treatments were based on harvests after
leaffall, at the end of each season, of 400 trees in the
center of each plot. Moisture contents were dete ined on
subsamples to convert fresh weight to ovendry (105 C)
weight. Yields for the wider spaced plots were estimated
from dry-weight biomass equations applied to diameter and
The equations were
height measurements of the trees.
developed via destructive sampling of trees representative
of the spectrum of sizes in each spacing of each clone, and
accounted for 9 5t or more of the variation in weight.
Estimated above-ground woody dry matter of all trees on
each plot were summed, and the resulting plot dry weights
expanded by appropriate multipliers to provide yield per
hectare.
RESULTS AND DISCUSSION
Establishment Year (General)
Survival at the end o f the first growing season
Average heights
averaged 96% for D-01 and 98t for H-11.
In
for D-01 and H-11 were 1.44 and 1.80 m, respectively.
the two woodgrass treatments, mean heights of the two
cl ones were very similar, averaging 1.2 m in the 0. 18-m
spacing and 1.4 m in the 0.30-m spacing.
As spacing
widened from 0.18 to 1.00 m, mean height of clone H-11
increased to 2.3 m and mean height of D-Ol increased to
1.7 m. Mean heights for the clones at spacings of 0.50-,
Effects of
1.00-, and 2.00-m differed by 50 em or more.
spacing on basal diameter were similar to those for
Both clones had similar diameters at the 0.18- and
height.
0.30-m spacings; mean diameter of both clones was greater
at wider spacings and gains were greater for H-11.
The substantially reduced first-year growth in the
woodgrass spacings as compared with growth at wider
414
spacings indicated that competition among plants was
Contrasted with
sufficient to depress growth processes.
trees in the 1.0-m spacing, trees in the densest woodgrass
spacing averaged about 40% shorter in height, 70% smaller
in basal diameter, and 80% lower in leaf area per tree.
Because of intense competition in the woodgrass plots, and,
in accord with Dula's (5) procedures, we cut these dense
plots at the end of the growing season to establish
coppice.
These growth patterns and competitive stresses
observed in the establishment year were harbingers of
differences among clones and spacings in subsequent years.
Woodgrass
Yields from the first (non-coppice) harvest of the
woodgrass spacings and those of four subsequent (true
coppice) harvests are shown in Table 1. First-year yields
of the 0.18-m spacing were significantly higher than those
of the 0.30-m spacing but did not vary by clone.
Dry-matter production averaged 4.0 tonnes per hectare in
the 0.18-m spacing, and 3.0 tonnes per hectare in the
0.30-m spacing. Although yields were about 30% higher in
the denser spacing, three times as many cuttings (200%
more) had been planted. This decreased growth efficiency
per tree was caused by increased competitive stress in the
denser spacing.
Table 1.
Year
DRY YIELD OF WOODGRASS DURING 5 YEARS AFTER
PLANTING.*
Clone D-01
SEacing
0.18-m
0.30-m
Clone H-11
SEacing
0.18-m
0.30-m
- -tonnes per hectare1
2
3
4
5
Total
Mean
-
3.9
9. 6
8. 6
7.0
5. 9
3.1
8.7
7.9
5.0
7.3
4.1
8.3
8.3
7.8
5.9
3.0
7.7
7.5
7.3
6.6
35.0
32.0
34.4
32 1
7.0
6. 4
6.9
6.4
* Above-groung , leafless biomass dried to constant
weight at 105
c.
415
.
Vigorous sprouts began to develop on the stumps in
early April, and growth was excellent throughout the second
year. Yields from the second cutting were more than double
those of the first cutting, and ranged from 7.7 to 9. 6
Production was significantly
tonnes per hectare (Table 1).
greater in the denser spacing (9.0 vs. 8.2 tonnes per
hectare).
Clone D-Ol tended to produce higher yield than
H-11, but considerable variation existed within the clonal
treatments and differences were not significant at
P < 0.05.
Increased production of the woodgrass spacings
in the second harvest is associated with increased growth of the dominant stem on each root stock and, for the D-01 clone, a greater number of living stems per plant (Table 2). Averaged over both spacings, mean heights of D-Ol and H-11 were 2.0 and 2.2 m in the second year as compared to 1.3 and 1.4 m in the first year: diameters were also
greater in the second year for both clones.
Although
dominant sprouts of H-11 were larger than those of D-01,
the tendency of D-Ol to produce higher yield per hectare
resulted from its dramatically greater production of
coppice sprouts. Averaged across spacings, D-Ol produced
more than seven sprouts per rootstock but H-11 had less
than five.
In number of live sprouts at the end of the
growing season, even greater differences existed between
the clones. Such differences were especially evident in
the 0.30-m spacing where D-Ol averaged seven living sprouts
per rootstock and H- 11 averaged only one (Table 2).
Table 2. EFFECTS OF CLONE AND SPACING ON CHARACTERISTICS
OF WOODGRASS OVER 5 -YEAR PERIOD.
- Year
3
5
Clone
Spacing
Characteristic*
D-01
0.18 m
Survival (%)
Height (m)
Live sprouts (no.)
100
1.1
1
79
90
96
1.8 1.8 1.6
2
3
4
1.6
Survival (%)
Height (m)
Live sprouts (no. )
100
1.4
1
98
2.1
7
98
2.0
9
98
1 5
12
97
2.1
8
Survival (%)
Height (m)
Live sprouts (no. )
100
1. 2
1
90
1.9
1
66
2.2
1
40
2.3
1
31
2.4
2
Survival (%)
Height (m)
Live sprouts (no.)
100
1.6
1
97
2.6 1
82
2. 6
1
56
2.9
2
49
2.8
2
0.30 m
H-11
0.18 m
0. 30 m
1
2
*
Height refers to tallest sprout per rootstock;
living sprouts is also per rootstock.
416
4
.
77
4
number of
Sprouting of stumps was also vigorous after the second
harvest (or first coppice harvest), but it became
increasingly less so with each successive harvest.
Third­
and fourth-year yields continued to be greater in the
densest woodgrass spacing (Table 1). Production declined,
however, and by the fifth harvest, average yield was about
25% lower than that obtained in the second harvest.
The
reductions in yield were greater for clone D-Ol (28%) and
in the densest (0. 18 m) spacing (3 4%). For the first time,
production in the 0. 3 -m spacing exceeded that in the 0. 18-m
spacing.
Data on sprouting characteristics (Table 2)
provide some explanation for these trends in biomass
production.
Rootstock survival declined overall and the
losses differed substantially by treatment; mortality
ranged from 3 % for clone D-Ol at the 0. 3 -m spacing to 6 9%
for clone H-11 at the 0.18-m spacing.
Such mortality
losses were accompanied by enhanced sprout growth on
surviving rootstocks of clone H-11 and similar (in some
instances, declining) sprout growth on surviving rootstocks
The superior performance of clone
of clone D-Ol (Table 2).
· ­ D-01 at 0.3-m spacing in the fifth year is a notableexception to such general trends; its fifth-year height growth was about 40% greater than that of the previous year, its yield of 7.3 tonnes was the highest of all clone-spacing treatments, and it was the only treatment that increased rather than decreased in yield as compared with that of the previous year. Total 5-year production in the woodgrass treatments
ranged from 32. 0 to 35.0 tonnes per hectare (Table 1).
Mean annual increment at age 5 ranged from 6 . 4 to 7.0
tonnes per hectare. Annual production averaged about 0.5
tonnes per hectare more in the denser (0.18-m) spacing, but
average yields of the two clones were essentially equal.
Wider spacings
Tree growth in the wider spacings (0.5-, 2.0-, and
2. 0-m) also accelerated during the second year, and even
more so in the third year in the two widest spacings.
Although subsequent height and diameter growth slowed as
competition increased in all clonal and spacing treatments,
survival and tree size at age 5 years were excellent (Table
Survival remains at 100% in all spacings of clone
3).
D-01, but 11% and 2% of the trees in 0.5- and 1.0-m
spacings, respectively, of clone H-11 have died and many
more are now suppressed. Even the 2.0-m spacing provides
less than adequate growing space for these Populus clones,
as indicated by superior growth of trees in the border
(non-measured) rows of the plots.
Trees of clone H-11 were substantially larger at age 5
in diameter (14%), height (32%), and woody biomass (88%)
than those of clone D-01. Such differences between clones
•
417
I
'
'
generally increased with spacing.
Clonal differences in
biomass accumulation have also been influenced by
differences in branch retention; clone D-Ol tends to retain
its branches much longer than clone H-11.
Table 3. CHARACTERISTICS OF OTHER (NON-WOODGRASS) SHORT
ROTATION SPACINGS AT AGE 5 YEARS.
:::lone
D
-
01
H-11
s12acing
Mean tree size
Lower
diameter
Height Dry weight
-mm-mkg
Survival
-%-
-
-
0.50
1.00
2.00
100
100
100
33
65
115
6.6
9.7
11.1
1.37
5.47
16.25
Mean
100
71
9.1
7.70
0.50
1.00
2.00
89
98
100
40
73
130
8.0
12.3
1 5.6
2.65
9. 43
31.30
Mean
96
81
12 .0
1 4. 46
Mean tree woody biomass has continued to increase
substantially with spacing, and at age 5 is 12 times
greater in the 2.0-m spacing than in the 0.5-m spacing.
As
a result, biomass accumulation per hectare has become much
more similar among spacings.
Woodgrass vs. Wider s12acings
Cumulative 5-year woody biomass production is shown in
Table 4 for all treatments. Yields for woodgrass spacings
nclude l ive woody biomass from five harvests; values for
the 0.5-, 1.0-, and 2.0-m spacings represent live woody
biomass standing after each growing season. Although
production increased in the two woodgrass treatments in the
second year, it accelerated even more in the wider,
non-coppiced treatments.
The growth advantages of wider
spacings became even greater in subsequent years.
Mean
annual production of woodgrass over the 5-year period was
similar for both spacings and both clones--6.4 to 7.0
tonnes per hectare. Mean annual production of clone D-Ol
in the 0.5- and 1.0-m spacings averaged 10.9 tonnes per
hectare, and annual production of clone H-11 averaged over
all years and all wider spacings was 17.6 tonnes per
hectare. Cumulative 5-year yield of clone D -01 in the
2.0-m spacing is about 17% greater than that of the
418
woodgrass treatments and yield in the 0.5- and 1.0-m
spacing is about 60% greater.
Cumulative 5-year yields of
the three wider spacings of clone H-11 are two to three
times greater than those of the woodgrass treatments.
Thus, the wider spacings are unquestionably superior to
woodgrass for growing woody biomass with both Populus
clones. Because the growth characteristics of these two
clones are so different, it seems reasonable to conclude
that such superiority of the wider spacings is likely to be
indicative of responses for Populus in general.
Table 4. CUMULATIVE
5-YEAR OVEN-DRY WOODY BIOMASS YIELDS
OF TWO POPULUS CLONES GROWN UNDER WOODGRASS AND
WIDER SPACED SHORT-ROTATION REGIMES.
Short-rotation
regime
D-01
Clone
H-11
- tonnes per hectare-
Woodgrass
0.18-m
0.30-m
Wider spacings
0.50-m
1.00-m
2.00-m 35. 0
32.0
34.4
32.1
54.7
54.5
40.6
94.0
92.4
78.3
IMPLICATIONS AND CONCLUSIONS
What is the potential role of woodgrass in the
production of biomass for conversion to energy?
If yield
and cost of production are the primary criteria for
selection of a short-rotation density regime, spacings
other than woodgrass are overwhelmingly superior.
Yields
of clone H-11 in the wider spacings are two or three times
greater than woodgrass. Moreover, establishment costs are
Differences in cutting
substantially higher for woodgrass.
costs alone are tremendous; at 10¢ per cutting, such
costs would be $31,000 and $11,000 per hectare for the two
woodgrass spacings as compared to $1000 per hectare for the
1.0-m spacing.
Even if cuttings were only 1¢ each,
total cutting costs per hectare for the woodgrass spacings
would be $3100 and $1100 versus only $100 per hectare for
the 1.0-m spacing--differences still amounting to $1000 to
$3000 per hectare.
Considerable savings would therefore be
needed in other management, maintenance, harvest, or
interest costs to overcome such differences in
establishment costs. Despite the apparent disadvan ages of
woodgrass in terms of yield and production costs, the
system could be desirable if characteristics of the
419
produced biomass were superior in value to those of biomass
grown by other short-rotation systems.
Because of its
younger age and smaller size, woodgrass will have higher
contents of bark, extractives, nutrients, and moisture and
a lower content of cellulose than an equal biomass produced
in a wider spacing on a somewhat longer rotation (1,2).
:1any of these differences are considered negative traits in
various systems of conversion (3), but they might be
beneficial for some uses.
Even so, the characteristics
;vould have to be superior by many, many fold and the
advantages derived therefrom reflected in raw material
prices paid to the grower by the processing or conversion
industry.
The conclusion of our experiment, coupled with other
current knowledge, is that woodgrass has little promise as
a viable system for growing Populus biomass for energy.
Other wider-spaced, short-rotation density regimes,
especially those involving clone H-11 (and other P.
· trichocarpa hybrids), are producing higher
deltoides x
yields than expected, and possibilities for commercial
application of these systems seem much brighter.
ACKNOWLEDGEMENT
This work was supported and coordinated by the Short
Rotation Woody Crops Program (now Biofuels Feedstock
Development Program) of the u.s. Department of Energy
through Interagency Agreement No. DE-A105-810R20914.
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421
ENERGY FROM
BIOMASS AND
WASTES XV
Edited by Donald L. Klass Institute of Gas Technology, Chicago, Illinois, U.S.A. INSTITUTE OF GAS TECHNOLOGY CHICAGO 1 9 9 1