Reproduced by USDA Forest Service
for official
use.
844
Effects of irrigation, pulp mill sludge, and repeated coppicing on growth and yield of
black cottonwood and red alder'
CONSTANCE A. HARRINGTON2 AND DEAN S. DEBELL Pacific Northwest Forest and Range Experiment Station, Forestry Sciences Laboratory, 3625 - 9Jd Avenue SW, Olympia, WA, U.S.A. 98502 Received December 20, 1983'
Accepted July 18, 1984
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HARRINGTON, C. A., and D. S. DEBELL. 1984. Effects of irrigation. pulp rnill sludge, and repeated coppicing on growth and
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Growth and yield of black cottonwood (Populus trichocarpa Torr. and Gray) and red alder (Alnus mbra Bong.) were
measured at four successive 2-ycar coppice harvests. Three levels of Jmendcd pulp mill sludge (450, 225, and 0 t·ha-1) were
applied before planting, and one-half of the plots were irrigated during tht• 2-year establishnu:.nt period prior to the first coppice
cycle. Yields of black cottonwood were generally much higher th;m yields of red alder; moxnnum ovendry yields obtained were
13.8 t ha -I· year .. 1 for black cottonwood and 7. I t·ha 1 ·year -I for red alder. The species differed in their responses to
treatment. Growth and yield of black cottonwood was increased by the initial sludge applications, but the reverse was true for
red alder. The initial irrigation treatment had carryover effects on the yields of st bsequcnt growth cycles for both species. For
all treatments, the greatest yields were usually obtained at the first or second coppice harvc:.t. On the average, red alder yields
declined substantially between the third and fotu1h harvests; black cottonwood yields were generally more consistent between
these two harvests. Both species sprouted after all four harvests; however, rootstock mortality generally increased with each
harvest. Mortality was mu<:h higher for red alder than for black cottonwood; after four coppice harvests. mort11lity on red alder
treatments averaged 55-79%. Average mortality on black cottonwood treatments ranged from 12 to 38%. The management
implications of the results are discussed.
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HARRINGTON, C. A., et D. S. Dr-.BELL. 1984. Effects of irrigation, pulp mill luJgc, anJ rern atcJ ..:oppicing on growth and
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yield of black cottonwood and red alder. Can. J. For. Res. 14: 844--849 .
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yield of black cottonwood and red alder. Can. J. For. Res. 14: 844-R49.
La croissance et Ia production ligneuse du peuplier baumier de I'Ouest (Populus trichocarpa Torr. and Gray) et de l'aulne
de !'Oregon (Alm1s ruhra Bong.) furent mesurcs au moment de quatre rccoltes successivcs espacees de 2 ans. Trois niveaux
de boues provenant d'usines de pate de bois, aux taux de 450, 225 ct 0 t ha 1, furcnt appliques avant Ia mise en terre, tandis
qu'une moitie de chaque place d'etude etait irriguee durant Ia phase d'etablis crnent de 2 ans qui prcceda Ia premiere rotation
du taillis. La production ligneuse du peuplier fut en general bien superieure a celle de l'aulne: le premier produisit au maximum
13,8 t ha -I· an -I (poids sec), Ia production du second etant de 7, I t ha- 1 ·an· 1• Les reactions au traitement furent egalcment
differentes chez l'une et !'autre espece. Ainsi, Ia croissance et Ia production ont augmente chez le peuplier suite aux applications
de boues, alors que It; contrairc s'est produit chez l'aulne. Chez les deux essences, !'irrigation initialc a affecte Ia production
durant lcs rotations subsequentes. En general, quelque soit le traitement, Ia plus for1e production est survcnue au moment de
Ia premiere ou deuxieme rccolte. En moyenne, Ia production de l'aulne fut substantiellement reduite entre Ia troisieme et Ia
quatricme recolte; celle du peuplier demeura plus stable entre ces deux memes rccoltes. Les deux essences ont forme des rejets
aprcs chaque recolte: toutefois, Ia mortalite des systemes racinaires a eu tendance a s'accentuer avec chaque recolte successive.
La mortalite fut Ia plus elevee avec l'aulne: elle a atteint 55-79% au bout de quatre recoltes. Chez le peuplier, elle varia en
moyenne de 12 a 38% suivant les traitements. Les implications de ces resultats sont discutes dans une perspective
d'amenagement.
[Traduit par le journal]
·
·
·
Introduction
Black cottonwood (Populus trichocarpa Torr. anu Gray) and
red alder (Alnus rubra Bong.) are the two native tree species in
the Pacitic Northwest that appear to be the most promising
candidates ·for short-rotation intensive culture. Both species
have rapid juvenile growth and high prouuctivity, and can be
coppiced on short rotations. Short-rotation plantations are of
nterest to some pulp and paper manufacturers as a possible
way to increase the availability of raw material close to the
mills. Many pulp mills are also faced with waste disposal
problems. Using the same land to grow short-rotation fiber
:
crops and to dispose of mill wastes may be a viable alternative
under some circumstances. In addition, short-rotation intensive
culture of hardwood plantations is being evaluated for its appli­
cability in pmducing wood for energy.
This paper documents the growth and yield of an experi­
1 This article was written and prepared by United States Government
employees on official time, and it is therefore in the public domain.
2Current addre·ss: Southern Forest Experiment Station, Box 3516,
Monticello. AR, U.S.A. 71655.
3 Revised mapuscript received July 4, 1984.
mental biomass planting of black cottonwood and red alder
established in 1973 along the Columbia River in southern
Washington. The study was designed to compare the two spe­
cies and to evaluate the effects of amended pulp and paper mill
sludge, irrigation, and repeated harvest on each species.
Results from four consecutive coppice harvests are presented
and discussed. An earlier report (Harrington et al. 1979)
presented results of the establishment period, initial (non­
coppice) harvest, and the first two coppice harvests. This report
presents the results of four coppice harvests and expands the
analyses and discussion of the earlier report.
Materials and methods
Location and site
The study area was located on Lady Island in the lower Columbia
River near Camas, Washington. The area is flat with an elevation of
approximately 5 m. Annual precipitation averages 1000 mm with 80%
falling from October I through March 31, primarily as rain. Mean
annual temperature is 11.4°C; the average number of frost-free days
is 225. Soil is a silty clay loam of alluvial origin. (Alluvial deposits
in this area have not yet been mapped by soil series.) Native vegetation
on the island includes black cottonwood, willow (Salix sp.), teasel
HARRINGTON AND DEBELL
TABLE I. Distribution of plots by treatment
TABLE 2. Mean height of the tallest stem per plant by species,
coppice harvest, and treatment
======= - --
No. of plots
Treatment
Black cottonwood
Treatment
2
2
2
2
2
2
2
2
2
2
2
2
Total no. by species
12
12
•Amended pulpmill sluage.
(Dipsacus sy/vestris Huds.), Canadian thistle (Cirsium arvense (L.)
Scop.), stinging nettle ( Urtica dioica var. lyallii (Wats.) Hitchc.), and
several other herbaceous and grassy species. Red alder apparently
does not occur naturally on the island but is common in the general
area.
Trees in the immediate vicinity were removed for a powerline
right-of-way several years prior to the establishment of the study. In
the spring of 1972 the area was rototilled; developing herbaceous
vegetation was sprayed with amitrole in early fall. In early 1973,
24 6 x 6 m plots were established. Plots were separated by 2 m wide
unplanted strips. The treatments were species (black cottonwood and
red alder), level of pulp and paper mill sludge (0, 225, and 450
ovendry t ha ··I ) , and level of irrigation (none and irrigated). There
were two replications of each treatment (Table I); treatment assign­
ment to the plots was random.
Each plot consisted of five rows of nine trees. Trees within a row
were planted at 0.6-m intervals with rows 1.2 m apart. Red alder
seedlings and black cottonwood cuttings were planted in March 1973.
The alder seed and cottonwood cuttings were collected in natural
stands located within 30 km of the study area. Collections were made
"
from severa! plants per species; thus, the planting material used was
not clonal. At the time of planting, the container-grown alder seed­
lings were 15-25 em tall and about 6 months old. Cottonwood
cuttings were 60 em long and planted to a 40 em depth. Initial survival
of the alder was excellent, but mortality of cottonwood was high
during the I st year in most of the sludge-treated plots. All the cotton­
wood cuttings (dead or alive) were replaced in February 1974 with
new cuttings. Weed vegetation in the plots was hoed and mowed
during the first two growing seasons. The buffer strips between the
plots were mowed for the first 4 years.
The sludge used in this study came from a Crown Zellerbach pulp
and paper mill at Camas, Washington. It was composed primarily
(80%) of short wood fibers that had fallen through paper machine
screens. The other 20% included soil material brought into the mill
with logs, and various additives (e.g., aluminum sulfate, resins, and
clays) used in the pulping and papermaking processes. Previous work
(Aspitarte et al. 1973) had shown that the sludge had a carbon­
nitrogen (C; N) ratio of more than 200: I and had evaluated the growth
of agricultural crops under three sludge treatments (450, 900 , and
1350 ovendry t ha -I , each modified by additions of N fertilizer to
C:N ratios of 10: I, 50: I. and 100: I. The best 1st-year growth of
crops receiving sludge treatment was obtained in the 450 t·ha ·I treat­
ment with a C:N ratio of 100: I. Thus, nitrogen fertilizer was applied
to the 225 and 450 t · ha -I sludge-treated plots in amounts sufficient
to reduce the C:N ratio to 100: I (1450 kg N/ha in low-sludge plots,
and 2900 kg N/ha in high-sludge plots). In addition, fertilizers con­
taining 50 kg P/ha and 100 kg of K/ha were applied to low-sludge
plots and 100 kg P/ha and 200 kg K/ha to high-sludge plots. Plots
without sludge were fertilized with the equivalent of 170, I00, and
200 kg/ha of N, P, and K, respectively. Sludge and fertilizers were
·
·
Height (m)
Red alder
Nonirrigated plots
No sludge*
225 t · ha- 1 sludge
450 t·ha- 1 sludge
Irrigated plots
No sludge
225 t·ha 1 sludge
450 t·ha - 1 sludge
Plot establishment and mailllenance
845
Irrigation
No
No
No
Yes
Yes
Yes
Black cottonwood
coppice harvest
Sludge
(t·ha-1)
I*
2
3
4
0
225
450
0
225
450
3.3
4.5
4.7
4.4
4.6
5.0
4.3
4.7
4.3
4.8
4.2
5.0
4.3
4.8
4.4
4.7
4.3
5.0
4.1
4.3
3.5
4.2
3.7
4.3
Red alder
coppice harvest
3.8
3.7
3.1
3.6
3.7
3.7
•coppice material was harvested at 2-year intervals beginning in
2
3
4
3.8
3.6
3.2
3.8
3.3
3.2
4.1
3.9
3.9
4.4
3.6
3.8
3.8
3.4
3.0
3.4
3.2
3.3
1976.
TABLE 3. Mean diameter of the tallest stem per plant by species,
coppice harvest, and treatment
Diameter (em)
Treatment
Irrigation
No
No
No
Yes
Yes
Yes
Black cottonwood
coppice harvest
Sludge
(t·ha -I )
I*
2
3
4
0
225
450
0
225
450
3.0
3.8
4.0
3.7
4.3
4.1
3.1
3.3
3.1
3.4
3.2
3.4
2.7
3.2
2.8
2.9
3.0
3.1
2.5
2.7
3.4
2.8
2.6
2.6
Red alder
coppice harvest
2.9
3.1
2.5
2.8
2.8
3.1
*Coppice material was harvested at 2-year intervals beginning in
2
3
4
2.7
3.0
2.4
2.9
2.6
2.4
2.9
2.8
3.1
3.2
3.2
3.0
2.9
2.7
2.2
2.8
2.8
2.6
1976.
rototilled into the soil prior to planting. No additional sludge or fertil­
izer was applied in subsequent years.
Half of the plots at each sludge level were irrigated. The water used
for irrigation came from a small pond adjacent to the study area;
because the study area is on an island in the Columbia River, the
irrigation water would be similar in most characteristics to river water.
Plots receiving the irrigation treatment were watered three to four
times per week during the first two growing seasons. Water was
applied using overhead sprinklers. None of the plots were irrigated
after the first 2 years.
Coppice growth was established by cutting all plants 15 em above
the ground in March 1975. Two-year-old coppice or sprout growth
was harvested in late winter following the 1976, 1978, 1980, and 1982
growing seasons (hereafter referred to as the first, second, third, and
fourth coppice harvests). All cutting was done at the same or slightly
higher height as the original cutin 1975.
Growth and yield measurements and analyses
The outside row on each side of each plot was used as a buffer strip
and just the 21 interior plants on each plot were measured at each
harvest. Height and diameter of the tallest sprout and the number of
sprouts greater and less than I em in diameter were tallied for each
plant. Diameter was measured 10 em above the level of sprout origin.
The measured plants were then cut, bundled, and weighed to deter­
mine green yields by plot. At each harvest, subsamples of fresh
material from both species were uried to constant weight at 65°C to
determine the ratio between green and ovendry yields. Two-year green
yields per plot were then converted to mean annual ovendry yields per
hectare. Yields were expressed on a per hectare basis to facilitate
comparisons with yields from other studies; caution is recommended,
however, in extrapolating from yield figures based on small plots with
narrow buffer strips to yields obtainable from large-scale plantings.
CAN. J. FOR. RES. VOL.
R46
TABLE 4. Mean number of sprouts greater than I em in diameter, by
species, coppice harvest, and treatment
14. 1984
TABLE 5. Ovendry yield by species, coppice harvest, and treatment
(harvests were at 2-year intervals)
No. of sprouts per plant
Black cottonwood
Treatment
Irrigation
No
No
No
Yes
Yes
Yes
I*
2
3
4
0
225
450
0
225
450
1.4
1.5
1.8
1.6
1.8
1.8
3.6
2.9
5.0
3.4
3.8
4.9
3.5
4.9
5.4
6.5
6.3
6.4
7.8
6.9
8.6
tCoppicc material
3.4
was haptc\.lc<i' at
5.6
4.4
•
Treatmen t
Red alder
coppice harvest
coppice harvest
Sludge
(t·ha ·I)
Ovendry yield (I·ha -I year-1)
1.8
1.9
1.0
2.2
2.0
2.4
2
3
4
4.7
4.4
4.1
4.9
4.6
3.7
3.9
5.1
5.8
5.1
6.3
4.3
2.6
3.0
3.8
5.5
3.4
3.4
2-year inlervals begmning in llJ76.
Sludge
Irrigation (t· ha- 1 )
No
No
No
Yes
Yes
0
225
450
0
225
450
Yes
•coppice malerial
was
Black cottonwood
coppice harvest
I*
5.7
8.4
10.8
8.6
9.8
13.8
Red alder
coppice harvest
2
3
4
9.9
6.6
5.7
6.1
7.6
6.0
7.9
7.8
5.8
6.7
7.4
5.7
5.3
4.2
6.4
6.2
7.2
5.1
6.1
3.8
4.8
2.7
3.2
9.6
9.6
13.0
10.4
12.1
3
4
7.1
6.6
5.2
4.2
6.2
5.1
3.8
6.9
4.3
3.5
2.3
2
4.0
2.9
2.0
harvc,lcd at l.-year intervals beginning in 1976.
TABLE 6. Significance ofF values for the treatment sources of variation, by variable and harvest
Sources of variation
Species
X
Variable
Height of the
tallest sprout
per plant
Diameter of the
tallest sprout
per plant
Number of sprouts
per plant
greater than
l cm
Yield per plot
Coppice
harvest
I
2
3
4
2
3
4
Species
Species
Sludge
sludge
X
X
X
X
Species
Irrigation
Sludge
irrigation
sludge
irrigation
irrigation
0.001
0.001
0.008
0.017
0.027
0.422
0.521
0.956
0.069
0.200
0.822
0.460
0.184
0.199
0.804
0.705
0.004
0.122
0.366
0.906
0.261
0.023
0.177
0.282
0.101
0.366
0.549
0.720
0.001
0.001
0.240
0.515
0.094
0.351
0.115
0.749
0.079
0.164
0.191
0.836
0.238
0.398
0.680
0.368
0.099
0.253
0.641
0.174
0.935
0.136
0.H80
0.694
0.149
0.861
0.209
0.129
0.242
0.985
0.372
0.109
0.578
0.253
0.848
0.662
0.136
0.465
0.200
0.589
0.382
0.602
0.422
0.189
0.581
0.276
0.127
0.372
0.413
0.002
0.080
0.042
0.294
0.054
0.044
0.466
0.824
0.001
0.553
0.249
0.518
0.174
0.491
0.844
0.744
0.962
0.952
0.802
0.958
I
2
3
4
0.117
0.006
0.053
0.225
0.262
0.650
I
2
3
4
0.001
0.001
0.002
0.001
0.001
0.213
0.466
0.991
0.148
The data collected at each harvest were analyzed using three-way
and did not appear to be related to sludge or irrigation. Height
analysis of variance (SPSS Manova program; Hull and Nie 1981 ) ; the
analyses were done separately for each harvest. Treatment effects
were considered significant at p :s 0.10.
of both species in all treatments was substantially less at the
fourth harvest than at the third harvest, and height at the fourth
Results and discussion
general pattern as height (Tables 3 and 6). The similarity
in response of diameter and height to treatment was not
Height of the tallest sprout per plant differed between species
and was intluenced by the initial applications of sludge or
irrigation (Tables 2 and 6). Cottonwood sprouts were signifi­
cantly taller than alder sprouts at all harvests. Irrigation signifi­
cantly increased height at the first coppice harvest. In addition,
the effects of sludge and the species-sludge interaction were
significant at the first harvest when sludge increased the height
of cottonwood plants and either decreased or had no effect on
the height of alder plants. The sludge-· irrigation interaction
was significant at the second harvest. Most of the alder plants
had their maximum height at the third harvest, but the harvest
at which cottonwood treatments had their greatest height varied
harvest was generally the lowest of any harvest.
Diameter of the tallest sprout per plant followed the same
unexpected as height and diameter are correlated. There were
significant differences in diameter between the two species at
the first two harvests with cottonwood plants larger than alder.
Effects of irrigation and sludge were significant only at the first
harvest. The species-sludge interaction was significant at the
first harvest: cottonwood diameters were increased by sludge
and alder diameters were unaffected. In the cottonwood plots,
diameter was usually greatest at the first harvest and decreased
with each successive harvest. In the alder plots without sludge;
diameter varied little among harvests. Alder plots with sludga
did not exhibit a consistent trend of increasing or decreasing
HARRINGTON AND DEBELL
·
Fro. I. Stump morphology at the time of the fourth coppice harvest. (A) Black cottonwood. (B) Red alder.
847
848
CAN.
J.
FOR. RES. VOL. 14, 19&4
TABLE 7. Mortality by species,
Black co!!onwood (no. dead)
T rea tment
Cumulative
mortality
(1974-1982)
Sludge
Irrigation
(t ha ' )
No
No
No
·
0
225
450
0
225
450
Yes
Yes
Yes
1979
1980
1981
1982
(%)
I
0
4
11.9
I
I
I
5
35.7
35.7
I
4
2
2
0
2
4
4
2
8
5
0
1974
1975-1976
1977-1978
0
0
0
0
0
0
0
0
0
6
0
0
0
0
0
0
5
I
6
5
I
I
2
28.6
26.2 38.1 diameter with harvest number; however, the alder sludge plots
between the third and fourth harvests. Alder plots with sludge
had more variable mean diameters than alder plots without
·
sludge.
have generally had declining yields; alder plots without sludge,
however, had increasing or stable yields for the first three
At the first coppice harvest, the alder and cottonwood stumps
harvests. At the fourth harvest, yields on most alder piots were
looked quite similar. Both species sprouted along the stem,
although cottonwood sprouts tended to originate higher on the
stump than did alder sprouts. At the first harvest the mean
number of sprouts greater than I em in diameter was similar for
the two species and was significantly higher on irrigated than
ern
Mortality was minor the first few years but began increasing
1976 h rvest (Table 7). Some of the mortality from
1976 to 1979 appeared to be related to sludge, especially on the
after the
alder plots treated with sludge. Most of the alder mortality
6). The number of small
occurred when stumps did not resprout following harvest (i.e.,
in diameter) averaged I 3. 3 per
odd-numbered years). Mortality of cottonwood, however, was
on nonitTigated plots (Tables 4 and
sprouts per plant (less than I
markedly reduced.
plant and was not significantly affected by treatment.
At each succeeding harvest, the differences in stump appear­
fairly evenly divided between stumps that did not resprout
following harvest and stumps that sprouted but later died. Mor­
ance between the two species became more pronounced. Black
tality was generally greater on plots that included sludge appli­
cottonwood coppice always originated in new wood (i.e.,
cations. For cottonwood the greatest cumulative mortality was
toward the base of the sprouts from the previous coppice cycle):
in the plots that had the highest yields; thus, this mortality may
this caused the stumps to develop an inverted-pyramid shape
have been related primarily to competition. Anderson ( 1979)
with a fairly narrow stump at ground level and an increasingly
found that rootstock mortality increased with planting density
wider cross-sectional area at the height of the harvest cuts (i.e.,
in hybrid poplar coppices and, therefore, also suggested that
at 15 em, see Fig. lA). At the fourth harvest, cottonwood
competition was involved. The reasons for the apparent associ­
ation between mortality of red alder and the sludge treatment
stumps had an average of 7. I sprouts that were greater than
I em and 4. 1 sprouts less than I em. Red alder sprouts, in
are unknown; it was not competition related, however, as the
contrast, originated in old wood, predominantly at or near the
highest mortality was associated with the treatments having the
root collar. The alder stumps therefore had a greater cross­
lowest yields. In fact. the high mortality on alder plots with
sectional area at ground line than the cottonwood stumps, but
sludge treatments was probably a major factor in the steadily
declining
decreased substantially. Mortality was often clumped leaving
change much in size with succeeding harvests. At the fourth
portions of the aboveground growing space unutilized.
harvest, alder plants averaged 4.8 large sprouts and 2.0 small
sprouts per stump. The differences in sprout numbers between
species were significant at this last harvest.
Effects of treatments on ovendry yield were more clearly
yields.
Once
mortality
exceeded
yields
a much smaller cross-sectional area at the height where the
sprouts were harvested (Fig. I B). The alder stumps did not
50%,
Implications
This study has demonstrated that it is possible to grow trees
on land also used for disposal of pulp and paper mill sludge.
defined thim on any other measured plant variable (Tables 5
Initial survival of black cottonwood on sludge-treated plots was
6). The two species had significantly different yields at all
poor, however, perhaps indicating this combination of primary
and
harvests. Sludge effects were significant at the first three har­
sludge and fertilizer should he aged or weathered before
vests and the species-sludge interaction was significant at the
planting. Once they were established, black cottonwood plant
first harvest. Irrigation was significant only at the first harvest;
had increased growth with initial sludge-fertilizer applica·
the species-in·igation interaction was significant at the first
tions. Early survival of red alder was not affected by the sludge
and second harvests. In general, cottonwood yields were higher
treatments but growth was reduced. Sludge from most pulp and
than those of alder, and irrigation and sludge increased yields
paper mills is now processed differently and has a much lower
have declined over time; cottonwood yields on the no- and
C: N ratio; thus, large amounts of N fertilizer are not con­
were to he used for
sludge Jisposal in the future, black cottonwood should cer·
tainly be considered a candidate species. In this study, black
low-sludge treatments were greatest at the second harvest.
cottonwood clearly demonstrated its ability to produce high
Anderson ( 1979) also reported short-rotation .yields to be
yields in association with sludge application and to vigorously
of black cottonwood. Sludge decreased alder yields at all har­
vests, and effects of irrigation were minor. Cottonwood yields
on high-sludge treatments were greatest at the first harve ;t and
sidered necessary. If this or similar sites
greatest at the .first or second harvest and to decline over time.
resprout following repeated short growing cycles.
Within a treatment, cottonwood yields did not vary much
testing of red alder on other sites and with other treatments
Future
HARRINGTON AND DEBELL
849
treatment, and growing season
Red alder (no. dead)
1973-1974
1975-1976
1977-1978
1979
1980
1981
1982
Cumulative
mortality
( 1973-1982)
(%)
0
I
2
5
0
0
0
0
I
I
4
5
7
II
5
3
9
6
8
19
13
9
15
14
2
0
2
0
0
7
3
3
2
3
I
I
0
0
4
2
0
5
54.8
78.6
71.4
59.5
76.2
71.4
should be done, however, as the relative performance of the
two species is site specific. Further work with other cotton­
wood sludge treatments appears warranted as the black cotton­
wood yields obtained on sludge plots at the first two harvests
(8 .4- 13.8 t ha -I year-1) were higher than those that have
been reported by researchers testing other treatment combina­
tions (Heilman et al. 1972; Heilman and Peabody 1981).
Irrigation during the first two summers had surprisingly
long-lasting effects on plot yields, especially on the cotton­
wood plots. Growth during the establishment period was
significantly greater for both cottonwood and alder (Harrington
et al. 1979). At the first coppice harvest, irrigation was highly
significant in its effects on yield of both species. Irrigation was
still significant in its effects on cottonwood yield at the second
coppice harvest. Later cottonwood harvests still had greater
yields on formerly irrigated plots, but the differences were no
longer statistically significant. The long-lasting effects of irri­
gation during establishment warrant further research. If similar
carryover effects are found on other sites, they could influence
the profitability of irrigation, especially irrigation during the
establishment period. In addition, the effects of continuing
irrigation beyond the establishment period need to be
evaluated.
The short coppice cycle of 2 years used in this study was
selected to ensure that plants did not grow close to overhead
power lines. This study has demonstrated that repeated 2-year
cycles are possible and there may be other specialized situ­
ations when short cycles would be useful. Even with a 2-year
coppice cycle, our 0.6 x 1.2 m planting density was probably
greater than necessary to achieve full site occupancy. Yields
from the third harvest for alder plots without sludge were equal
to or better than yields in the first harvest, despite the fact that
mortality had progressed from 5 to 40%. As well, there wasn't
a strong relationship between mortality and yield of black cot­
tonwood. We therefore suspect that the initial planting density
of both species could have been reduced substantially (i.e., as
much as 50%) without markedly affecting yields. Certainly,
wider spacings would be more appropriate for longer rotations.
The use of both longer rotations and lower planting densities
could result in equal or greater yields being obtained with lower
establishment costs. In general, short-rotation intensive culture
research has focused on short coppice cycles (5 years or less).
If such systems are to be implemented on a broad scale in the
Pacific Northwest, however, we suspect lower planting densi­
ties with longer rotations (10-15 years) will be adopted.
·
•
Longer cycles might be more productive for both species (c.f.
Heilman and Peabody 1981; Zavitkovski et al. 1979). In addi­
tion, material harvested after longer rotations would not require
the specialized equipment necessary for efficient harvesting of
small-diameter material. This would allow use of a wider range
of terrain.
Acknowledgements
This study was established while one of the authors (D. S.
DeBell) was employed by Crown Zellerbach Corporation,
Camas, Washington. Several Crown Zellerbach employees
from Central Research Division at Camas and Forestry
Research, Wilsonville, Oregon, helped in study establishment
and measurement; their assistance is gratefully acknowledged.
This work was supported, in part, by an Interagency Agree­
ment with the United States Department of Energy. We thank
James Wilcox, Forestry Sciences Laboratory, Olympia,
Washington, for his assistance and patience in data analysis.
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a hybrid cottonwood minirotation as intluenced by spacing and
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