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BOT. GAZ. 140(Suppl.): S97-S101. 1979.
Copyright is not claimed for this article.
GROWTH AND NITROGEN RELATIONS OF COPPICED BLACK COTTONWOOD
AND RED ALDER IN PURE AND MIXED PLANTINGS
D. S. DE BELL AND M. A. RADWAN
Forestry Sciences Laboratory, Pacific Northwest Forest and Range Experiment Station, USDA Forest Service, Olympia, Washington 98502 Growth and nitrogen status were assessed in pure and mixed plantings of black cottonwood and red
alder. Annual dry-matter production of 2-yr-old coppice (stump sprouting) in the mixed plots was higher
than production in pure cultures of cottonwood and alder. Twigs of cottonwood from mixed plantings con­
tained 18% more nitrogen than twigs from pure plots. Soil collected in the mixed cottonwood-alder and
pure alder cultures contained, respectively, 9% and 23% more nitrogen than soil from the pure cottonwood
planting. For the upper 15 cm of mineral soil, average annual nitrogen accretion under the mixed stand
was estimated at 32 kg/ha and under pure alder, 80 kg/ha. The beneficial effects of red alder in pure and
mixed stands, obtained in such a short time, may have application for silage cellulose production, other
short-rotation forest systems, and for proposed "biomass farms for energy. "
The climate is humid with most of the annual
precipitation (120 cm) occurring as rain during the
winter. The soils are silty clay loams of alluvial
origin. Native vegetation on the island includes
black cottomvood, willow (Salix spp.), teasel (Dip­
sacus spp.), Canada thistle (Cirsium arvense [L.J
Scop.), nettle (Urtica spp.), and several grass and
other herbaceous species. Alder does not occur nat­
urally on the island but is very common in the
general area.
Trees in the immediate vicinity of the study area
had been removed several years previously for a
powerline right-of-way. During the year prior to
planting, the area was rototilled in spring, and
developing herbaceous vegetation was sprayed with
Amitrol in early fall. In February 1973, the area
was again rototilled, and urea and 0-20-20 fertilizers
were applied in amounts equivalent to 168 kg N
and 224 kg P205 and K20 per hectare.
PLOT ESTABLISHMENT AND MAINTENANCE.-Plots
were 6.1 X 6.1 m and were surrounded by a lane
1.8 m wide. Treatments were pure red alder, pure
black cottonwood, and a mixture of alder and cotton­
wood planted in alternating spots within rows. Two
replications of each treatment were planted at 0.6 X
1.2-m spacing, and each plot contained five rows,
each with nine plants. Alder seedlings and cotton­
wood cuttings were planted in March 1973; the
container-grmvn alder seedlings were 15-25 cm tall
and about 6 mo old, and cottonwood cuttings were
60 cm long and planted to a 40-cm depth. Alder
survival was excellent, but first-year mortality was
high in some cottonwood plots. Therefore, cotton­
wood was replanted in the two pure cottonwood
plots and in one of the mixed-species plots in
February 1974. The plots were hoed and mowed to
reduce weed competition during the first and second
growth seasons. Coppice growth was established by
cutting all plots back to 15-cm stumps in January
1975 and allowing them to sprout. The 2-yr-old
coppice was harvested in January 1977.
Introduction Interest in intensive culture of hardwood coppice
(stump sprouts) on short rotations has increased
steadily since the "silage sycamore" concept was
proposed by McALPINE et al. (1966). In essence,
such culture involves establishing plantations at
dense spacings of 2 X 2 m or closer and harvesting
on cutting cycles of 10 yr and less. Stump sprouting
provides for several successive crops. In this system,
the goal is maximum fiber or biomass production.
During the early 1970s, research in the Northwest
suggested that two native hardwoods, black cotton­
wood (Populus trichocarpa Torr. & Gray) and red
alder (Alnus rubra Bong.), would be particularly
suitable for intensive coppice management (DEBELL
1975). Both species have rapid juvenile growth
rates and good fiber properties. High yields have been
reported for densely spaced cottonwood plantations
(HEILMAN et al. 1972) and for natural thickets of
red alder. Red alder appeared especially attractive
because of its capacity to fix atmospheric nitrogen
and the beneficial effect it may have on other species
in mixed culture (TARRANT 1961). TARRANT and
TRAPPE (1971) suggested that wood fiber production
might be substantially increased by using mixtures
of red alder and black cottonwood.
To evaluate cultural possibilities with these spe­
cies, we established black cottonwood and red alder
in pure and mixed plantings in 1973 on land suitable
for coppice management. A preliminary examination
of initial coppice yields harvested 4 yr later revealed
that mixed plantings substantially outproduced pure
cultures of either species. We, therefore, made a
more detailed analysis of growth and yield charac­
teristics of the two species and assessed the nitrogen
status of plants and soils in pure and mixed cultures.
Material and methods
STUDY AREA.-The study area was located on
Lady Island in the lower Columbia River near
Crown Zellerbach's mill at Camas, Washington.
S97
BOTANICAL GAZETTE
S98
GROWTH AND YIELD MEASUREMENTs.-Height and
diameter were measured annually on plants selected
from the 21 interior plants on each plot. On plots
with only one species, every other plant was mea­
sured; on mixed plots, data were collected on 10
plants of each species. Thus, 10 plants were measured
in pure plots and 20 plants were measured in mixed
plots.
During the harvest of the 2-yr-old coppice in
January 1977, (1) total fresh weight of all coppice
(stems and branches minus leaves) on the central
21 plants, (2) moisture content of the coppice based
on a representative subsample, and (3) number of
sprouts in various diameter classes were measured.
From this information and from height and diameter
measurements, the relative production of alder and
cottomvood in both pure and mixed cultures ,vas
determined.
PLANT NITROGEN STATuS . -Ten vigorous terminal
twigs, 12-15 cm long and 3-5 mm in diameter, were
cut in January 1977 from five randomly selected
plants from the interior of each plot. Twigs from
each plot were combined, and after oven drying to
constant weight at 65 C, the samples were ground
in a Wiley mill to pass through a 40-mesh sieve. The
ground samples were analyzed for total nitrogen
content by standard micro-Kjeldahl technique.
SOIL NITROGEN STATus.-Mineral soil was sampled
in February 1977 to a 15-cm depth at nine repre­
sentative locations in each plot. After thorough
mixing, a portion of the composite sample was
placed in a plastic bag, sealed, and placed in a
portable cooler with dry ice. The soils were thawed
and passed through a 2-mm stainless steel sieve. A
portion of each sample was extracted fresh with
2N KCI and analyzed for ammonium and nitrate
nitrogen by semimicro-Kjeldahl and steam-distilla­
tion methods (BREMNER 1965). Another soil portion
was dried at 65 C and used to determine total
nitrogen by micro-Kjeldahl procedure. The remain­
der of each sample was used for moisture determina­
tion by drying to constant weight at 105 C.
STATISTICAL ANAL YSES . -Annual height growth,
annual diameter growth, 1976 height, 1976 diameter,
number of sprouts, and twig nitrogen were analyzed,
using separate one-way analysis of variance for each
species. In each of these analyses, the compared
treatments were pure culture and mixed culture.
Information on other variables such as fresh weight,
moisture content, and soil nitrogen was collected on
a plot basis. The three treatments (pure cottonwood,
pure alder, mixed cottonwood/alder) were compared
using analysis of variance followed by Tukey's test
(MENDENHALL 1968) when differences were signifi­
cant at the 5% level.
Results and discussion
ANNUAL HEIGHT AND DIAMETER GROWTH. - COt­
tonwood height growth during the first rotation
[MARCH (SUPPL.)
averaged about 1.3 m/yr and d id not differ signifi­
cantly between pure and mixed cultures (table 1).
Height growth of cottonwood coppice was substan­
tially higher, ranging between 1.48 and 2.62 m/yr.
In mixed plantings, height growth during the first
(1975) and second (1976) coppice-growing seasons
was significantly greater than growth in pure culture
(table 1). Although first-year height growth of alder
seedlings was only 0.36 and 0.37 m in pure and mixed
culture, growth in the second through fourth growing
seasons averaged nearly 2 m/yr (table 1). No con­
sistent differences in alder height growth between
pure and mixed cultures were detected.
The effects of mixed culture on diameter growth
of the two species are similar to those observed for
height growth (table 2). First-year diameter growth
of cottonwood averaged 1.2 cm in the pure and
mixed plots. The apparent beneficial effect of the
species mixture on second-year (1974) diameter
growth of cottonwood in plot 25 is undoubtedly due
to the fact that cottonwood in this plot had not been
replanted that spring, as it had been in the pure plots
and the remaining mixed plot (no. 26). Following the
initial harvest, annual diameter growth of the tallest
cottonwood coppice in the mixed plantings averaged
2 cm and was significantly greater than growth in
pure plantings. Earlier diameter growth differences
between mixed plots due to root-stock age were no
longer apparent at the end of the second year of
coppice growth. Diameter growth of red alder
seedlings was only about 0.6 cm during the first
growing season. Growth in subsequent years was
substantially higher, but mixed culture had no de­
tectable influence on diameter growth of alder.
YIELD AND SPROUT CHARACTERISTICS OF 2-YR-OLD
TABLE 1
ANNUAL HEIGHT GROWTH OF BLACK COTTONWOOD AND
RED ALDER IN PURE AND MIXED PLANTINGS
SPECIES, TREATMENT,
AND PLOT NO.
FIRST ROTATION
(m)
------1973
Cottonwood, pure culture:
2. . .. .. . .. . .. . .. . . . . . . 1. 82
6. . . .. . . . . .. . . . ... . .. . . 80
Average. . . .. . ......... 1.31b
Cottonwood, mixed culture:
25.. . ...... ...... . .. . . 1. 52
26. . . .... . . ...... .. ... 1. 10
Average.. . . . . . ........ 1. 31b
Alder, pure culture:
. 29
8.. . .. .. . . . . . . . . . . .. . .
. 42
9.. . .. ... ..... ... . ... .
. 36x
Average... , . . . ... . .. ..
Alder, mixed culture:
25 . . . .. . . .. . . . . .. ... . . . 37
. 37
26. . .. .. . . . .. . ........
. 37x
Average.. . . ... ... . ... .
COPPICE ROTATION
(m)
----1976
1974
1975
1. 52
. 99
1 . 26b
1. 91
1.55
1. 73c
1. 66
1.31
1. 48c 1 . 80
.84
1 . 32b
2. 66
1. 85
2.26b
2. 67
2. 58
2. 62b
1 . 63
2. 19
1. 91x
1. 58
1. 94
1. 76x
2. 10
1. 99
2. 04x
2 . 33
1. 69
2.01x
1.92
1. 73
1. 82x
1. 94
2.35 2. 14x
NOTE.-For each species, treatment means within a column followed by a
common letter are not significantly different at the 5% level. Comparisons
are not made between species.
S99
DEBELL & RADWAN-COTTONWOOD/ALDER
1979]
COPPlcE. -Fresh-weight yields (shoots minus leaves)
of the coppice averaged about 41,000 kg/ha in the
mixed-species plots, but were only 25,000 kg/ha
in pure plots of both species (table 3). The moisture
content (50%) of red alder was slightly lower (P ::;
.15) than that of black cottonwood (54%). As a
result, dry-weight yields (shoots minus leaves) for
pure alder plots averaged about 1,000 kg/ha higher
than for pure cottonwood plots. Dry-weight yields
for mixed-species plots averaged 19,510 kg/ha, rep­
resenting an average ,yield increase of 71% when
compared with pure cottonwood or 55% when com­
pared with pure alder. Expressed on a per annum
TABLE 2
ANNUAL DIAMETER GROWTH OF BLACK COTTONWOOD
AND RED A LDER IN PURE AND MIXED PLANTINGS
FIRST ROTATION
(em)
SPECIES, TREATMENT,
AND PLOT NO.
Cottonwood, pure culture:
2 .....................
6.....................
Average.. . . . .. ... . . . . .
Cottonwood, mixed culture:
25....................
26....................
Average . . . ......... ...
Alder, pure culture:
8.....................
9.....................
Average. .. . .... . . . . . . .
Alder, mixed culture:
25....................
26....................
Average. .... .. . . . . ....
COPPICE ROTATION
(em)
1973
1974
1975
1976
1.7
.8
1.2b
1.2
.9
1.0 c
1.6
1.1
l 4c
1.8 1.4 1.6c 1.4
1.1 1.2b
2.8
.9
1.8b
2.3
1.6
2.0b
2.0 2.0
2.0b .4
.8
.6x
1.7
2.1
1.9x
1.6
1.9
1.8x
1.0
1.3
1.2x
.6
.6
.6x
1.6
2.6
2.1x
1.6
1.8
1.7x
1.2
1.5
1.3x
,
basis, dry-matter production averaged 5,720, 6,280,
and 9,755 kg/ ha per year, respectively, on the pure
cottonwood, pure alder, and mixed-species plots.
Although there was considerable within-treatment
variation, productivity of both mixed-species plots
was higher than that of any pure plot, and the
treatment means differed significantly at the 20%
level.
Sprout numbers and size (table 4) provide addi­
tional insight on structure and productivity of pure
and mixed plots for the 2-yr-old coppice rotation.
These data are useful in understanding effects of
mixed culture on individual plant performance of
each species and also in evaluating possible effects of
differences in age of cottonwood root stocks in the
two mixed-species plots.
Though all coppice was the same age, there were
some residual effects of older root stocks on cotton­
wood plants in plot 25. Such root-stock effects were
most apparent in numbers of sprouts per stump
(table 4). Cottonwood in plot 25 averaged 28.9
sprouts per stump, with 3.1 of these being greater
than 1.2 cm in diameter. Cottonwoods in plots 2, 6)
and 26 replanted in the second year, however,
averaged only 12.2-14.7 sprouts per stump and fewer
attained 1. 2 cm diameter. Average sprout numbers
on coppiced alder plants ranged from 13.9 to 16.5 on
plots 8, 9, and 26, but were only 8.0 on plot 25.
Perhaps the increased vigor of cottonwood plants in
that plot reduced sprout development or survival of
the associated alder.
Alder had beneficial effects on height and diameter
of cottonwood grown in association with it (table 4) .
TABLE 4
NOTE.-For each species, treatment means within a column followed by a
common letter are not significantly different at the 5% level. Comparisons
are not made between species.
SPROUT CHARACTERISTICS OF 2-YR-OLD COPPICE OF
BLACK COTTONWOOD AND RED ALDER
TABLE 3
SPROUTS/STUMP
YIELD OF 2-YR-OLD COPPICE OF BLACK COTTONWOOD
Diameter
>1.2
em
(no.)
Height
(m)
Diameter
(em)
14.5 14.7 Average. . .. . . .. . . . ... . 14.6c
1.9
1.0
1.4b
3.57
2.86
3.22c
3.4
2.6
3.0c
25....................
26....................
28.9
12.2 20.6b
3.1
.7
1.9b
5.24
4.52
4.88b
4.4
3.6
4.0b
8..................... 13.9
9..................... 16.5
Average. . .. . .. . . . . . . . . 15.2x
1,4
1.7
1.6x
3.68
3.94
3.81x
2.6
3.2
2.9x 25.................... 8.0
26.................... 14.2 Average.... . .. . . . . .. . . 11.1y
.9
2.3
1.6x
3.86
4.17
4.02x
2.8
3.3
3.0x AND RED ALDER IN PURE AND MIXED PLANTINGS
DRY WEIGHT YIElD
FRESHWEIGHT
YIELD
(kg/ha)
M or STUllE
CONTENT
(%)
Total
(kg/ha)
Mean annual
production
(kg/ha/yr)
2...........
6 ...........
29,890 20,040 24,870
55
53
54
13,450
9,420
11,440
6,725
4,710
5,720
8...........
9...........
21,040
29,020
25,030
51
49
50
10,310
14,800
12,560
5,155
7,400
6,280
SPECIES
COMPOSITION
AND PLOT NO.
Black cottonwood:
Average/ plot
Red alder:
Average/ plot
Mixed alder and
cotton­
wood:
25..........
26..........
Average/ plot
SPECIES TREATMENT
AND PI.OT NO.
I
I
----------
Cottonwood, pure culture:
2 .....................
6.....................
Cottonwood, mixed culture:
47,230 35,040
41,140
Total
(no.)
TALLEST SPROUT
53
52
52
22,200
16,820 19,510
11,100
8,410
9,755
Average. . .. . . ... ......
Alder, pure culture:
Alder, mixed culture:
NOTE.-For each species, treatment means within a column followed by
a common letter are not significantly different at the 5% level. Comparisons
are not made between species.
BOTANICAL GAZETTE
S100
The largest cottonwood sprouts were produced in
plot 25, and their size is partially related to root­
stock age. Among the replanted plots with root
stocks of the same age, cottonwoods in mixed-species
plot 26 averaged 40% taller and 20% larger in
diameter than plants in pure plots 2 and 6. Mixed
culture, however, had no significant effect on height
and diameter of alder.
The higher yields obtained in the mixed plantings
result primarily from enhanced growth of cotton­
wood. The increased cottonwood growth had no
adverse impact on gwwth of associated alder. Sprout
numbers, however, were lowest for alder grown in
mixed plot 25. Because cottonwood root-stock age
also had some influence on growth and yield in
mixed-species plot 25, data from plot 26 provide a
more appropriate indication of the benefits of ad­
mixed alder per se to cottonwood gwwth. Thus,
yield increases associated 'with mixed-species culture
might be estimated at 34%-47% above yields ob­
tained in pure alder and cottomvood culture, re­
spectively.
PLANT NITRO GEN.-Nitrogen contents of alder
twigs from the pure and mixed plantings were
higher than those of cottonwood twigs (table 5).
Alder-twig nitrogen in the pure alder plantings
(1.44%) was similar to that in the mixed planting
(1.49%).
Beneficial effects of alder were reflected in nitrogen
concentrations of the cottonwood twigs. On the
average, twigs from cottonwood sprouts growing in
pure culture contained 1.09% nitrogen, while cotton­
wood twigs from the mixed planting had 1.29%
nitrogen, an increase of 18%, which was statistically
significant. Similar findings were obtained by PLASS
TABLE 5
[MARCH (SUPPL.)
(1977) for foliar nitrogen of several pine and hard­
wood species planted with and without European
black alder (Alnus glutinosa [L.] Gaertn.) on coal
spoils in Kentucky. Increased nitrogen concentra­
tions in tissues of trees growing in mixture with
Alnus species have also been documented by TAR­
RANT (1961), HE I LMAN (1966), and DE LVER and
POST (1968).
SOI L N ITRO GE N . -The concentration of total ni­
trogen in soil was significantly greater in the pure
alder plots (1,050 ppm) than in the pure cottonwood
plots (855 ppm); values for the mixed-species plots
fell between those for the pure plots (table 6).
Ammonium nitrogen varied little among the plant­
ings (3. 2-4.0 ppm), but the amount of nitrate nitro­
gen in soil beneath pure alder was more than double
that from either pure cottonwood or mixed cotton­
wood-alder plantings.
Other investigators have reported increased soil
nitrogen beneath alder (see review by TARRANT and
TRAPPE [1971]). In general, these studies were con­
ducted in stands much older than 4 yr. LAWRENCE
(1958) and BO LLEN et al. (1969), however, detected
positive effects of Sitka alder (Alnus sinuata [Reg. ]
Rybd.) on nitrogen contents of glacial or avalanche
debris after 5 yr. ZAVITKOVSKI and NEWTON (1968)
reported high increases in soil nitrogen contents in
young (2-14 yr) alder thickets. BO LLEN and Lu
(1968) had shown that soil beneath alder is higher in
nitrate nitrogen and in nitrifying capacity than that
under conifers.
Data on total nitrogen concentrations were con­
verted to amount per hectare using a soil bulk
density value of 1. 1 g/cm3 (1,650,000 kg per is-em
hectare slice). Compared with pure cottonwood, the
amount of nitrogen in the upper 15 em of soil in
TWIG NITROGEN OF B LACK COTTONWOOD
AND RED ALDER IN PURE AND
TABLE 6
MIXED P LANTINGS
NITROGEN CONCENTRATIONS IN SOIL B ENEATH B LACK
S p ecies, treatment,
and plot no.
Cottonwood, pure culture:
2 . . .... .... .. . ... . .. . . ..
6. ,..,,, '" ,,,,.. ...... ,
Average. " ...," ',. ,.. . .
Cottonwood, mixed culture: 25,,,. ... ... ..... .. , .. . .
26. . .. ,. , . .. .,,.... . ... .
Average.,,,. ..,,.... ,,. .
Alder, pure culture:
8.. ... .. . . .. . . .. . .. .. .. .
9" , . ..,. ...," ,.." . . .,
Average. " " " ,.,. ......
Alder, mixed culture:
25, . ... . ... .,.. .,.,. . . . .
26.. ,. ,,,,,. ..... ,. . . ...
Average" ,.. ... ,.,.,." .
Twig nitrogen
(%)
1.08
1,10
1. 09c
1. 30 1. 28
1 .29b
COTTONWOOD, RED ALDER, AND MIXED COTTONWOOD /ALDER P LANTINGS SOIL
Total
(ppm)
SPECIES COMPOSITION
AND PLOT NO.
Black cottonwood:
2" .. , . .... . .. ...... .
6" . ... .,. . ......." ., .
Average,, , , , .... .. . ..
Red alder:
8" .....,. " " "
. ... ,
9" " ,. ,',. ,. ,
, .
Average.. ,.. ... ... . . ,
Mixed alder and cottonwood:
25" ,... .......... .......
26" . ,." ,. " " " " .. . , ,
Average,,,... ... . . .. .. .
"
,
,
,
1. 44 1,44
1,44x
'
,
,
,
.
1. 52
1. 46
1.49x
,
.
.
,
,
'
,
.
.
,
,
,
.
.
.
,
,
,
NOTE.-For each species, treatment means within a
column followed by a common letter are not significantly
different at the 5% level. Comparisons are not made be­
tween species.
,
'
NITROGEN
Ammonium
(pp m)
Nitrate
(ppm)
850
860
8SSs
3,6
3,2
3,4r
0,8
,8
8
1 , 030
1 , 070
1 , 050r
3,2
3,6
3,4r
2,0
2,0
2,0
3,6
4,0
3,8r
,8
890
980
935rs
,8
,8
NOTE,-Treatment means within a column followed by a common letter
are not significantly different (Tukey's test, P " .05),
1979)
DE BELL & RADWAN-COTTONWOOD/ALDER
mixed-species plots was 130 kg/ha greater; in pure
alder plantings the increased nitrogen amounted to
320 kg/ha. Expressed in terms of average annual
accretion rate, this amounted to 32 and 80 kg of soil
nitrogen per hectare per year. Although these values
are lower than the very high rates of up to 300 kg/ha
per year reported by ZAVITKOVSKI and NEWTON
(1968), they are similar to those reported for pure
and mixed stands of red alder (TARRANT and MILLER
1963; TARRANT et aL 1969; BERG and DOERKSEN
1975).
Conclusions
Red alder substantially increased soil nitrogen
content in only 4 yr; average accretion rates under
pure alder and mixed cottonwood-alder were esti­
mated at 80 and 40 kg/ha per year. Although other
workers have also reported effects in a short time on
"new" soils (LAWRENCE 1958; BOLLEN et aL 1969)
or on soils 'which had been deeply scarified by har­
vesting or site preparation (ZAVITKOVSKI and NEW­
TON 1968), our findings reveal that rapid increases
in nitrogen content can also occur on more produc­
tive soils. Thus, additional credence may be placed
in the suggestion that red alder crops grown for
short duration-perhaps 10 yr or less-might be
used to improve soil fertility on lands dedicated
primarily to conifer production.
Dry-matter production of mixed cottonwood-alder
plantings was higher than yields obtained in pure
plantings of either species. The increased yield re-
S101
suits primarily from enhanced growth of cottonwood
and presumably is due to beneficial effects of alder
on nitrogen status as reflected in soil and cottonwood
twigs. Similar mixtures of Alnus and Populus species
have been documented in other countries, especially
in Europe, as compatible and productive (TARRANT
and TRAPPE 1971). Beneficial effects of alder may
have application in coppice management and short­
rotation forests involving other species also. Inter­
planting with alder may have particular importance
in proposed "biomass farms for energy," where nitro­
gen fertilizer will account for a greater expenditure
of energy than the combined inputs of all other
phases of the operation, excepting irrigation (INMAN
1977).
Admixtures and alternate cropping systems involv­
ing alder, therefore, appear to be feasible alternatives
to application of synthetic nitrogen compounds for
improving soil fertility and forest production. Great­
er use of alder in forestry would enhance the net
energy balance and energy efficiencies and possibly
the cost effectiveness of forest management.
Acknowledgments
These plantings were established when D. S.
DEBELL was employed by Crown Zellerbach Cor­
poration. We acknowledge with gratitude the work
of A. M. RODGERS, D. G. ECKMAN, and J. H.
HAGENSEN on plot establishment, maintenance, and
measurement.
LITERATURE CITED
BERG, A. , and A. DOERKSEN. 1975. Natural fertilization of a
heavily thinned Douglas-fir stand by understory red alder.
Oregon State Univ., School Forest., Forest. Res. Lab., Res.
Note no. 56. 3 pp.
BOLLEN, W. B. , and K. C. Lu. 1968. Nitrogen transformations
in soils beneath red alder and conifers. Pages 141-148 in
J. M. TRAPPE, J. F. FRANKLIN, R. F. TARRANT, and G. M.
HANSEN, eds. Biology of alder. Pacific Northwest Forest and
Range Experiment Station, Portland,Oreg. 282 pp.
BOLLEN, W. B., K. C. Lu,J. M. TRAPPE, and R. F. TARRANT.
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LAWRENCE,