Growth, Development, and Yield in
Pure and Mixed Stands of
Eucalyptus and Albizia
Dean S. DeBell, Thomas G. Cole, and Craig D. Whitesell
ABSTRACT. Productivity of
Eucalyptus saligna Sm. plantations is commonly limited by low levels of
available nitrogen (N), and synthetic N fertilizer applications are costly and sometimes impractical;
Albizia falcataria (L.) Fosberg [=
Paraserianthes falcataria (L.) Nielson]. Five ratios of Eucalyptus and Albizia were compared with each
other, with pure Albizia, and with pure Eucalyptus fertilized periodically with N in a randomized block
design on the wet Hamakua coast of the Island of Hawaii. Eucalyptus growth increased as the amount
of Albizia in the stand increased from 11 to 66%, and heights and diameters of Eucalyptus in stands
containing 34% or more Albizia were equal to or larger than those in fertilized, pure stands. Total
thus, we evaluated mixed species plantings in which N is added by
aboveground biomass, stem biomass, and stem volume per ha of mixed stands at age 10 were at least
Eucalyptus stands; total biomass of mixed stands with 50
Eucalyptus and 10 and 24% greater
than that of pure Albizia. Yield of the Eucalyptus component alone in these two mixtures was at least
equal to that of fertilized, pure Eucalyptus stands. Moreover, mean annual increment declined more
slowly after culmination in all mixed stands than in the fertilized, pure Eucalyptus stand. Over time,
the apparent benefits of mixed versus pure plantings of Eucalyptus and Albizia have increased, and
at age 10 include diversity in stand structure (and habitat) as well as the improvements in Eucalyptus
equal to yields produced in fertilized, pure
or 66% Albizia was 30 and 46% greater than that in fertilized, pure
tree growth and stand productivity recognized at younger ages. For. Sci. 43(2):286-298.
Additional
Key Words: Paraserianthes, stand dynamics, biomass, species mixtures, silviculture,
tropical forestry.
S
hart-rotation plantations of Eucalyptus and other spe­
cies offer a promising way to increase wood and
energy supplies in many tropical and subtropical areas
and thereby relieve some of the pressure on natural forests
throughout the world (Evans 1992). Hawaii is no exception;
eucalypt trees were first planted there more than a century ago
(LeBarron 1962), and high yields have been produced on
some sites (Pickford and LeBarron 1960, Walters 1980,
Whitesell et a1. 1992). Thousands of acres of agricultural land
are becoming available for Eucalyptus culture with the ap­
proaching demise of the Hawaiian sugar cane industry (Davis
1994). The establishment and culture of Eucalyptus and other
species on this land could bolster the agricultural sector of
Hawaii's economy and reduce the state's dependency on
imported oil and wood which currently provide 90% or more
of energy and wood needs.
Growth of Eucalyptus and other plants on much, if not
most, of the land previously occupied by sugar cane is limited
by low levels of available nitrogen (N). Responses to N
fertilizers have been excellent (Miyasaka 1984, Whitesell et
a1. 1987), and current management guidelines suggest 4 to 8
fertilizer applications per rotation, depending on rotation
length and site quality (Whitesell et a1. 1992). Synthetic N
fertilizer, however, is costly-it involves substantial energy
expenditures in manufacture, transport, and application, and
its use would be impractical in many developing countries.
Dean S. DeBell is Supervisory Research Forester, USDA Forest Service, Pacific Northwest Research Station, 3625 93rd Avenue SW, Olympia, WA
98512. (360) 753-7667; /s=d.debell/ou1=s26L09a@mhs-fswa.attmail.com. Thomas G. Cole is Forester and Craig D. Whitesell is Supervisory
Research Forester (retired), USDA Forest Service, Pacific Southwest Research Station, Honolulu, HI 96813.
Acknowledgments: We are grateful to Thomas H. Schubert and Thomas B. Crabb of BioEnergy Development Corporation, Hilo, Hawaii, for their
assistance with installation, maintenance, and measurement of the experimental plantings. Research was performed under Subcontract No. 19X09061C with Oak Ridge National Laboratory under Martin Marietta Energy Systems Inc., Contract DE-AC05-840R21400, with the U.S. Department
of Energy, and under Interagency Agreement Number DE-A105-860R21661 for the U.S. Department of Energy. C.A. Harrington provided the photos.
Helpful comments from Robert O. Curtis, Matthew J. Kelty, and Steven D. Tesch improved the manuscript.
Manuscript received October 16, 1995. Accepted April 10, 1996.
This manuscript was written by U.S. government employees and is therefore in the public domain.
286
Forest Science 43(2) 1997
Reprinted from Forest Science, Vol. 43, No.2, May 1997. Not for further reproduction.
Thus, considerable interest has developed in finding eco­
nomically effective ways to use N2-fixing plants to increase
growth in managed forests and plantations in Hawaii and
elsewhere.
Mixed species plantations in which N is provided by
N2-fixing trees have been evaluated on the island of
Hawaii for more than 15 yr. Our initial investigations
(1979-1984) demonstrated that 50:50 mixtures ofAlbizia
Jalcataria (L.) Fosberg [= ParaserianthesJalcataria (L.)
Nielson] or Acacia melanoxylon R. Br. with Eucalyptus
saligna Sm. and E. grandis Hill ex Maid. resulted in
greater height and diameter growth of Eucalyptus trees
than was obtained in pure stands and led to greater total
stand biomass yield at 5 112 yr in the mixed stands (DeBell
et al. 1985). Because best growth and yield were obtained
in the mixtures containing Albizia, a followup study was
established to compare mixed stands containing various
ratios of E. saligna and AlbiziaJalcataria with each other,
with pure Albizia, and with pure E. sa ligna fertilized
periodically with synthetic N fertilizer. At 4 yr, Eucalyp­
tus trees in mixed plantings containing 34% or more
Albizia were equal to or larger than those in repeatedly
fertilized, pure Eucalyptus stands, and total aboveground
dry biomass yields of mixed plantings with 34 to 66%
Albizia averaged about 10% greater than fertilized, pure
Eucalyptus plantings (DeBell et al. 1989). In addition,
levels of N and P in Eucalyptus foliage and total soil N
. were higher in mixed stands than in the pure Eucalyptus
plots. Supplemental studies conducted at age 6 revealed
that rates of litterfall and nutrient cycling, nutrient-use
efficiency of Eucalyptus, and light interception were en­
hanced by intermixed Albizia (Binkley et al. 1992); and
earthworm populations were 3 to 5 times greater in mixed
and pure stands containing Albizia than in pure Eucalyptus
stands (Zou 1993). Moreover, mean height and biomass
data summarized briefly at age 8 indicated that productiv­
ity benefits of the mixed plantings and height differences
between Eucalyptus and Albizia had increased over time
(DeBell and Harrington 1993).
This paper examines patterns of tree growth and stand
development of the Eucalyptus and Albizia plantings
through age 10 yr. Survival and periodic and cumulative
growth in height, diameter, and biomass; stand structural
characteristics; and absolute and relative yield at age 10
are reported. Growth trends in adjacent operational Euca­
l yptus plantings, growing under a minimal fertilizer re­
gime (similar to that of mixed species and pure Albizia
treatments), are compared with the experimental plantings.
In addition, implications of the findings in management
regimes for intensive wood and energy production as well
as multipurpose forestry are discussed.
Study Area
The experimental plantings are located near Hakalau, an
area typical of much land presently or formerly in agriculture
(primarily sugar cane) along the Hamakua coast (lat. 19030 'N,
long. 155015 'W) of the island of Hawaii. The test site is at
480 m elevation. Mean annual rainfall is about 4600 mm,
distributed fairly evenly throughout the year, with an occa­
sional dry season usually lasting no more than 3 months.
Slopes are gentle, ranging from 0 to 10%. The soil series is
Akaka silty clay loam (thixotropic isomesic Typic
Hydrandept) and is moderately acidic (pH 5.8-6.0). Nitrogen
concentration is similar to that of most soils of the Hamakua
coast, averaging about 0.5% in the 0 to 20 cm surface layer.
Sugar cane was produced on the land for more than 50 yr, but
its production was discontinued because of low yields fol­
lowing the October 1980 harvest. Immediately before the
study was started, the area was occupied by residual sugar
cane heavily infested with the very troublesome californiagrass
(Brachiaria l11utica [Forsk.] Stapf.) and smaller amounts of
other grasses and broad-leaved weeds. The site was prepared
for planting with a Rome cutaway harrow, which flattened
and cut up the sugar cane and grass to form a mulch.
Reinvading and resprouting vegetation was sprayed with
glyphosatel prior to planting in January 1982.
Methods
The experimental design was a randomized complete
block with 7 species mixture-fertilizer treatments repli­
cated in 4 blocks. The blocks and treatment plots con­
tained therein were generally contiguous and were sur­
rounded by a large stand of Eucalyptus of about the same
spacing and age.
The 7 treatments, expressed as the percentage of Euca­
lyptus andAlbizia, respectively, were 100:0, 89:11, 75:25,
66:34, 50:50, 34:66, and 0:100. Five of the treatments
were applied on 0.09 ha plots (30 x 30 m); the other two
treatments (34% Eucalyptus-66% Albizia and 100%
Albizia) were applied on 0.045 ha plots ( 15 x 30 m). Each
plot was planted with 3-month-old container seedlings at
2 x 2 m spacings (2500 trees/ha) in January 1982. The
planting stock was produced via procedures described in
Whitesell et al. ( 1992); in addition, the growing media in
containers for Albizia were inoculated with Rhizobium to
ensure that Albizia seedlings were nodulated prior to
planting. The various combinations of Eucalyptus and
Albizia were established systematically with uniform dis­
tribution of the two species throughout the plot (i.e., the
species were not planted in pure rows or clumps).
Each seedling was fertilized with 1 15 g of mixed fertilizer
containing nitrogen (N), phosphorus (P), and potassium (K)
at outplanting and at 4 and 8 months later; each application
was equivalent to 40 kg N, 18 kg P, and 33 kg K per ha. Trees
in the pure Eucalyptus plots received the same amounts ofN­
P-K fertilizer at 12, 18, 24, and 36 months. Trees in the
variously mixed Eucalyptus-Albizia plots and in the pure
Albizia plots received P-K fertilizer at the same times. Thus,
identical amounts of P and K were applied to all trees in all
treatments; Eucalyptus grown in pure stands received about
160 kg N/ha more than did trees in the other plots through age
36 months. No fertilizer was applied after 36 months to mixed
This publication does not contain recommendations for herbicide uses
reported, nor does it imply that such uses have been registered by the
appropriate government agencies.
Forest Science 43(2) 1997
287
species and pure Albizia plots. The pure Eucalyptus plot,
h,owever, received an application of N fertilizer equivalent to
130 kg N ha-1 at age 55 months, thus bringing the additional
fertilizer received to a total of 290 kg N ha-1. This treatment
is the most intensive fertilizer regime we have tested in our
studies, and amounts applied slightly exceed those currently
recommended for soils of average N status on the Hamakua
coast (Whitesell et al. 1992).
Measurements were taken on 9 trees in each of 4
subplots (36 trees), each located in a different quadrant in
the interior of each plot. For most treatments, there were 5
rows of buffer trees between measurement trees and trees
in other treatments. Survival and tree size were recorded
biennially. Tree heights were measured to the nearest 0.1
m with a telescoping rod until trees were 15 m tall; after
that time, they were measured with an Abney level and
tape. Diameters at breast height were measured to the
nearest 0.1 cm with a diameter tape.
At 4 yr, one 36-tree plot was established in operational
plantings of pure Eucalyptus growing adjacent to each of the
4 replicate blocks. Mean height and diameter at 2 yr were
estimated from growth trends on nearby sites. The fertiliza­
tion regime in these field plantings was similar to that of the
mixed species and pure Albizia treatments [i.e., no fertilizer
was applied after the first year, and other work has indicated
that applications of P and K beyond the establishment year do
not benefit Eucalyptus growth on these abandoned sugarcane
sites (Whitesell et al. 1992)] and thus differed substantially
from that of the fertilized, pure Eucalyptus plots in the
original experimental design.
Tree survival and size (height and diameter) data were
averaged for each plot and species and results summarized
and displayed by treatment and measurement year (age). Tree
growth patterns were further examined by plotting periodic
height or diameter increments as a function of mean tree size
(height or diameter) at the beginning of the growth period.
Such trends were graphed for each treatment, thus providing
an additional comparison of growth for trees and stands at
similar developmental stages (rather than ages).
Biomass equations were developed through destructive
sampling of trees at the study site and similar locations
(Schubert et al. 1988, Whitesell et al. 1988). Trees selected
for equation development included the range of sizes encoun­
tered in our study, and subsequent unpublished work by the
second author (Cole) verified applicability of the allometric
equations for older (though similar size) trees and in mixed as
well as pure plantings. Total aboveground, dry biomass and
stem biomass were estimated for each Eucalyptus andAlbizia
tree measured in each treatment and measurement period via
the following equations:
Eucalyptus
Total dry weight
=
0.08360
*
2
diameter .1554
11
Stem dry weight
=
0.03260
*
diameter1 .8130
n
288
Forest Science 43(2) 1997
heighto.2864
= 283, R2 = 0.98
*
heighto .8565
2
= 286, R = 0.98
*
Albizia
Total dry weight
=
0.03621
*
diameter 2.3146
11
Stem dry weight
=
0.01795
*
heighto.3600
= 95, R2 = 0.94
diameter2.2026
n
*
heighto .6660
2
= 95, R
0.94
*
=
with dry weight expressed in kg, dbh in cm, and height in m.
Stem volumes at 10 yr were estimated from stem dry
weights using the following wood density values (g cm-3)
determined from stems sampled in the study: 0.482 for
Eucalyptus in pure stands; 0.430 for Eucalyptus in mixed
stands with Albizia; and 0.296 for Albizia in all treatments.
The estimated weights and volumes of surviving measure­
ment trees in each treatment of each block were summed and
expanded to Mg or m3/ha based on the area occupied by the
measurement plots.
Standard analyses of variance were conducted to assess
the significance of treatment differences in selected tree and
stand characteristics at age 10. When treatments were signifi­
cantly different (i.e., P < 0.05), the means were separated by
Duncan's multiple range test.
The effects of combining the species were evaluated as
a replacement series by comparing the yield of each
species in mixture with its yield in pure culture as per
Harper (Harper 1977). Thus, the relative yield (RY) of
each species and the relative yield total (RYT) were
calculated for each treatment by:
RY Eucalyptus
_
RYAlbizia
_
RYT
-
-
=
yield of Eucalyptus in mixture
.
' pure cu1ture
Yleld 0f Eucalyptus III
yield of Albizia in mixture
.
. .
III pure cu Iture
Yleld 0f AlblZza
'
RYEucal)'lJtus + RY Albizia
For calculation of relative yield of Eucalyptus in various
mixtures at age 10, yield of field-planted Eucalyptus was
used as the pure culture index because it had received fertil­
izer treatment similar to that of the Eucalyptus in mixture.
Results and Discussion
General
Tree growth and survival were excellent in all treat­
ments throughout the 10 yr study period. Growth and yield
of Eucalyptus in the least productive "treatment" (i.e., the
plots established in adjacent plantings in which trees were
fertilized only during the establishment year) were quite
acceptable. At 10 yr, survival averaged 85%; diameter, 12
cm; height, 21 m; and total aboveground biomass totaled
132 Mg ha-1 . In general, the performance of both Eucalyp­
tus and Albizia at the Hakalau site equaled or exceeded that
observed in pure plantings at comparable ages and spac­
35
30
ings in other locations (Walters 1973, Schubert and
Whitesell 1985, Parrotta 1990).
Patterns of Mortality
Survival at age 2 yr ranged from 96 to 98% for Eucalyptus
and from 99-100% for Albizia. Little change occurred for
either species until age 4 when minor amounts of competi­
tion-related mortality began to occur in Eucalyptus (Figure
1). Survival of Eucalyptus at age 10 was 85% in the pure
Eucalyptus plantings and ranged from 84 to 94% in the mixed
Eucalyptus:Albizia treatments. Eucalyptus survival increased
with greater amounts of Albizia despite larger average tree
size and higher levels of biomass (Eucalyptus alone and both
species combined) per ha, presumably because spacing be­
tween Eucalyptus trees was correspondingly increased (thus,
intra-species competition was decreased) and nutrient status
was enhanced. Survival of Albizia ranged from 97 to 100%
across all treatments at age 10, even though it had been
overtopped by Eucalyptus in some treatments for several
years. Although Albizia is generally considered a light­
demanding species that cannot tolerate suppression (Parrotta
1990), it obviously survives well under moderate shade at
least through the small pole stage as suggested by Browne
(Browne 1955).
Patterns of Height Growth
Early height growth of Eucalyptus was very rapid during
the first 4 yr after outplanting, averaging nearly 4 m/yr. At age
4, differences among treatments had become established;
these differences were maintained or strengthened during the
next 6 yr even though periodic growth declined (Figure 2a).
In general, Eucalyptus height increased with increasing
amounts of Albizia; its height in mixtures was comparable to
that in fertilized, pure stands when plantings contained 34%
Albizia. By age 6, however, mean height of Eucalyptus in all
mixed stands exceeded that in the field plantings of pure
Eucalyptlls stands that had not been fertilized after the first
year. Height growth of Eucalyptus in the 50 and 66% Albizia
treatments tended to be much greater than that in other
treatments, resulting in mean tree heights at age 10 of 28 and
31 m, respectively. Plottings of periodic height increment in
%
Albizia
100
0, Fert.
-0, Field
-Go-
C
11
.........
25
90
"-G-'
34
. - .. 50
::s
en
........
66
80
o
1.
2
-b-
4
6
8
10
Age (years)
Figure 1. Survival of Eucalyptus trees through age 10 yr in pure
stands and in various mixtures with Albizia.
Age
10
08
(a) Cumulative
6
25 mJ4
02
I20
...
..c:
.
:z:
15
10
5
O. Fert.
10
I
1:
I!!
u
11
25
34
50
% Albizia in mixture
66
%
Albizia
(b) Periodic
0, Fert.
0, Field
8
-Go11
,..,.....
25
6
.E
4
N
2
...
O. Field
., E3"
34
- .. 50
.
. ... ..
O L---__-L____
5
10
L_____
15
20
_J____
____
25
Height at Beginning of Period (m)
30
Figure 2. Height growth of Eucalyptus in pure stands and in
various mixtures of Eucalyptusand Albizia: (a) cumulative and (b)
periodic as related to tree height.
relation to tree height indicated that Eucalyptus trees in most
treatments (other than those with 50 or 60% Albizia) grew
similarly after they attained a height of 15 m (Figure 2b).
Height growth in the fertilized treatment and field planting
"treatment" of pure Eucalyptus were similar after 4 yr (Figure
2a), with trees in each treatment growing about 7.5 m during
the next 6 yr period. This similarity in subsequent growth of
the 2 Eucalyptus plantings, despite striking differences in
earlier fertilizer application, probably is related to several
matters: (1) no additional N fertilizer was applied after 55
months in either treatment, (2) gradual acquisition of soil N
by trees in the field planting may have been sufficient, (3)
internal nutrient cycling of Eucalyptus is very efficient (Flo­
rence 1986), and (4) intertree competition for all resources
was lower in the field planting because trees were smaller.
Growth of Eucalyptus trees in the 50 and 66% Albizia
treatments, however, was substantially greater than that in
other treatments beyond the second year and at heights
exceeding 10 m (Figures 2a and 2b); moreover, growth in the
66% Albizia treatment was decidedly superior to that in the
50% Albizia treatment.
Albizia grew nearly as rapidly as Eucalyptus during the
first few years, averaging about 3.3 m per year through age 4.
During that period, growth tended to be best in the 34%
Albizia treatment, declining with lesser or greater amounts of
Forest Science 43(2) 1997
289
Albizia in the stand (Figure 3a). Beyond 4 yr, however, height
growth in the mixed treatments generally decreased with
increasing amounts of Albizia in the stand. At age 10, Albizia
trees in the 11, 25, and 34% Albizia treatments had heights
which were similar to each other and to trees in the pure
Albizia stand. Trees in the 50 and 66% Albizia treatments
were substantially shorter. Plottings of height increment in
relation to tree size showed that Albizia (as well as Eucalyp­
tus) trees in most treatments grew similarly after they attained
a height of 15 m (Figure 3b). Trees in the 50 and 66% Albizia
treatments, however, grew much less throughout the life of
the stand, and averaged only 15 m at lOyr, about 3.0 m shorter
than heights attained in all other treatments.
The net result of such height growth patterns among
treatments is the development of striking differences in
stand canopies. Differences in vertical structure of the
canopies are shown schematically for pure Eucalyptus;
mixtures with 11, 50, and 66% Albizia; and pure Albizia
(Figure 4). In pure Eucalyptus and pure Albizia stands,
there is a single layer canopy; and some stratification
(differences of 4 or 5 m in mean heights of the species) has
developed in the 11, 25, and 34% Albizia treatments. The
50 and 66% Albizia treatments, however, have developed
distinct two-storied canopies; mean heights of Eucalyptus
were 13 and 16 m taller than those of Albizia.
35
Age
10
08
6
0114
02
(a) Cumulative 30
25
K
20
.
15
....
.r::
::I:
10
E
.E...
m
>;-
N
25
3
4
50
1;:
o
c
cu
<.> m
10 ::E 11
o
50
%Albizia in mixture
100
66
Figure 4. Canopy stratification of Eucalyptus and Albizia in pure
and mixed stands at age 10 yr. Pure Eucalyptus heights shown
are the "fertilized" treatment.
Patterns of Diameter Growth
Diameter growth of Eucalyptus was much more rapid
during the first 2 yr after outplanting than in subsequent
periods (Figure Sa). Minimal differences in diameter among
treatments at age 2 (ranging from 7 to 9 cm) widened
substantially by age 4 (9 to 14 cm). Eucalyptus diameters
25
Age
10
08
6
0114
02
(a) Cumulative
20
E
...
15
E
10
% Albizia in mixture
0
100
66
%
Albizia
(b) Periodic
11
25
- 0­
'
'
34
-.­
6
_.
50
.. -..
66
4
100
O. Fer!.
7
.........
8
E
....
c
.!:.
O. Field
11
25
3
4
50
% Albizia in mixture
66
%
Albizia
(b) Periodic
0, Ferl
6
0, Field
-e11
5
.........
Q)
E 4
i!!
U
.E... 3
cu
-'
---8
--
L--10
----J 4----J ----
12
1
16
Height at Beginning of Period (m) 1
8--
20
Figure 3. Height growth of Albizia in pure stands and in various
mixtures of Eucalyptusand Albizia: (a) cumulative and (b) periodic
as related to tree height.
Forest Science 43(2) 1997
0
-·
-.­
50
2
0
25
34
66
N
2
°'
6
290
20
Q)
:z:: 5
11
1:
..c:
.
C'O
5
I
Euc. Alb.
I
....
i5
10
h
30 5
10
15
20
Diameter at Beginning of Period (cm)
25
Figure 5. Diameter growth of Eucalyptus in pure stands and in
various mixtures of Eucalyptus and Albizia: (a) cumulative and (b)
periodic as related to tree diameter.
increased with increasing amounts of Albizia, and differ­
ences established at age 4 were maintained or strength­
ened during the next 6 yr even though periodic growth
declined. Ten-year mean diameters of Eucalyptus in mixed
stands containing 34% or more Albizia were equal to or
larger than those in the fertilized, pure stand. Diameters of
Eucalyptus in mixed stands containing 11 to 25% Albizia
were somewhat smaller than those of the fertilized, pure
stand but not significantly so (Table 1). Moreover, even
with these low amounts (11 and 25%) of Albizia, the
Eucalyptus had larger diameters than those in the pure
Eucalyptus field planting. Eucalyptus diameters grown in
stands with 50% Albizia averaged nearly 20 cm at 10 yr
and were significantly greater than those in pure, fertilized
stands and mixed stands with lower amounts of Albizia;
diameters of Eucalyptus in the 66% Albizia treatment were
larger yet (23.5 cm) and differed significantly from those
of all other treatments.
Plottings of periodic diameter increment in relation to
diameter at the beginning of the period indicated that trees
in most treatments grew similarly as size increased (Figure
5b). Diameter increments of Eucalyptus trees in the 50 and
66% Albizia treatments, however, were markedly greater
at similar sizes.
Diameter growth patterns ofAlbizia in pureAlbizia stands
were very similar to those of Eucalyptus in fertilized, pure
Eucalyptus stands (Figures Sa and 6a), both species attaining
14.7 cm at age 10 yr (Table 1). In mixed stands, diameter
growth decreased with increasing amounts of Albizia. Peri­
odic diameter growth of Albizia in mixed stands containing
only 11, 25, and 34% Albizia was greater than in pure Albizia
stands; growth in mixed stands with 50 and 66% Albizia,
however, was lower than that in pure stands (Figure 6b).
Apparently, intraspecies competition among Albizia reduces
diameter growth of Albizia more than interspecies competi­
tion between Albizia and Eucalyptus when the two inter­
mixed species do not differ drastically in height. At the 50 and
60% levels of Albizia, however, Eucalyptus growth was so
great that average height differences between the two species
exceeded 12 m, and growth of associated Albizia was re­
duced. In general, diameter growth of Albizia in mixed
treatments was lower than that in pure Albizia stands when
Eucalyptus trees were 6 m or more taller than Albizia in a
treatment.
Patterns of Dry Biomass Accumulation per Hectare
Treatment effects on height and diameter growth of Euca­
to influence total aboveground
lyptus and Albizia combined
Table 1. Survival, tree size, and yield at age 10 yr for various combinations of Eucalyptus and Albizia.a
Stand yield
Species combinationb
Survival(%)
Dbh(cm)
Height(m )
Total
biomass
(Mg ha-I)
Stem
biomass
(Mg ha-I)
Stem volume
(m3 ha-I)
334C-a
100% E(fertil)
Eucalyptus
84.7
l4.7c
23.7bc l69B-a
l61B-a
Eucalyptus
8S.4
12.2
21 128
122
Eucalyptus
Albizia 83.6
100.0 13. lc
19.8a
22.0c IS. la
132a
40b
InB
129a
36b
16SB
299a
122b 42IBC Eucalyptus
Albizia 86.1 100.0 14.2c
17.0ab
22.2c
IS.4a
134a
Sib
ISSB
12Sa
4Sb
170B
291a
IS3b
444BC
Eucalyptus
Albizia 89.6 100.0 14.7c
16.Sab
23.0bc
IS.4a
139a
70b
209AB'
13la
63b
I94AB
304a
214b
SISAB
Eucalyptus
Albizia 90.1 98.6 19.7b
12.6b
27.9ab
IS.2a
169a
SIb
220AB
IS9a
4Sb
204AB
369a
IS3b
S22AB
Eucalyptus
Albizia 93.8 96.8
23.Sa
11.Sb
31.1a
IS.Oa
182a
66b
24SA
Ina
S9b
23lAB
400a
199b
S99AB
Albizia
97.2
14.7ab
17.9a
200AB-a
ISSAB-a
63SA-a
100%E (field)
.2S3
89%E:II%A
Total
7S%E:2S%A
Total
66%E:34%A
Total
SO%E:SO%A
Total
34%E:66%A
Total
100% A
a
b
Eucalyptus and Albizia sizes were compared statistically within species only; biomass and volume yields were compared within
species (lower case letters) and for the total plot (both species combined-upper case letters). Values followed by the same letter
do not differ significantly at P= 0.05. Characteristics of the field planting of pure Eucalyptus were not compared statistically with
other treatments contained in the original experimental design.
E = Eucalyptus; A = Albizia.
Forest Science 43(2) 1997
291
25
Age
10
08
6
0014
02
(a) Cumulative
20
5
% Albizia in mixture
%
Albizia
(b) Periodic
E
....
c
2!
...
cv
N
11
6
•....
25
....
-'{'j-'
34
5
- -+50
4
3
2
_ ..
,." ...
66
-100
-+-
o6L- -L8 --- 1Lo
--
12L ----14L ----16L ----18L -- 20
----
Diameter at Beginning of Period (cm)
Figure 6. Diameter growth of Albizia in pure stands and in various
mixtures of Eucalyptusand Albizia: (a) cumulative and (b) periodic
as related to tree diameter.
biomass (Figure 7). Beneficial effects of admixed Albizia
increased with time. At age 2, biomass production in all
treatments was at best equal to that in the fertilized, pure
Eucalyptus stand and, in many instances, was lower. By age
4, mixed stands containing 34% or more Albizia produced as
much biomass as the fertilized, pure Eucalyptus stands.
Similar statements could be made at age 8 and 10 yr for mixed
stands with only 25 and 11% Albizia, respectively. By age 10,
total aboveground biomass in mixed stands with 50 and 66%
Age
10
08
6
0014
02
300
250
'7
Cl
:i:
cv
iii
200
150
100
50
0,
Fert.
0,
Field
11
25
%Albizia i n mixture
Figure 7. Accumulation patterns of total aboveground biomass
of Eucalyptus and Albizia grown in pure and mixed stands.
292
Forest Science 43(2) 1997
was, respectively, 30 and 46% greater than that of
pure, fertilized Eucalyptus and 10 and 24% greater than that
of pure Albizia stands. In fact, biomass yield of Eucalyptus
alone in these two mixed stands was equal to or greater than
(though not significantly so at P < 0.05) that of the fertilized,
pure Eucalyptus stand (Table I ).
The relative contribution of Albizia to total stand biomass
changed over time, more so in some treatments than others
(Table 2). The proportion of yield from Albizia in the 11%
Albizia treatment increased from 7 to 23% from age 2 to age
10, whereas the Albizia contribution in the 50 and 66%
Albizia treatments declined over time, accounting for only
about 114 of the biomass in both mixed stand treatments at age
10. In the 25 and 34% treatments, the Albizia component of
biomass yield approximated its proportion to number of
stems in the stand. Such findings are consistent with previ­
ously described effects of treatments on patterns of height
and diameter growth.
Trends in periodic annual increment (PAl) and mean
annual increment (MAl) of total aboveground biomass are
displayed in Table 3. In addition to differences in mean
productivity, there were substantial differences among treat­
ments in trends in biomass increment over time. On average,
P AI culminated at 4 yr, and MAl culminated at 6 yr. The
peaks differed somewhat among treatments, however. PAl
peaked slightly later in the pureEucalyptus field planting and
in the 11% Albizia treatment, whereas MAl peaked slightly
earlier in the pure, fertilizedEucalyptus stands and somewhat
later in the mixed stand with 66% Albizia. Furthermore, the
decline in biomass growth following culmination of both PAl
and MAl was substantially less in mixed stands and in pure
Albizia stands than in pure, fertiIizedEucalyptlls stands. Such
differences in MAl trends are illustrated for the individual
species and the total stand in Figures 8a, b, and c. In fact, the
Eucalyptus component of biomass increment in the 66%
Albizia treatment has not culminated (Figure 8a) although the
total stand may have done so (Figure 8c). The Albizia com­
ponent of MAl in mixtures with 25 to 66% Albizia declined
after age 4 yr (Figure 8b), whereas MAl in the pure Albizia
has remained on the plateau attained at 4 yr and Albizia
growth in the 11% Albizia mixture was still increasing at age
10. Thus, there is a longer period of high productivity as well
as higher productivity per se in the mixed stands with 34% or
more Albizia.
Albizia
Table 2. Contribution (%) of Albizia to total biomass accumula­
tion in pure and mixed stands through age 10 yr.
Age (yr)
Species
combination
100% E (fertilized)
100 %E (field)
89%E:ll%A
75%E:25%A
66%E:34%A
50%E:50%A
34%E:66% A
100% A
2
4
6
8
10
0
0
6.8
24.1
31.9
35.7
49.8
100
0
0
15.5
36.3
37.9
33.1
42.1
100
%
0
0
17.9
32.3
35.6
26.7
32
100
0
0
21.2
29.4
35.2
24.8
28.6
100
0
0
23.4
27.5
33.5
23.3
26.5
100
Table 3. Periodic annual increment (PAl) and mean annual increment (MAl) in aboveground biomass in pure and mixed
stands through age 10 yr.
Age (yr)
Species combination
100%E (fertilized)
100%E (field)
89%E: l1%A
75%E:25%A
66%E:34%A
50%E:50%A
34%E:66%A
100% A
PAl
(MAl)
2
4
6
8
10
PAl
(MAl)
PAl
(MAl)
PAl
(MAl)
PAl
(MAl)
PAl
(MAl)
PAl
(MAl)
PAl
(MAl)
PAl
(MAl)
18.1
(18.1)
10.0
(10.0)
16.1
(16.1)
14.5
(14.5)
16.4
(16.4)
18.0
(18.0)
13.8
(13.8)
12.6
(12.6)
27.9
(23.0)
15.0
(12.5)
19.1
(17.6)
24.4
(19.4)
29.2
(22.8)
30.5
(24.2)
31.6
(22.7)
28.6
(20.6)
Mg ha-1 yr-l
21.2
(22.4)
16.0
(15.4)
20.6
(18.6)
23.1
(20.7)
24.6
(23.4)
26.6
(25.0)
31.4
(25.6)
24.5
(21.9)
11.0
(19.5)
10.1
(14.0)
15.2
(17.8)
17.2
(19.8)
18.8
(22.3)
18.0
(24.3)
26.2
(25.7)
20.2
(21.4)
6.4
(16.9)
8.7
(13.0)
15.0
(17.2)
13.3
(18.5)
15.2
(20.9)
17.0
(22.0)
20.8
(24.8)
14.4
(20.0)
Stand Characteristics at Age 10
By age 10, substantial differences had developed among
species combination treatments in mean tree size, stand
structure and biomass and volume yield (Table 1). Mortality
appeared related to competitive stresses; it averaged 15% in
the two pure Eucalyptus treatments and for the Eucalyptus in
mixed stands with 11 and 25% Albizia; and it was only 6 to
10% for Eucalyptus in the mixed stands with higher amounts
of Albizia. Mortality in Albizia was negligible in all treat­
ments. Diameters of Eucalyptus trees in mixed stands with 50
and 60% Albizia were significantly larger (34 and 60%,
respectively) than those in all other treatments, and diameters
of Eucalyptus trees in stands with lower amounts of Albizia
did not differ significantly from those of the fertilized, pure
stand (Table 1); similar trends occurred for Eucalyptus tree
height. In general, diameters of Albizia trees in the 11%
Albizia treatment were significantly larger than those in the
50 or 66% Albizia treatments, but they did not differ from
trees in pure Albizia stands and intermediate mixtures, and
the latter did not differ significantly from each other. Trends
among treatments for Albizia heights were similar to those for
diameter, but differences were not statistically significant.
't;·!Tree form differed greatly between the two species (Fig­
ures 9a-e). Eucalyptus trees were invariably tall, single­
stemmed, slender, and straight; branching habit was excur­
rent, producing relatively narrow crowns that did not overlap.
Albizia trees, on the other hand, were substantially shorter,
yet larger in diameter at comparable heights; branching
height was decurrent and resulted in wide, umbrella-shaped
crowns of light foliage. Branch and stem wood of Albizia is
somewhat brash and is easily broken or damaged by wind or
other forces. One-fourth to one-third of theAlbizia trees in the
experimental plots had two or more stems. The presence of
multiple stems, however, was unrelated to treatment and is
presumed related primarily to conditions and operations
associated with nursery production and early tending in the
plantation.
Differences between mean heights and diameters of the
two species within mixed stand treatments changed markedly
with increasing amounts of Albizia (Table 1). Diameters of
the Albizia were about 50% larger than those of Eucalyptus
in the 11% Albizia treatment. Albizia diameters were fairly
similar to those of Eucalyptus in the 34% Albizia treatment,
but they were only half as large as Eucalyptus in the 66%
Albizia treatment. Although heights of Eucalyptus were
greater than those of Albizia in all treatments, the height
difference between the two species increased from 4 m to 16
m as the proportion of Albizia increased from 11 to 66% (see
also Figure 4).
The Eucalyptus component differed among treatments in
terms of size of largest trees, degree of differentiation, and
diameter distribution (Table 4). Diameters of the largest 100
Eucalyptus trees/1m provide an indication of the size of domi­
nants (or site trees). Trends are similar to those for mean
diameter; that is, diameter of the 100 largest Eucalyptus trees
increased with the proportion of Albizia in the stands (cf. Table
1). Coefficients of variation for diameter (Table 4) provide
another measure of stand differentiation or size diversity among
trees of the same species within each treatment. Coefficients of
variation for Eucalyptus diameter were higher in the pure field
planting and in most mixed stands than in fertilized, pure stands.
Variation in Eucalyptusdiameters declined as Albizia increased
from 25 to 66%, and, in the 66% Albizia treatment, the variation
for Eucalyptus was essentially equal to that in pure, fertilized
stands despite the mean diameters being 60% larger (23.5
vs. 14.7 em). Coefficients of variation for Albizia diameter
in the mixed plantings averaged 42% as did those for
Eucalyptus, but the former were unrelated to stand compo­
sition treatments. Diameters in pure stands of Albizia,
however, had a much larger coefficient of variation (56%).
Increasing amounts of Albizia in mixed stands therefore
not only enhanced the size of Eucalyptus trees and the size
differential between Eucalyptus and Albizia, it also in­
creased the uniformity among the large Eucalyptus trees.
Forest Science 43(2) 1997
293
30
%
Albizia
(a) Eucalyptus
0, Fert.
-+-
0, Field
25
--e-11
';" 20
...
. .....
. ..
>-
';"
ctI
.c
Cl
34
.. -
-
10
25
n·
··
15
50
........ 66
5
-fr-
0
4
2
6
8
10 Age (years) %
Albizia
(b) Albizia
30
11
•....
....
25
··
- .. -
>-
ctI
.c
Cl
G··
34
';"... 20
';"
25
50
........
15
66
---fr100
10
...-4-
, e" : :: :�: -:;_ ' ": :=' ::::=:-:::�::1
O
-L______-L______-L______-L
______
2
6
4
8
10
Age (years)
30
%
Albizia
(c) Combined
0, Fert.
-+-
25
0, Field
--e--
...
11
20
•....
....
>-
';"
ctI
.c
25
" G"
15
Cl
-
10
34
..50
.. . .....
66
5
-fr-
0
...-4-
100
2
4
6
8
10
Age (years)
Figure 8. Trends in mean annual increment in total aboveground
biomass in pure and mixed stands of Eucalyptus and Albizia: (a)
Eucalyptus, (b) Albizia, and (c) both species combined.
Diameter distributions of the 500 largest Eucalyptus trees/
ha also differed substantially by treatment (Table 4). Such
trees contained most of the volume in all treatments at age 10
and represent the crop trees for extended rotations. All 500
trees in all treatments were larger than 15 cm dbh, the size
below which handling costs rise markedly (Kluender 1980).
Their average size and the number of trees in the largest
diameter classes increased substantially with the amount of
Albizia in the stand: the 50 and 66%Albizia treatments had 87
294
Forest Science 43(2) 1997
and 209 trees/ha that were equal to or greater than 30 cm dbh.
Conversely, none of the trees in the pure Eucalyptus stands
had attained this size.
Total aboveground biomass, stem biomass, and stem
volume yields of all mixed species stands were equal to or
greater than fertilized, pure Eucalyptus at age 10 yr (Table 1,
Figure lOa). Yields of the mixed stands and of the Eucalyptus
component thereof increased with increasing amounts of
Albizia; at 50 and 66% Albizia, Eucalyptus biomass or stem
volume was equal to or greater than that in fertilized, pure
stands despite the fact that there were only 1/3 or 1/2 as many
Eucalyptus trees. Apparently, added nitrogen, reduced com­
petition, and other effects associated with the companion
Albizia enhanced the growing environment for Eucalyptus
more than enough to make up for reduced numbers of trees.
Pure Albizia stands outproduced pure Eucalyptus stands
and mixed stands with only 11 and 25% Albizia; but biomass
production in mixed stands with 34, 50, and 66% Albizia was
equal to or greater than that in pure Albizia stands (Figure
lOa). Volume production was highest in pure Albizia, how­
ever (Table 1). Trends in volume yields differ from those of
biomass because the wood density of Albizia is 30 to 40%
lower than that of Eucalyptus.
Relative aboveground yields for each species and combi­
nations thereof are displayed for each treatment in Figure
lOb. Trends for stem biomass follow nearly identical patterns
(not shown). If both species use resources in identical ways,
the expected RY of each species will be equivalent to its
proportional representation in each mixture and the expected
RYT will equal 1.0. Values greater than those expected
indicate either niche separation (the 2 species are using
resources differently) or some beneficial relationship be­
tween the species; values lower than those expected indicate
antagonistic or competitive relationships between the spe­
cies. When the similarly fertilized field planting of Eucalyp­
tus is used as the index to evaluate the effects of Albizia on
Eucalyptus, the relative yield of Eucalyptus was higher than
expected in all mixed species plantings. Although RY in­
creased only slightly as the proportion of Albizia increased
from 11 to 34%, it rose markedly with higher levels of
Albizia. Relative yield ofAlbizia was disproportionately high
(i.e., benefited) at 11% Albizia, roughly proportional to its
presence in the mixed stands with 25 and 34% Albizia, and
disproportionately low (i.e., adversely affected) at 50 and
55% Albizia. The relative yield total, however, increased
greatly as the proportion of Albizia in the mixed stands
increased. At 11% Albizia, the relative yield total was 1.25 as
compared to 1.00 in the pure Eucalyptus planting; and the
relative yield advantage of the mixture increased to 1.75
whenAlbizia made up 66% of the stand. Thus, the mixture of
the two species clearly benefits yield of Eucalyptus and the
species mixtures in all combinations.
Implications and Conclusions
The growth-enhancing benefits of Albizia in Eucalyptus
plantations documented at age 4 (DeBell et al. 1989) and
described briefly in subsequent reports at age 6 (Binkley et al.
1992) and age 8 (DeBell and Harrington 1993) have in­
Figure 9. Treeform and stand characteristics in Eucalyptus and Albizia plantings: (a) pure Eucalyptus, (b) pure Albizia,
(c) mixed stand with 34% Eucalyptus (dark stems) and 66% Albizia (light stems), (d) Albizia crowns extend over road,
(e) straight and crooked stems of Albizia, and (f) understory development, including Hawaiian treefern.
Forest Sciellce 43(2) 1997
295
Table 4. Additional diameter characteristics of Eucalyptus component of various combinations of Eucalyptus
and Albizia.
Coefficient
of variation
(dbh)
Species
combinationa
100%E (fertilized)
100% E (field)
89%E:ll%A
75%E:25% A
66%E:34%A
50%E:50%A
34%E:66%A
a
E
=
Diameter of
largest 100
trees ha-1
Diameter distribution of largest
500 trees ha- 1 diameter class (cm)
:2: 15
<
:2: 20
20
cm
26.4
23.8
28.3
29.8
30.3
31.8
36.5
122
328
209
213
165
48
57
Total
Eucalyptus
III
iii
2
.
..
.
. .
. . .
.
.
..
50
.
.
.
...
. .. . ... ...
.
. .
..
.. . . . .
.
.
.
. ._
. . .. . ...
.. .. .. .
.
..
..
.
.
.
.
100
.
.
.
:2: 35
100
17
61
78
126
139
191
0
0
30
35
48
74
148
0
0
0
9
9
13
61
..
..
..
... ..
.... ,/
.
.
Total
Eucalyptus
1.5
i
i
'C
Qj
>=
i
0.5
......
0, Field
11
25
% Albizia
Albizia
.5
-
..
.
'C
.
.. .
.
o LL----L----3�4--5=0�--�66�--1�00
0, Fert.
:2: 30 < 35
ing, or light construction, and it does have promise as a
source of pulpwood for certain grades of paper (National
Academy of Sciences 1979), but rarely is it superior to
Eucalyptus for such uses. In most cases, the values pro­
vided by Albizia in terms of soil improvement, habitat
diversity, and enhanced growth of Eucalyptus are para­
mount and can be achieved even if Albizia trees are felled
and left on site after Eucalyptus is harvested. If significant
amounts of Albizia wood are desired for commodity pur­
poses, the species should be established either in pure
stands or in mixtures with lower amounts of Albizia (34%
or less) where its contribution to yield will at least equal its
representation in the stand. In such cases, managers prob­
ably will want to select and establish a combination of both
mixed Eucalyptus-Albizia and pure Albizia stands to
achieve overall land management objectives, including
production goals for both Eucalyptus and Albizia.
The yields estimated in this study were within the range
of yields determined in other studies of each species
(Parrotta 1990, Skolmen 1960), but some "fall-down" or
reduction in research-plot yields should be anticipated in
large-scale operations (Bruce 1977). Because measure­
ment trees were in the interior of each treatment plot
(which was contained within a much larger Eucalyptus
plantation), they were well buffered from growing condi­
Albizia
.
-150
30
(b) Relative Yield
200
:i!:
<
Eucalyptus, A = Albizia.
250
..c: Cl
:2: 25
278
155
200
165
152
226
43
(a) Absolute Yield
ia
25
trees ha-1
%
34
42
43
51
48
35
31
creased through age 10 yr. Moreover, it has become apparent
that the advantages of mixing Albizia in the plantations
extend beyond simply substituting biologically fixed nitro­
gen for synthetic nitrogen fertilizer. Other aspects of the
growing environment are affected, and other values and
conditions of the forest are improved.
Our 10 yr analyses show that trees in some of the mixed
plantings have attained sizes (dbh of 20 to 24 cm; ht of 23
to 31 m) and have provided stand yields (210 to 250 Mg
ha-1 ) that are greater than in pure plantings of either
species. Despite the fact that mixed plantings contained
fewer Eucalyptus trees, yield of the Eucalyptus compo­
nent alone was equal to or greater than that in both
fertilized and field plantings of pure Eucalyptus. In fact,
the highest Eucalyptus biomass and volume yields were
achieved in stands with the fewest Eucalyptus trees (i.e.,
highest Albizia components).
Although total stand yields and the Eucalyptus compo­
nent thereof were enhanced in mixed plantings, yields of
Albizia were disproportionately low in the 2 most produc­
tive mixtures (50 and 66% Albizia) as compared with pure
Albizia plantings. Generally, reduced yield of Albizia is of
minor concern because its wood characteristics -are unfa­
vorable for major commodity uses such as structural tim­
ber and fuel. Albizia wood can be used for pallets, shelv­
300
<
..
. ...
O LL----L-----L
0, Fert. 0, Field
11
25
34
50
66
100
% Albizia
Figure 10. Absolute and relative yields of aboveground biomass at age 10 for Eucalyptus and Albizia grown in pure
and mixed stands: (a) absolute and (b) relative.
296
Forest Science 43(2) 1997
tions markedly different from those of each treatment;
thus relative differences among treatments in measured
yields can be considered indicative of those likely to occur
in operational plantings.
Rotation length can have a major influence on the nature
and degree of benefits that may be realized from mixed
species plantings. At least three aspects of the growing
environment improved with time in mixed plantings, and the
change apparently was greater with increasing amounts of
Albizia. Continued improvement in both chemical and physi­
cal aspects of the soil environment in mixed plantings is
indicated by data on litter fall and nutrient cycling (Binkley
et al. 1992) and on earthworm concentrations (Zou 1993).
Benefits associated with improved soil nutritional status are
likely to be further enhanced by efficient internal nutrient
cycling mechanisms of Eucalyptus (Florence 1986) which
become more significant at older stages of stand develop­
ment. Secondly, the development of two-storied canopies
with considerable horizontal as well as vertical stratification
may improve the interception of light and the efficiency with
which it is used in mixed stands (Binkley et al. 1992).
Furthermore the obvious aboveground differences in niche
separation-and, thus, reductions in crown competition­
may be paralleled by stratification of root systems and other
differential use of soil resources. Some of the stands have
developed a luxuriant understory which includes Hawaiian
treeferns (Cibotium spp.) as well as many herbaceous and
woody species (Figure 9f). Thirdly, spacing between Euca­
lyptus trees obviously increased with increasing amounts of
Albizia in the planting. With time, height differences between
Eucalyptus an dAlbizia trees widened, thus further enhancing
spacing effects. As trees grew older and larger, benefits with
increased spacing became more important and were mani­
fested in significantly greater Eucalyptus tree growth. Dis­
tances between Eucalyptustrees may be more critical than for
many other species because buds on Eucalyptus branches
lack bud scales and are easily abraded by windsway; such
damage may substantially limit lateral crown development.
Presumably, this problem is minimized if not avoided in the
mixtures containing 50 and 66% Albizia because of much
greater distances between the emergent Eucalyptus.
In terms of ecological theory, it appears that the "competi­
tive reduction principle" (i.e., reduced competition in mix­
tures) and the "facilitative production principle" (i.e., one
species positively affects growth of the other) (Vandermeer
1989, Kelty 1992) have both contributed significantly to
superior yields of the mixed plantings. Biological fixation of
nitrogen and enhanced rates of nutrient cycling associated
with Albizia has facilitated growth of Eucalyptus, with ef­
fects observable after age 2 (Figures 2a and Sa). At later ages,
however, reductions in net competition presumably were
also important. Compared to pure plantings, Albizia growth
was enhanced or unaffected in mixed plantings if it repre­
sented only 11 to 34% of the stand. At higher levels ofAlbizia
(50 and 66%), Albizia growth was reduced in the presence of
the fewer but larger Eucalyptus trees. Eucalyptus growth,
however, increased with increasing amounts of Albizia; at the
higher levels of Albizia, Eucalyptus trees were markedly
larger in diameter and height than Albizia. Competition from
Albizia in these stands was therefore probably lower than the
intraspecies competition occurring in pure Eucalyptus stands.
Overall, the increased growth of Eucalyptus in the mixed
plantings more than made up for any reduction in growth of
the companion Albizia.
Could Eucalyptus yields equal to those achieved in the 50
and 66% Albizia treatments be achieved by planting pure
Eucalyptus at wider spacing and applying larger amounts of
N fertilizer? Possibly, but costs of establishment and man­
agement would be substantially greater. In fact, such a regime
and a mixed species regime were identified as promising
management alternatives and compared in recently pub­
lished guidelines for short rotation management of Eucalyp­
tus (Whitesell et al. 1992). Although estimated planting costs
were somewhat lower for wide spacings than for denser,
mixed species plantings ($168 vs. 247 ha- 1 ), subsequent
costs for mowing and fertilizing were estimated to be much
higher ($647 vs. 207 ha- I ). Moreover, many of the additional
benefits associated with mixed species plantings would be
foregone.
Other attributes of mixtures-related to both commodity
and noncommodity values-can be very significant. Two
important attributes are the larger tree size and equal or
greater tree uniformity (lower coefficient of variation) that
accompanied the Eucalyptus yield in stands with 50 and 66%
Albizia. Larger tree size (or piece size) is strongly correlated
with lower harvesting and processing costs, higher wood
quality, and greater recovery during manufacturing pro­
cesses. Uniformity is likewise associated with lower costs
and increased recovery. Thus, quality and economic return as
well as quantity of production are likely to be greater in the
mixtures. The broader plateau of the mean annual increment
curve (Figure 8c) is another important benefit associated with
the mixtures; growth of pure plantings of Eucalyptus de­
clined rapidly after mean annual increment culminated at age
4, whereas mean annual productivity remained nearly con­
stant through age 10 in the mixed plantings. This growth
pattern provides owners and managers with greater flexibil­
ity in scheduling harvest to optimize financial returns, pro­
vide other values, and otherwise respond to changing condi­
tions of the physical and social environment as well as the
marketplace. In addition, the continued period of maximum
growth means that other commodity values (e.g., larger piece
size) as well as noncommodity values associated with longer
rotations and mixed species stands can be attained without
sacrifice in wood yields. Such values might include visual
appearances and wildlife habitats provided by two-storied
stands; development of a diverse understory; increased accu­
mulation of nitrogen, organic matter, and other components
of the soil and forest floor; and other factors generally
associated with longer versus shorter rotations (Curtis and
Marshall 1993).
We therefore conclude that benefits of mixed plantings of
Eucalyptus and Albizia versus pure plantings of Eucalyptus
are mUltiple and substantial. The benefits have increased in
number and degree with time, and at age 10 include flexibility
in harvest age and diversity in stand structure and associated
Forest Science 43(2) 1997
297
habitats as well as the improvements in Eucalyptus tree
growth, stand productivity, and soil properties identified at
earlier ages. In those tropical and subtropical areas where
inadequate N limits production but climate and soil condi­
tions are otherwise suitable for growth of both Eucalyptus
and Albizia, mixtures of the two species offer attractive, cost­
effective alternatives to periodic application of synthetic N
'
fertilizers beyond the establi shment year in pure Eucalyptus
plantations. Choice of a specific mixture or selection of a
combination of mixtures for any forestry endeavor, however,
will vary with rotation length, wood production goals, and
other management considerations.
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