S hart-rotation plantations of Eucalyptus and other spe­cies 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 ap­proaching demise of the Hawaiian sugar cane industry (Davis 1994). The establishment and culture of Eucalyptusand 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. 19X-09061C 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.

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 Sm. plantations is commonly limited by low levels of

available nitrogen (N), and synthetic N fertilizer applications are costly and sometimes impractical;

thus, we evaluated Albizia falcataria [=

other,

Paraserianthes with pure

falcataria mixed species plantings in which N is by (L.) Fosberg

(L.) Nielson]. Five ratios of Eucalyptus added

Albizia, and Albizia

Eucalyptus were compared with each

and with pure fertilized

Eucalyptus periodically with N in a randomized block

design

Albizia on the wet Hamakua coast of the Island of Hawaii. growth increased

Eucalyptus as the amount

of in the stand increased

Albizia from 11 to 66%, and heights and diameters of in stands

containing 34% or more were equal to or larger than those in fertilized, pure stands. Total

aboveground biomass, stem biomass, and stem

Eucalyptus volume per ha of mixed stands at age 10 were at least

equal to yields produced in fertilized, pure stands; total biomass of mixed stands with 50

or 66% Albizia was 30 and 46% greater than that in fertilized, pure Eucalyptus Albizia. Eucalyptus

and 10 and 24% greater

than that of pure Yield of the component alone in these two mixtures was at least

equal to that of fertilized, pure stands. Moreover, mean annual increment declined more

slowly after culmination in all mixed

Eucalyptus stands than in the fertilized, pure stand. Over time,

the apparent benefits of mixed versus pure plantings of and

Eucalyptus have increased, and

at age 10 include diversity in stand structure (and habitat)

Eucalyptus as well as the improvements

Albizia in

tree growth

Key and stand productivity

Paraserianthes, recognized at younger ages. For. Sci. 43(2):286-298.

Eucalyptus

Additional Words: stand dynamics, biomass, species mixtures, silviculture,

tropical forestry.

Eucalyptus saligna

Thus, considerable interest has developed in finding eco­nomically 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, Eucalyp­tus 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 en­hanced by intermixed Albizia (Binkley et al. 1992); and earthworm populations were 3 to 5 times greater in mixed and pure stands containingAlbizia than in pure Eucalyptus stands (Zou 1993). Moreover, mean height and biomass data summarized briefly at age 8 indicated that productiv­ity 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 Euca­lyptus plantings, growing under a minimal fertilizer re­gime (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 occa­sional 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 fol­lowing 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 repli­cated in 4 blocks. The blocks and treatment plots con­tained therein were generally contiguous and were sur­rounded by a large stand of Eucalyptus of about the same spacing and age.

The 7 treatments, expressed as the percentage of Euca­lyptus 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 dis­tribution 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 ofN­P-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 Euca lyptus growing adj acent to each of the 4 replicate blocks. Mean height and diameter at 2 yr were estimated from growth trends on nearby sites. The fertiliza­tion 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 encoun­tered 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 Euca lyptus andAlbizia tree measured in each treatment and measurement period via the following equations:

Eucalyptus

Total dry weight = 0.08360 * diameter2.1554 * heighto.2864

11 = 283, R2 = 0.98

8565Stem dry weight = 0.03260 * diameter1.8130 * heighto. n = 286, R2 = 0.98

Albizia

3600Total dry weight = 0.03621 * diameter2.3146 * heighto.

11 = 95, R2 = 0.94

Stem dry weight = 0.01795 * diameter2.2026 * heighto.6660

n = 95, R2 = 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 measure­ment 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 signifi­cantly 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:

yield of Eucalyptusin mixture RYEucalyptus

_ .

l 1 - Yleld 0f Euca yptus III' pure cu ture

yield of Albizia in mixture RY

_Albizia . . .lZza III pure cu ture - Yleld 0f Alb' I

RYT = RYEucal)'lJtus + RYAlbizia

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 fertil­izer treatment similar to that of the Eucalyptus in mixture.

Results and Discussion

General

Tree growth and survival were excellent in all treat­ments 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 Eucalyp­tus andAlbizia at the Hakalau site equaled or exceeded that observed in pure plantings at comparable ages and spac­

288 Forest Science 43(2) 1997

1.

--

8

O L---__ -L ____ L_ ____ ____ _J ____

...

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 competi­tion-related mortality began to occur in Eucalyptus (Figure

1). Survival of Eucalyptus at age 10 was 85% in the pure Eucalyptusplantings and ranged from 84 to 94% in the mixed Eucalyptus:Albiziatreatments. Eucalyptussurvival 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 be­tween 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 light­demanding 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

100 0, Fert.

0, Field -Go-

11 .........

C 90 25 "-G-'

. 34

- .. -::s en 50

........ 80 66

-b-

o 2 4 6

Age35 (a) Cumulative 10 30 08

6

25 mJ4 02I20

.....c: . 15 :z:

10 5

O. Fert. O. Field 11 25 34 50 66 % Albizia in mixture

% Albizia10 (b) Periodic

0, Fert.

I 0, Field -Go-

111: , .. , ..... 6 25 I!! ., E3" .Eu 34

4 - .. -50

.......N 2

5 10 15 20 25 30 Height at Beginning of Period (m)

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 Euca lyptus 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

% Albizia

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 (Flo­rence 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 8 10 first few years, averaging about 3.3 m per year through age 4. Age (years) During that period, growth tended to be best in the 34% Figure 1. Survival of Eucalyptus trees through age 10 yr in pure

stands and in various mixtures with Albizia. Albizia treatment, declining with lesser or greater amounts of

Forest Science 43(2) 1997 289

°6' ---8 -- 10L--- 12----J14----J16---- 18-- 20

25

15

0114 02

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 Eucalyp­tus)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.

Age 35 (a) Cumulative 10

0830

30

hEuc. Alb. I ......c:. 20 Q):z:: 1;:occu<.> m 10 ::E

o 11 50 66 100 %Albizia in mixture

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

Age 25 (a) Cumulative 10

0866 20

0114 E

1502K 20 ... .....r:: . 15 ::I:

10

EC'O i5

10

5 5

11 25 34 50 66 100

0 O. Fer!. O. Field 11 25 3

4 50 66

% Albizia in mixture % Albizia in mixture

% % 10 (b) Periodic

Albizia 11

7 (b) Periodic Albizia

0, Ferl

I 1: E .E ... m>;-N

8

6

4

2

......... 25

- ' 0­' 34

-.­50 _... -.. 66

100

E.!:. ....cQ)Ei!!U.E ... cu

N

6

5

4

3 2

0, Field -e-

11 .........

-' 25 0-· 34

-.­50

66

0 5 10 20 25

Height at Beginning of Period (m) Diameter at Beginning of Period (cm)

Figure 3. Height growth of Albizia in pure stands and in various Figure 5. Diameter growth of Eucalyptus in pure stands and in

mixtures of Eucalyptusand Albizia: (a) cumulative and (b) periodic various mixtures of Eucalyptusand Albizia: (a) cumulative and (b)

as related to tree height. periodic as related to tree diameter.

290 Forest Science 43(2) 1997

increased with increasing amounts of Albizia, and differ­ences established at age 4 were maintained or strength­ened during the next 6 yr even though periodic growth declined. Ten-year mean diameters of Eucalyptusin 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 Eucalyptusin 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. Peri­odic 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 competi­tion between Albizia and Eucalyptus when the two inter­mixed 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 re­duced. 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 Euca­lyptus and Albizia combined to influence total aboveground

Table 1. Survival, tree size, and yield at age 10 yr for various combinations of Eucalyptus and Albizia.a

Species combinationb

100% E (fertil) Eucalyptus

100% E (field) Eucalyptus

89% E:II% A Eucalyptus

Albizia Total

7S% E:2S% A Eucalyptus

Albizia Total

66%E:34%A Eucalyptus

Albizia Total

SO%E:SO%A Eucalyptus

Albizia Total

34%E:66%A Eucalyptus

Albizia Total

100% A Albizia

Stand yield

Total Stem biomass biomass Stem volume

Survival (%) Dbh (cm) Height(m ) (Mg ha-I) (Mg ha-I) (m3 ha-I)

84.7 l4.7c 23.7bc l69B-a l61B-a 334C-a

8S.4 12.2 21 128 122 .2S3

83.6 13. lc 22.0c 132a 129a 299a 100.0 19.8a IS.la 40b 36b 122b

InB 16SB 42IBC

86.1 134a 12Sa 291a 100.0 Sib 4Sb IS3b

ISSB 170B 444BC

89.6 139a 13la 304a 100.0 70b 63b 214b

209AB' I 94AB SISAB

90.1 169a IS9a 369a 98.6 SIb 4Sb IS3b

220AB 204AB S22AB

93.8 182a Ina 400a

24SA 23lAB S99AB

97.2 14.7ab 17.9a 200AB-a ISSAB-a 63SA-a

14.2c 22.2c 17.0ab IS.4a

14.7c 23.0bc 16.Sab IS.4a

19.7b 27.9ab 12.6b IS.2a

23.Sa 31.1a 96.8 11.Sb IS.Oa 66b S9b 199b

a 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.

b E = Eucalyptus; A = Albizia.

Forest Science 43(2) 1997 291

o6L----8L---- 1Lo----1L2----1L4----1L6----1L8-- 20

Albizia was, respectively, 30 and 46% greater than that of 25 (a) Cumulative

20

5

6 E

5 ....c

4 2! 3

... cv 2 N

% Albizia in mixture

(b) Periodic

Diameter at Beginning of Period (cm)

Age 10

086

001402

% Albizia

11 ....•.... 25

-'{'j-'34

- -+-50 _ .. ,." ... 66

--100

-+-

Figure 6. Diameter growth of Albizia in pure stands and in various mixtures of EucalyptusandAlbizia: (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%

300 250

'7 200 Cl:i: 150cv 100

iii 50

0, Fert. 0, Field 1 1 25 %Albizia in mixture

Age 10

086

0014 02

Figure 7. Accumulation patterns of total aboveground biomass of Eucalyptus and Albizia grown in pure and mixed stands.

292 Forest Science 43(2) 1997

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 previ­ously 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 treat­ments 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 pure Eucalyptus field planting and in the 11 % Albizia treatment, whereas MAl peaked slightly earlier in the pure, fertilized Eucalyptus 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 com­ponent 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.

Table 2. Contribution (%) of Albizia to total biomass accumula­tion in pure and mixed stands through age 10 yr.

Age (yr) Species combination 2 4 6 8 10

% 100% E (fertilized) 0 0 0 0 0 100 % E (field) 0 0 0 0 0 89% E:ll% A 6.8 15.5 17.9 21.2 23.4 75% E:25% A 24.1 36.3 32.3 29.4 27.5 66% E:34% A 31.9 37.9 35.6 35.2 33.5 50% E:50% A 35.7 33.1 26.7 24.8 23.3 34%E:66% A 49.8 42.1 32 28.6 26.5 100% A 100 100 100 100 100

8.7

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) PAl

Species combination (MAl) 2 4 6 8 10

Mg ha-1 yr-l 100% E (fertilized) PAl 18.1 27.9 21.2 11.0 6.4

(MAl) (18.1) (23.0) (22.4) (19.5) (16.9) 100% E (field) PAl 10.0 15.0 16.0 10.1

(MAl) (10.0) (12.5) (15.4) (14.0) (13.0) 89% E: l1% A PAl 16.1 19.1 20.6 15.2 15.0

(MAl) (16.1 ) (17.6) (18.6) (17.8) (17.2) 75% E:25% A PAl 14.5 24.4 23.1 17.2 13.3

(MAl) (14.5) (19.4) (20.7) (19.8) ( 18.5) 66%E:34%A PAl 16.4 29.2 24.6 18.8 15.2

(MAl) (16.4) (22.8) (23.4) (22.3) (20.9) 50% E:50%A PAl 18.0 30.5 26.6 18.0 17.0

(MAl) (18.0) (24.2) (25.0) (24.3) (22.0) 34% E:66% A PAl 13.8 31.6 31.4 26.2 20.8

(MAl) (13.8) (22.7) (25.6) (25.7) (24.8) 100% A PAl 12.6 28.6 24.5 20.2 14.4

(MAl) (12.6) (20.6) (21.9) (21.4) (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 treat­ments. Diameters of Eucalyptustrees 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, single­stemmed, slender, and straight; branching habit was excur­rent, 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 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 domi­nants (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 compo­sition 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 in­creased the uniformity among the large Eucalyptus trees.

Forest Science 43(2) 1997

were

293

e" : :: :�: -:;_ ' ": :=' ::::=:-:::�::1 O ______ -L ______ -L ______ -L ______ -L

" G"

-fr-

25

20

25

10

5

......... 34

....•....

50

- .. -... .....

% and 209 trees/ha that were equal to or greater than 30 cm dbh. Albizia 30 Conversely, none of the trees in the pure Eucalyptus stands(a) Eucalyptus 0, Fert. -+- had attained this size. 25 0, Field Total aboveground biomass, stem biomass, and stem

--e--

';" 20 ...>-

11

25

volume yields of all mixed species stands were equal to or greater than fertilized, pure Eucalyptus at age 10 yr (Table 1,

equal to or greater than that in pure Albizia stands (Figure

·· n - ..-

· Figure lOa). Yields of the mixed stands and of the Eucalyptus';"ctI 15.c component thereof increased with increasing amounts of Cl

5010 ........

66 5 -fr-

0 2 4 6 8 10

Age (years)

% Albizia

30 11(b) Albizia

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 com­petition, 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

25·· G·· lOa). Volume production was highest in pure Albizia, how­34';"... ever (Table 1). Trends in volume yields differ from those of - .. ->- biomass because the wood density of Albizia is 30 to 40% ';"ctI ........15.c 66Cl ---fr-

10010 ...-4-

, 2 4 6 8 10

Age (years)

% Albizia

30 (c) Combined 0, Fert. -+-0, Field --e--

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 be­tween the species; values lower than those expected indicate antagonistic or competitive relationships between the spe­cies. When the similarly fertilized field planting of Eucalyp­

20... 11 tus is used as the index to evaluate the effects of Albizia on. ...•....

';">- 25 Eucalyptus, the relative yield of Eucalyptus was higher than ctI .c 15 expected in all mixed species plantings. Although RY in­34Cl

50 creased only slightly as the proportion of Albizia increased from 11 to 34%, it rose markedly with higher levels of

66 Albizia. Relative yield ofAlbizia was disproportionately high

100 0

2 4 6 8 10 ...-4-

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% Albiziatreatments had 87

294 Forest Science 43(2) 1997

(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 Eucalyptusand 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

i i

oLL----L----L----L----3�4----5=0�--�66�--1�00 OLL----L----L----L----L----L----L-----L

Species of variation largest 100 combinationa (dbh) trees ha-1 :2: 15 < 20 :2: 20 < 25 :2: 25 < 30 :2: 30 < 35 :2: 35

% cm

100% E (fertilized) 34 26.4 100% E (field) 42 23.8 89% E:ll% A 43 28.3 75%E:25% A 51 29.8 66% E:34%A 48 30.3 50% E:50%A 35 31.8 34% E:66%A 31 36.5

a E = Eucalyptus, A = Albizia.

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 nitro­gen 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

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 compo­nent thereof were enhanced in mixed plantings, yields of Albizia were disproportionately low in the 2 most produc­tive 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 unfa­vorable for major commodity uses such as structural tim­ber and fuel. Albizia wood can be used for pallets, shelv­

(a) Absolute Yield 300 Total Eucalyptus Albizia 250

... . .. . .. .. ........... .... .......... ..._.. ... . .

. .. . .. .. .

. .

.. ....-

.... ,/

50 . .... . ....... .... .. ... .. .. ... ..

0, Fert. 0, Field 11 25 % Albizia

122 328 209 213 165

48 57

trees ha-1

278 100 0 0 155 17 0 0 200 61 30 0 165 78 35 9 152 126 48 9 226 139 74 13

43 191 148 61

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 pro­vided by Albizia in terms of soil improvement, habitat diversity, and enhanced growth of Eucalyptus are para­mount 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 pur­poses, 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 prob­ably 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 measure­ment 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

(b) Relative Yield 2

1.5 'C.5 'CQj>=

i 0.5

Total Eucalyptus Albizia

...... .. ....

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.

Table 4. Additional diameter characteristics of Eucalyptus component of various combinations of Eucalyptus and Albizia.

Diameter distribution of largest Coefficient Diameter of 500 trees ha-1 diameter class (cm)

nent

ia 200..c: Cl:i!:

-150

III 100

iii

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 physi­cal 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 competition­may 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 Euca­lyptus trees obviously increased with increasing amounts of Albiziain 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 mani­fested in significantly greater Eucalyptus tree growth. Dis­tances 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 "competi­tive reduction principle" (i.e., reduced competition in mix­tures) 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 ef­fects 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 repre­sented 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 man­agement would be substantially greater. In fact, such a regime and a mixed species regime were identified as promising management alternatives and compared in recently pub­lished guidelines for short rotation management of Eucalyp­tus (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 pro­cesses. 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 de­clined rapidly after mean annual increment culminated at age 4, whereas mean annual productivity remained nearly con­stant through age 10 in the mixed plantings. This growth pattern provides owners and managers with greater flexibil­ity in scheduling harvest to optimize financial returns, pro­vide other values, and otherwise respond to changing condi­tions 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 accu­mulation 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 condi­tions 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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B ROWNE, F.G. 1 955. Forest trees of Sarawak and Brunei and their products. Government Printing Office, Kuching, Sarawak. 369 p.

B RUCE, D. 1977. Yield differences between research plots and managed forests. J. For. 75 : 14-17.

CURTIS, R.O., AND D.O. MARSHALL. 1993. Douglas-fir rotations-time for reappraisal? West. J. Appl. For. 8(3):8 1-85.

DAVIS, N.D. 1 994. Hawaii: Forestry's best kept secret. Am. For. 1 00(9 & 1 0):42-44, 58-59.

D EBELL, D.S., AND C.A. HARRINGTON. 1993. Deploying genotypes in short­rotation plantations: Mixtures and pure cultures of species and clones. For. Chron. 69(6):705-7 1 3 .

DEBELL, D.S., C.D. WHITESELL, AND T.H. SCHUBERT. 1 985. Mixed plantations ofEucalwtus and leguminous trees enhance biomass production. USDA For. Servo Res. Pap. PSW-1 75. 6 p.

D EBELL, D.S., C.D. WHITESELL, AND T.H. SCHUBERT. 1989. Using N2-fixing Albizia to increase growth of Eucalyptlls plantations in Hawaii. For. Sci. 35(\ ):64-75.

EVANS, 1. 1 992. Plantation forestry in the tropics. Ed. 2. Oxford University Press, New York. 403 p.

FLORENCE, R.G. 1 986. Cultural problems of Eucalyptus as exotics. Commonw. For. Rev. 65(2): 1 4 1 -65.

HARPER, J.L. 1977. Population biology of plants. Academic Press, New York. 892 p.

KELTY, M.1. 1 992. Comparative productivity of monocultures and mixed species stands. P. 1 25- 1 4 1 ill The ecology and silviculture of mixed species forests, Kelty, M.1., et al. (eds.). Kluwer Academic Publishers, Dordrecht, The Netherlands. 287 p.

KLUENDER, R.A. 1 980. Whole tree chipping for fuel: The range of diameter limits. Am. Pulpwood Assoc., Washington, DC. 6 p.

LEBARRON, R.K. 1 962. Eucalypts in Hawaii: A survey of practices and research programs. USDA For. Servo Misc. Pap. No. 64. 24 p.

MIYASAKA, S.C. 1 984. Comparison of quick- and slow-release fertilizers in young plantings of Eucalyptlls species. Tree Plant. Notes 35(2):20-24.

NATIONAL ACADEMY OF SCIENCES. 1 979. Tropical legumes: Resources for the future. Nat. Acad. of Sci., Washington, DC. 3 3 1 p.

PARROTTA, J .A. 1990. Pa/'{/seriallfizesjalcataria (L.) Nielsen-Batai, Moluccan sau. USDA, For. Serv., SO-ITF-SM-3 1 . 5 p.

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SCHUBERT, T.H., R.F. STRAND, T.G. COLE, AND KE. McDUFFIE. 1 988. Equa­tions for predicting biomass of six introduced tree species, Island of Hawaii. USDA For. Servo Res. Note PSW-40 1 . 6 p.

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VANDERMEER, J.H. 1989. The ecology of intercropping. Cambridge Univer­sity Press, Cambridge, England. 237 p.

WALTERS, G.A. 1 973. Growth of saJigna eucalyptus, a spacing study after ten years. J. For. 7 1 :346-348.

WALTERS, G.A. 1 980. Saligna eucalyptus growth in a 1 5-year-old spacing study in Hawaii. USDA For. Servo Res. Pap. PSW- 1 5 1 . 6 p.

WHITESELL, C.D., D.S. DEBELL, AND T.H. SCHUBERT. 1987. Six-year growth of Eucalyptus saligna plantings as affected by nitrogen and phosphorus fertilizer. USDA For. Servo Res. Pap. PSW- 1 88. 5 p.

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298 Forest Science 43(2) 1997