Growth dynamics in a mixed-species plantation ofEucalyptus globulus and Acacia mearnsii

David I. Forrestera,b,*, Jurgen Bauhusa,b, Partap K. Khannac,1

aSchool of Resources, Environment and Society, The Australian National University, Canberra, ACT 0200, AustraliabCooperative Research Centre for Greenhouse Accounting, GPO Box 475, Canberra, ACT 2601, Australia

cCSIRO Forestry and Forest Products, P.O. Box E4008, Kingston, ACT 2611, Australia

Abstract

Previous work has shown greater productivity in mixed than in mono-specific stands of Eucalyptus globulus and Acacia

mearnsii at age 3 and 6.5 years. To assess how long the synergistic effects of acacias on eucalypts in mixed stands would last, and

what future trajectory growth might take, we investigated the growth dynamics of mixed and mono-specific plantations over the

first 11 years since establishment.

Monocultures of E. globulus (E) and A. mearnsii (A) and mixtures (75E:25A, 50E:50A, 25E:75A) of these species were

planted following a species replacement series. At the tree level, eucalypt and acacia heights, diameters, volumes and above-

ground biomass were higher in mixtures than in monocultures 3–4 years after planting. Similarly, at the stand level, volumes and

above-ground biomass were significantly greater in mixtures than monocultures after 3–4 years. The difference in productivity

between mixed plots and mono-specific eucalypt stands increased with time from 3 to 11 years after establishment. Litterfall was

higher in the mixed stands than the monocultures, and this led to an increase in N and P cycling through litterfall in stands

containing A. mearnsii.

The study indicated that above-ground biomass accumulation in E. globulus plantations can be increased by acacia admixture.

This can partially be explained by canopy stratification and improved nutrition of eucalypts. Although the biomass production in

acacias peaked early, the synergistic effect of the acacias appears to be long lasting as was indicated by the increasing differences

between mixed and pure stands.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Acacia mearnsii; Competition; Eucalyptus globulus; Mixed-species plantations

1. Introduction

Mixed-species plantations have the potential to

improve nutrient cycling (Binkley et al., 1992), soil

fertility (Montagnini, 2000), biomass production

(DeBell et al., 1985; Parrotta, 1999; van Winden,

2001) and carbon sequestration (Kaye et al., 2000;

Resh et al., 2002) while providing other benefits

through a diversification of products (Montagnini

et al., 1995; Khanna, 1997; Montagnini, 2000),

improved risk management and protection from pests

and diseases (Ewel, 1986; FAO, 1995; Montagnini,

2000). In addition, they can function as a silvicultural

system for growing high value timber (Ewel, 1986;

Keenan et al., 1995; Montagnini et al., 1995; DeBell

et al., 1997).

Forest Ecology and Management 193 (2004) 81–95

* Corresponding author. Tel.: þ61-2-6125-2623;

fax: þ61-2-6249-0746.

E-mail address: david.forrester@anu.edu.au (D.I. Forrester).1 Present address: Institute of Soil Science and Forest Nutrition,

Gottingen University, Busgenweg 2, 37077 Gottingen, Germany.

0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2004.01.024

Several authors have demonstrated the growth

advantages of mixed-species plantations containing

Eucalyptus and discussed the interactions that may be

responsible for these to occur (Binkley et al., 1992;

DeBell et al., 1997; Khanna, 1997, 1998; Bauhus et al.,

2000; Binkley et al., 2000). However, there are also

examples where there has been little or no growth

advantage (DeBell et al., 1987) or where one species

has suppressed the growth of another (Turvey et al.,

1984; Binkley and Gardina, 1997; Hunt et al., 1999;

Parrotta, 1999). To understand why some species

combinations have yielded less than mono-specific

stands, it is necessary to examine the ecological

interactions in mixed species plantations more closely.

Theoretically there are three types of interactions in

mixed stands of trees: competition, competitive reduc-

tion and facilitation (Vandermeer, 1989; Kelty, 1992).

Competition occurs when two or more plants or

populations interact so that at least one exerts a

negative effect on the other (growth or mortality)

(Vandermeer, 1989). Competitive reduction in a mix-

ture, also known as complementarity or the competi-

tive production principle, occurs when the inter-

specific competition for a limiting resource is less

than that in the monocultures (Kelty and Cameron,

1995). This often occurs when there is a partitioning of

either above-ground (light) or below-ground resources

(water or nutrients) (Kelty and Cameron, 1995) that

leads to niche separation and ultimately a more effi-

cient use of site resources. Facilitation occurs when

one species has a positive effect on another (Vanderm-

eer, 1989), for example, when a nitrogen-fixing spe-

cies increases the growth of the other by increasing

nitrogen (N) availability.

When positive interactions are dominant (facilita-

tion or competitive reduction), mixed stands should be

more productive than monocultures. However, if the

inter-specific competition is greater than these positive

interactions, mixtures will be less productive. It is

difficult to forecast the outcome of the various inter-

actions in mixed stands, and the nature of these

interactions may change as the stands develop. This

makes predictions of potentially successful species

combinations, optimal proportions and suitable sites

for the combinations difficult until a rotation has been

completed. Few studies have examined the growth

dynamics of mixed-species plantations of Eucalyptus

with a N-fixing species and these have been in

the tropics (DeBell et al., 1997; Parrotta, 1999;

Wichiennopparat et al., 1998). The aim of this

paper is to examine the growth dynamics and inter-

actions within a mixed stand of Eucalyptus globulus

ssp. pseudoglobulus (Naudin ex Maiden) Kirkpatr. and

Acacia mearnsii De Wild. in temperate Australia to

identify which processes may lead to an increase in

productivity, and how the ecological interactions

between the two species may change over time. Earlier

studies have shown that the mixtures in this trial were

more productive than the monocultures at age 3 years

(Khanna, 1997) and 6.5 years (Bauhus et al., 2000).

This paper provides the combined results of the trial at

age 11 years. Litterfall data from age 9 to 10 years are

also included.

2. Materials and methods

2.1. Site characteristics

The experimental trial is located 5 km south-east of

Cann River in East Gippsland, Vic., Australia. Prior

to establishment the vegetation on the site was dry

sclerophyll forest dominated by Eucalyptus sieberi

(L. Johnson). The site is about 100–120 m above sea

level on undulating terrain with slopes of 10–32%. The

soil is a yellow podzolic with a high content of coarse

sand (Stace et al., 1968). Initial soil (0–5 cm depth)

N concentration was 1.10 g kg�1 and Bray I-P (Bray

and Kurtz, 1945) concentration was 1.6 mg kg�1. The

average annual rainfall at Cann River (1951–1973) is

1009 mm and evenly distributed throughout the year

(Bureau of Meteorology, 2002). The mean daily mini-

mum and maximum temperatures are 7.8 and 20.5 8C,

respectively (Bureau of Meteorology, 2002).

2.2. Experimental design

Eucalyptus globulus ssp. pseudoglobulus and

Acacia mearnsii were planted at five species propor-

tions: 100% E. globulus (100E), 75% E. globulus þ25% A. mearnsii (75E:25A), 50% E. globulus þ 50%

A. mearnsii (50E:50A), 25% E. globulus þ 75%

A. mearnsii (25E:75A) and 100% A. mearnsii

(100A). This replacement series was planted at two

densities (2 m � 3:3 m and 3 m � 3:3 m). Plots were

arranged in a randomised block design with four

82 D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95

replicate blocks. Plot size was 23 m � 28 m including

a surrounding row of buffer trees. Excluding buffer

trees there were 60 trees per plot at the 2 m � 3:3 m

spacing and 35 trees per plot at the 3 m � 3:3 m

spacing. In the mixed-species plots trees were mixed

within rows, resulting in a checkerboard arrangement.

E. globulus seedlings were planted in early July 1992

and A. mearnsii in early October 1992. The stands

were fertilised with 25 kg P ha�1 in the form of super-

phosphate in November 1992. More detail about the

site and plantation establishment is provided by

Khanna (1997) and Bauhus et al. (2000).

It is important to note that by using a replacement

series design it is not possible to separate the inter- and

intra-specific interactions (Sackville Hamilton, 1994).

However, it is possible to determine whether inter- and

intra-specific competition are equal or unequal (Sack-

ville Hamilton, 1994), which is the more important

question in this study.

2.3. Growth measurements

To assess the effect of mixture and density treatments

on growth, height, diameter at breast height (1.3 m)

over bark (D), volume, biomass and survival were

compared at about 3–5, 6.5, 9 and 11 years of age.

Tree heights (H) were measured for a subset of trees and

allometric equations were constructed to estimate the

height of the remaining trees (van Winden, 2001).

Similarly, allometric equations were used to estimate

total stem volume (under bark) and above-ground

biomass. The allometric equations for A. mearnsii were

derived from a sample of eight trees (D from 3.9 to

20.6 cm) harvested at age 10 years. Additional destruc-

tive sampling would have compromised the integrity of

the trial before it reached maturity.

The regression equations relating total A. mearnsii

stem volume under bark (m3) and total A. mearnsii

above-ground biomass (kg) to diameter at breast

height over bark (D; cm) were:

lnðtotal above-ground biomassÞ¼ �1:29 þ 2:168 lnðDÞ ðAdj r2 ¼ 0:978;P < 0:001Þ

(1)

lnðstem volume underbarkÞ¼ �8:359 þ 2:265 lnðDÞ ðAdj r2 ¼ 0:992;P < 0:001Þ

(2)

Total stem volume and above-ground biomass of E.

globulus was estimated from allometric equations

developed by Bennett et al. (1997) for E. globulus

ssp. pseudoglobulus of age 6 years in East Gippsland

that spanned the same size range. Bennett et al. (1997)

found no significant effect of site or fertiliser on these

equations. DeBell et al. (1997) found that biomass

equations for Eucalyptus saligna and Albizia falca-

taria could be applied to older but similar sized

mixtures or monocultures. Thus it was assumed that

these equations could be applied across treatments and

to the older but similar sized trees in this trial. Growth

rates were calculated as the mean annual increment

(MAI) for a given variable, for example, biomass, as

follows:

MAI ¼ Biomass at n years

n(3)

2.4. Litter collection and nutrient content

Litterfall, including leaves, flowers, fruits, bark and

twigs (<10 mm diameter), was collected in circular

mesh traps (<1 mm) with a circumference of 2.5 m

(0.5 m2, and 30 cm deep). Traps were suspended 1 m

above the ground and litter was collected every 1–2

months for 1 year from 9.25 years (September 2001) to

10.25 years of age (September 2002). The litter traps

were placed systematically within the plots so that the

proportion of E. globulus and A. mearnsii surrounding

the traps corresponded to the proportion of E. globulus

and A. mearnsii in the plot. Three traps were used per

plot (12 per treatment) and these were bulked for

analysis (four per treatment).

Litter samples were sorted into the various litter

components; E. globulus leaves, E. globulus twigs, E.

globulus bark, A. mearnsii twigs and A. mearnsii other

(leaves, seed/seedpods and flowers). The components

of ‘A. mearnsii other’ were not separated due to the

small size of the leaflets, flowers and seed. Samples

were weighed after drying to constant weight at 50 8Cfor 1 week.

To determine N and P concentrations, ground

and dried litter components were digested in digestion

acid (potassium sulphate dissolved in concentrated

sulphuric acid) and hydrogen peroxide (modified

from Heffernan, 1985). Total N and P were deter-

mined simultaneously in a continuous flow system

D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 83

(Technicon1 TRAACS 800TM). N and P concentra-

tions were determined for bulked samples (of a given

litter component) consisting of sub-samples collected

at different times of the year.

2.5. Statistical techniques

Differences between treatments in tree and stand

characteristics were tested using ANOVA and Resi-

dual Maximum Likelihood (REML) in GenstatTM

(Genstat 5 Committee, 1997). REML was used to

analyse A. mearnsii data because the 100A treatment

at both the 2 m � 3:3 m and 3 m � 3:3 m spacings

were missing from one block, making the design

unbalanced. Treatment effects in the REML analysis

were assessed by Wald statistics, which are distributed

as chi-squared. The nature of the response to increas-

ing proportions of E. globulus or A. mearnsii was

examined by subdividing the treatment sums of

squares into linear and quadratic terms. The standard

errors of difference (S.E.D.s) for comparison of treat-

ment means are provided.

3. Results

3.1. Mortality

At age 8 months, survival ranged from 97 to 99%

for E. globulus and 95 to 100% for A. mearnsii

(3 m � 3:3 m spacing). Tree survival at age 11 years

was very high in all treatments averaging 93% (for

the 3 m � 3:3 m spacing survival was 92–94% for E.

globulus and 94 to 97% for A. mearnsii). The change

in species proportions was not more than 2% in any

plot and so the original species proportions have

been kept in the following sections. Survival was

not significantly different between the 3 m � 3:3 m

and 2 m � 3:3 m spacing for either species (P >0:05). Survival appeared to be higher in mixtures

than in monocultures although this difference was

significant for A. mearnsii (P ¼ 0:004) but not for E.

globulus (P ¼ 0:745). This effect of mixtures was

most pronounced for A. mearnsii at the 3 m � 3:3 m

spacing where A. mearnsii survival increased with

the percentage of E. globulus. There was no signifi-

cant interaction between spacing and mixture treat-

ments.

3.2. Height growth

The variability of stand characteristics in monocul-

tures at age 11 years of A. mearnsii was considerably

less than E. globulus as shown by the standard devia-

tions (Table 1). Acacia mearnsii admixture signifi-

cantly increased E. globulus height growth after only 2

years (P ¼ 0:045) (Fig. 1). Eucalyptus globulus height

tended to increase with the percentage of A. mearnsii;

the tallest E. globulus trees were in treatments

50E:50A and 25E:75A. The mean annual E. globulus

height increment peaked at 33 months, when it ranged

from 1.6 m year�1 in 100E to 2.2 m year�1 in

50E:50A. Eucalyptus globulus at the 3 m � 3:3 m

spacing were significantly taller (0.94 m; P ¼0:044) than those at the 2 m � 3:3 m spacing

(0.82 m) from age 0.7 years. The effects of density

and mixture were maintained to age 11 years but

there was no density � mixture interaction so only

data from the 2 m � 3:3 m spacing are provided

(Fig. 1).

Acacia mearnsii also benefited from growing in

mixture. Its height growth increased with the propor-

tion of E. globulus (Fig. 1) and these differences were

significant from age 2 years onwards (w2 ¼ 30:9 with

3 d.f.). This relative height difference between the

mixtures and A. mearnsii monoculture was maintained

to age 11 years. Mean annual A. mearnsii height

increment peaked at age 1.7 years, when it was

between 2.3 in 100A and 2.5 m year�1 in 75E:25A.

Acacia mearnsii trees at the 3 m � 3:3 m spacing were

significantly taller (3.2 m; w2 ¼ 4 with 1 d.f.) than

those at the 2 m � 3:3 m spacing (2.7 m) from age 1.7

years and this difference was maintained to age 11

years. There was no density � mixture interaction.

Table 1

Stand characteristics for E. globulus and A. mearnsii monocultures

after 11 years of growth

Parameter E. globulus A. mearnsii

Height (m) 14.1 (3.4) 10.5 (1.1)

Diameter (cm) 10.0 (3.9) 11.2 (2.9)

Stand basal area (m2 ha�1) 11.9 (3.4) 14.1 (0.1)

Stand volume (m3 ha�1) 71.3 (22.2) 76.3 (1.1)

Stand above-ground biomass

(Mg ha�1)

47.1 (14.5) 75.4 (0.5)

Data from 2 m � 3:3 m spacing. Standard deviations of means in

parentheses.

84 D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95

In mixture A. mearnsii trees were significantly taller

than E. globulus until age 5 years (Fig. 1). Subse-

quently A. mearnsii fell behind the height growth of

E. globulus resulting in the development of a stratified

canopy with E. globulus overtopping A. mearnsii.

In this study canopy stratification is defined as a

significant height difference between species in a

given treatment. The stratification of crowns at age

9 years was described in more detail by van Winden

(2001).

3.3. Diameter growth

Eucalyptus globulus diameters were significantly

larger in mixtures than monocultures from age 4 years

(P ¼ 0:022) (Fig. 2) and these differences increased

with time to age 11 years. Mean annual diameter

increment peaked around age 4 years from 1.3

(100E) to 1.6 cm year�1 (50E:50A). Similarly, A.

mearnsii diameter growth increased with the propor-

tion of E. globulus in the stand (Fig. 2). There were

significant differences between mixtures and mono-

cultures from age 3 years (w2 ¼ 113:7 with 3 d.f.) and

these were maintained or increased to age 11 years

(Fig. 2). The mean annual diameter increment of

A. mearnsii peaked early and was already declining

from between 2.4 cm year�1 (100A) to 3.1 cm year�1

(75E:25A) from the first diameter measurement

at age 3 years. Eucalyptus globulus trees at the

3 m � 3:3 m spacing were significantly larger

(3.7 cm; P < 0:001) than those at the 2 m � 3:3 m

spacing (2.9 cm) from age 2 years. Similarly, A.

mearnsii diameters were larger at the 3 m � 3:3 m

spacing (7.9 cm) than 2 m � 3:3 m spacing (7.3 cm)

(w2 ¼ 22:1 with 1 d.f.) from age 3 years. There was no

density � mixture interaction.

The skewness of the diameter distributions for both

species was generally more negative in the mixtures

than in the monocultures (Table 2; Fig. 3). Thus

monocultures contain a large range of sizes and

the mixtures have a higher proportion of trees in

the larger diameter classes.

0

2

4

6

8

10

12

14

16

18

100E 75E:25A 50E:50A: 25E:75A 100A

Hei

gh

t (m

)

2 years

1.3 years

3 years

5 years

9 years

11 years

E. globulusA. mearnsii

0.7 years

Fig. 1. Height growth of E. globulus and A. mearnsii in the monocultures and mixtures at the 2 m � 3:3 m spacing to age 11 years. Error bars

are standard errors of difference.

D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 85

In this trial a large tree is defined as a tree that has a

diameter at breast height (D) larger than the mean plus

one standard deviation of the D of the monocultures

(at age 11 years D > 3:9 cm for E. globulus and

>14.1 cm for A. mearnsii, both in the 2 m � 3:3 m

spacing). For A. mearnsii there is a significant increase

in the number of large trees as the proportion of

E. globulus increases (P ¼ 0:031; Table 2). For

0

4

8

12

16

0 2 4 6 8 10 12Age (yr)

Dia

met

er (

cm)

75E:25A50E:50A25E:75A100A

A.mearnsii

0

2

4

6

8

10

12

14

Dia

met

er (

cm)

100E

50E:50A25E:75A

75E:25A

E. globulus

(a)

(b)

Fig. 2. Diameter growth of (a) E. globulus and (b) A. mearnsii to age 11 years in the monocultures and mixtures at the 2 m � 3:3 m spacing.

Table 2

Stocking of large trees, skewness of diameter distribution and height to diameter at breast height (H/D) ratio for E. globulus and A. mearnsii at

the 2 m � 3:3 m spacing at age 11 years

Species proportion Large stems per hectare Skewness H/D (m cm�1)

E. globulus A. mearnsii E. globulus A. mearnsii E. globulus A. mearnsii

100E 246 �0.021 1.54

75E:25A 221 335 �0.157 �1.544 1.55 0.74

50E:50A 259 347 �0.47 �0.538 1.38 0.86

25E:75A 183 290 �0.615 �0.265 1.36 0.92

100A 236 �0.419 1.02

Significance (F probability) ns, P ¼ 0.334 P ¼ 0.031 P < 0.001 P < 0.001

Large trees are those with a D larger than the mean plus one standard deviation of the D of the monocultures (>13.9 cm for E. globulus and

>14.1 cm for A. mearnsii). For species proportion E ¼ E. globulus, A ¼ A. mearnsii and the number represents the percentage of the treatment

occupied by that species. Significance of linear trends indicated at F probability >0.05; ns, not significant.

86 D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95

E. globulus there was no significant trend in the

number of large trees (P ¼ 0:334; Table 2).

At age 11 years the height/diameter ratio (H/D,

in m cm�1) of E. globulus decreased as the proportion

of A. mearnsii increased (P < 0:001) (Table 2). The

H/D was also higher at the 2 m � 3:3 m spacing

(1.49 m cm�1) than at the 3 m � 3:3 m spacing

(1.37 m cm�1) (P < 0:001). Similarly the H/D of

A. mearnsii decreased as the proportion of E. globulus

increased (P < 0:001) and the H/D was also higher

at the 2 m � 3:3 m spacing (0.92 m cm�1) than at

the 3 m � 3:3 m spacing (0.82 m cm�1) (w2 ¼ 29:9

with 1 d.f.) (Table 2). There was no density � mixture

interaction for either species.

3.4. Above-ground biomass and stem volume

The above-ground biomass and stem volume cal-

culated on a single tree basis were greater in mixtures

than monocultures from about age 3 years for

A. mearnsii and 4 years for E. globulus (P < 0:05;

data not shown). Acacia mearnsii trees in the 75E:25A

were 119 and 120% larger in terms of above-ground

biomass and stem volume, respectively, than those in

0

100

200

300

400

500

1.5 4.5 7.5 10.5 13.5 16.5

Diameter class (cm)

Ste

ms

ha-

1

100E

75E25A

50E50A

25E75A

13.9

0

100

200

300

400

500

600

700

1.5 4.5 7.5 10.5 13.5 16.5 >18

Diameter class (cm)

Ste

ms

ha-

1

14.1

(a)

(b)

E. globulus

A. mearnsii75E25A

50E50A25E75A

100A

Fig. 3. Diameter distributions of (a) E. globulus and (b) A. mearnsii at age 11 years in the monocultures and mixtures at the 2 m � 3:3 m

spacing.

D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 87

the A. mearnsii monoculture at age 11 years. Tree

volume and biomass tended to increase with the

proportion of associated species.

At stand level, above-ground biomass (Fig. 4;

Table 3) was higher in mixtures than either mono-

culture at age 11 years. If there had been no overall

positive or negative interaction in mixture, then E.

globulus trees in the 50E:50A treatment would pro-

duce 50% of the volume produced in the 100E, how-

ever they produced about 80%. The growth advantages

in mixture increased with age; at age 3 years E.

globulus in 50E:50A produced 67% of the stand

above-ground biomass than in 100E and by age 11

years this increased to 79%. Despite the accelerated

growth rates, the total biomass production of a reduced

number of trees for a given species in mixture did not

reach the level achieved in monoculture after 11 years

of growth.

The MAI in A. mearnsii stem volume and biomass

started to decrease after about age 4 years when

expressed at the tree or the stand level (data for stand

biomass are shown in Fig. 5). In contrast, E. globulus

did not exhibit such a rapid, early culmination of

growth and the MAI of volume and biomass was still

increasing at age 11 years. At the stand level the MAI

of stand biomass was declining prior to age 4 years

0

20

40

60

80

100

120

100E 75E:25A 50E:50A: 25E:75A 100A

To

tal s

tan

d b

iom

ass

(Mg

ha-1

)

3 years4 years5 years6 years9 years11 years

Fig. 4. Development of above-ground stand biomass to age 11 years in the monocultures and mixtures of E. globulus and A. mearnsii at the

2 m � 3:3 m spacing.

Table 3

Above-ground stand biomass (Mg ha�1) at age 11 years in the monoculture and mixtures of E. globulus and A. mearnsii

Stand component Density Species proportion S.E.D.

100E 75E:25A 50E:50A 25E:75A 100A

E. globulus 2 m � 3.3 m 47.1x 39.6x 37.2x 20.6x 5.4

3 m � 3.3 m 47.0x 46.1x 38.9x 19.8x 5.4

A. mearnsii 2 m � 3.3 m 45.5x 61.4x 73.8x 75.8x 3.5

3 m � 3.3 m 38.0y 54.2y 63.2y 76.7x 3.5

Total 2 m � 3.3 m 47.1a,x 85.1b,c,x 98.5d,x 94.4c,d,x 75.5b,x 5.2

3 m � 3.3 m 47.0a,x 84.1b,c,x 93.1c,x 88.0c,x 76.7b,x 5.2

For species proportion E ¼ E. globulus, A ¼ A. mearnsii and the number represents the percentage of the treatment occupied by that species.

S.E.D., standard error of difference. Means sharing the same letters (a, b, c or d) are not significantly different at P < 0:05 across a given row.

Means sharing the same letters (x or y) are not significantly different at P < 0:05 for a given species proportion and stand component.

88 D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95

when there was 50% or more A. mearnsii (Fig. 5), and

the decline was more rapid when there was a higher

proportion of A. mearnsii in the stand.

After 11 years of growth and at the level of the

stand, there was no significant difference in E. glo-

bulus above-ground biomass between the 2 and 3 m

spacings for a given mixture (P ¼ 0:844); the larger

trees in the wider spacing compensated for the lower

number of stems (Table 3). However, while A.

mearnsii trees were larger in the wider spacing this

did not compensate for the higher number of stems at

the closer spacing that produced significantly higher

above-ground biomass in all mixtures than the wider

spacing at age 11 years (w2 ¼ 8:4 with 1 d.f.)

(Table 3). With both species combined the total stand

above-ground biomass was not significantly different

between the 2 m � 3:3 m or 3 m � 3:3 m spacing at

age 11 years (P ¼ 0:662). There was no density �mixture interaction for E. globulus, A. mearnsii or

total stand above-ground biomass at age 11 years.

0

2

4

6

8

10

12

0 2 4 6 8 10 12

Age (yr)

Ab

ove

-gro

un

d b

iom

ass

incr

emen

t (M

g h

a-1 yr

-1) 100E

75E:25A50E:50A25E:75A100A

Fig. 5. Mean annual increment of stand above-ground biomass (Mg ha�1 year�1) to age 11 years in the monocultures and mixtures of E.

globulus and A. mearnsii at the 2 m � 3:3 m spacing.

Fig. 6. Annual litterfall (kg ha�1 year�1) in the monocultures and mixtures of E. globulus and A. mearnsii from age 10.25 to 11.25 years, at

the 2 m � 3:3 m spacing. Means sharing the same letters are not significantly different at P < 0:05.

D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 89

The trends observed for stand above-ground biomass

were also observed for stand volume (data not

shown).

3.5. Litterfall and nutrient cycling

Higher quantities of litter were produced in treat-

ments that contained A. mearnsii (Fig. 6). A large

proportion of this litter consisted of E. globulus leaves

or A. mearnsii leaves, flowers, seeds and seed pods.

Unlike E. globulus, A. mearnsii had not shed its

branches, so A. mearnsii twigs formed only a small

component of litterfall. Acacia mearnsii produced

litter with higher N concentrations and the quantity

of N cycled through litterfall was higher in stands

containing A. mearnsii than E. globulus monocultures

(w2 ¼ 109:6 with 4 d.f.) (Fig. 7a). Nitrogen concen-

trations of E. globulus leaf litter (7.0 mg g�1 in 100E)

were lower than those of A. mearnsii foliage, flowers

and seeds/pods (15.0 mg g�1 in 100A). Similarly,

stands containing A. mearnsii cycled higher quantities

of P through litterfall than 100E (w2 ¼ 31:8 with 4 d.f.)

(Fig. 7b). This was mainly due to the higher quantity

of litter produced since P concentrations were not

higher in A. mearnsii (0.29 mg g�1 for A. mearnsii

foliage, flowers and seeds/pods in 100A) than in E.

globulus litter (0.34 mg g�1 for E. globulus leaf litter

in 100E).

Fig. 7. Nutrient content of annual litterfall (kg ha�1 year�1) in the monocultures and mixtures of E. globulus and A. mearnsii from age 10.25

to 11.25 years, at the 2 m � 3:3 m spacing; (a) nitrogen and (b) phosphorus. Means sharing the same letters are not significantly different at

P < 0:05.

90 D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95

4. Discussion

4.1. Ecological interactions

The net result of species interactions on growth

indicated the dominance of positive interactions (facil-

itation and competitive reduction), as evidenced by

increased productivity in mixtures, over negative

(competitive) interactions. While these interactions

rarely occur in isolation, it is important to know

how these interactions affected tree growth and how

their influence changed as the stands developed. The

interactions and their contributions to the growth

dynamics of E. globulus and A. mearnsii are discussed

below.

4.1.1. Competition and competitive reduction

The fact that survival of both species in mixtures,

despite the larger tree sizes, was as high if not higher

than in monocultures indicates that the inter-specific

competition in mixture was less than that in monocul-

tures. This has led to higher growth of trees of both

species in mixture and indicates that facilitation or

competitive reduction must have dominated species

interactions. Similarly, the survival and growth of

E. saligna increased when mixed with A. falcataria

in Hawaii (DeBell et al., 1997). This was explained by

the corresponding increase in spacing between E. sal-

igna trees, reducing intra-specific competition, and

enhancing tree nutrient status (DeBell et al., 1997).

Differences in carbon (C) allocation in trees can be

used to indicate the main source of growth limitation

(Waring, 1987). Increases in resource availability

(such as nutrients) can lead to a reduction in C

allocation to roots (Keith et al., 1997; Giardina and

Ryan, 2002) and increase stem taper (Waring, 1987),

while shading tends to reduce root growth and stem

taper (Waring, 1987). The height to diameter ratio (H/

D), which is a measure of tree shape, may be used to

indicate the level of competition in even-aged tree

populations (Abetz, 1976). As competition for light

increases relative to competition for below-ground

resources, trees will allocate more C to height than

to diameter or root growth to maintain their position

in the canopy, and the H/D will increase (Bauhus

et al., 2000). In this stand, below-ground competition

appeared to be similar between species and treatments

since fine-root biomass and fine-root length density in

the top 30 cm of soil were similar for all species

combinations, and fine root architecture and vertical

stratification were also similar for A. mearnsii and

E. globulus at age 6.5 years (Bauhus et al., 2000). Thus

it was assumed that the H/D was influenced more by

changes in competition for light than below-ground

resources.

The H/D was higher among E. globulus than

A. mearnsii (Table 2). Eucalyptus globulus appears

to be less shade tolerant than A. mearnsii since it did

not maintain foliage less than 9 m above ground while

the A. mearnsii canopies extended to 2–5 m above-

ground in the 9-year-old stands. Light levels beneath

the canopies of the E. globulus and A. mearnsii

monocultures were 44 and 18% of photosynthetically

active radiation in the open, respectively (van Winden,

2001). Thus the higher H/D in E. globulus may be

explained by stronger effects of light competition

on this species than A. mearnsii. For both species,

the H/D declined as the proportion of the associated

species increased, suggesting that the competition for

light declined for both species when planted with the

other. This trend was also reported for A. mearnsii by

Bauhus et al. (2000) at age 6.5 years. Given the greater

shade tolerance of A. mearnsii, it is not surprising that

an individual A. mearnsii tree will grow better next to a

E. globulus tree than another A. mearnsii.

For a shade intolerant genus such as Eucalyptus

(Florence, 1996) this competitive reduction interac-

tion could be important for their continued vigour in

mixed stands. Thus it will be important to ensure that

the height growth of E. globulus or other shade-intol-

erant species can match that of the N-fixing species

and eventually overtop it. Selecting species with

compatible height growth at a given site is important

so that both can establish and develop into a stand

where facilitative or competitive reduction interac-

tions are maximised and competitive interactions

minimised.

The competitive reduction for light for E. globulus

was probably influenced by canopy stratification at

later stages of stand development (by age 9 years

E. globulus was taller than A. mearnsii). Canopy

stratification also developed in mixtures of E. saligna

and A. falcataria in Hawaii. In these stands, the growth

of E. saligna increased with the proportion of

A. falcataria. Acacia falcataria, even though it is

considered to be a shade intolerant species, survived

D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 91

well under the E. saligna canopy for several years

(DeBell et al., 1997).

Stem form can change for mechanical reasons.

When exposed to high winds, stems can become more

tapered (decreased H/D) to enhance stability by

increasing diameter growth, decreasing height growth

or both (Larson, 1963). Smaller H/D may have devel-

oped in mixtures where there was a rougher, two-

storied canopy. However, since both diameter and

height increased in mixtures, mechanical strengthen-

ing alone does not explain the change in the H/D.

4.1.2. Facilitation

Often the primary objective of using N-fixing spe-

cies such as A. mearnsii and A. falcataria in mixed

species plantations is to increase the N available to the

main crop or companion species. The total N in the

plant-soil system can be increased by atmospheric N

fixation (Kelty, 1992; Kelty and Cameron, 1995;

Fisher and Binkley, 2000). In addition, N availability

can increase by accelerating the rate of N cycling

without an increase in total N levels in the plant-soil

system (Kelty, 1992; Kelty and Cameron, 1995).

Either way, N is transferred between the A. mearnsii

and E. globulus via the decomposition of litter (such as

foliage and fine roots, including root exudates) and the

subsequent release of N.

The quantity of litter produced annually in 100E

(2.8 Mg ha�1) and the N (14 kg ha�1) and P (0.58

kg ha�1) that it contained was low in comparison to

other Eucalyptus plantations (3.6–11.6 Mg litter ha�1;

31–85 kg N ha�1; 1.0–4.6 kg P ha�1) (Binkley et al.,

1992; Toky and Singh, 1993; Guo and Sims, 1999;

Turner and Lambert, 2002) and native eucalypt forests

(Ashton, 1975; Attiwill et al., 1978; Turner and Lam-

bert, 2002). This reflects the low above-ground biomass

produced at the site (47.1 Mg ha�1 at age 11 years)

compared with the Eucalyptus plantations mentioned

above (100–145 Mg ha�1 at 6–8 years). Higher quan-

tities of litter were produced in treatments that con-

tained A. mearnsii (2.8 Mg ha�1 in 100E compared to

4.7 Mg ha�1 in 50E:50A) (Fig. 6). The quantity of N

cycled annually through litter fall was about three- and

four-times higher in the 50E:50A (44 kg ha�1) and

100A (54 kg ha�1) treatments, respectively, than in

100E (14 kg ha�1) (Fig. 7a). Similarly, stands contain-

ing A. mearnsii cycled higher quantities of P through

litterfall than the 100E treatment (Fig. 7b). Consistent

with these observations, E. saligna stands containing

N-fixing A. falcataria cycled more N and P in litter than

pure E. saligna stands (Binkley et al., 1992). However,

while soil N availability increased with the proportion

of A. falcataria, soil P availability declined with

increasing proportions of A. falcataria (Binkley et al.,

2000; Kaye et al., 2000). The higher P uptake by

A. falcataria may have been due to a higher absorbing

area of the roots due to associations with mycorrhizae

(Binkley and Ryan, 1998; Binkley et al., 2000) and a

higher allocation of C below-ground (Binkley and

Ryan, 1998). The high demand of N-fixing species

for P (Marschner, 1986) may increase competition

for P for Eucalyptus in mixtures. Alternatively, Binkley

et al. (2000) suggested that A. falcataria might use P

unavailable to E. saligna. This may then be cycled and

become available to the Eucalyptus. To maintain or

maximise the benefits of N fixation in mixtures it may

be necessary to add P fertilisers (Khanna, 1997, 1998).

No data was available for a comparison of fine root

turnover in these plantations. However, Khanna

(1997) showed that plants in mixture were larger in

terms of height growth from as early as age 25 months,

and E. globulus in mixture had higher N concentra-

tions in fine roots and senescent foliage when com-

pared to monocultures at age 31 and 25 months,

respectively. Since leaf litterfall had not yet com-

menced, increased growth and N concentrations must

have been due to nutrient cycling through fine roots

(Khanna, 1997).

Improved N nutrition was maintained as the stands

developed with increased E. globulus fine-root N

concentrations in mixtures evident at age 6.5 years

(Bauhus et al., 2000) and elevated E. globulus foliar

N concentrations in mixtures evident at age 9 years

(van Winden, 2001). Despite an increase in P cycled

through litterfall, its concentration in E. globulus

foliage was lower in the 50E:50A than the 100E

treatment (van Winden, 2001). However, this may

have been a dilution effect in the larger trees in

mixture. Since E. globulus grew larger in mixture,

the positive effect of increased N must have out-

weighed any negative effect of reduced P availability.

4.2. Silvicultural implications

By age 9 years, E. globulus was significantly taller

than A. mearnsii in a given mixture. In the 50E:50A

92 D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95

and 25E:75A all the first order neighbours of the

E. globulus are A. mearnsii so the majority of the

E. globulus in these treatments are dominant trees.

This was demonstrated by the more negatively skewed

diameter distributions in the mixtures (Table 2;

Fig. 3), showing that they do not contain the small,

suppressed trees that are found in the monocultures,

and thus growth is focused on larger trees. Further-

more, trees in mixtures had lower H/D (Table 2),

which is characteristic of a more dominant tree

(Larson, 1963).

While the diameter distributions were more nega-

tively skewed in mixture, the number of large

E. globulus trees in the 50E:50A mixture (259 stems

ha�1) was only slightly greater than that in mono-

culture (246 stems ha�1) at age 11 years. However, the

mixtures may be an attractive alternative to mono-

cultures when the desired product is large logs as the

E. globulus in the 50E:50A mixture produced about

80% of the volume produced in 100E on only half the

number of trees. Furthermore if only the largest

200 stems ha�1 are considered, the growth at both

tree and stand level follows the trends described

above, with higher growth in mixtures than mono-

cultures (data not shown). This trend is likely to

continue since E. globulus in mixtures has overtopped

A. mearnsii and thus should experience less competi-

tion for light. While A. mearnsii remains healthy it

may continue to fix N and the advantages of mixtures

could increase. If the goal was more than about 40 cm

in diameter at breast height, then the mixtures would

easily contain enough large trees to fully stock a

mature plantation. Thus the volume of the final pro-

duct at the end of the rotation may be the same as in the

monocultures, but produced over a shorter period of

time. The larger and more homogenous piece size

of eucalypts in mixture will also increase harvesting

and processing efficiency. In contrast, the yields of

E. saligna in mixtures with A. falcataria in Hawaii

were greater than those of monocultures even though

they contained only one third the number E. saligna

trees. In this stand both tree size and the number

of large E. saligna trees increased when growing in

treatments with high proportions of A. falcataria

(DeBell et al., 1997).

The slow growth (MAI 6.5 m3 ha�1) when com-

pared to other E. globulus plantations in Australia

(MAI from 8 to 45 m3 ha�1 from stands aged between

5 and 9 years) (Bennett et al., 1997; Hingston and

Galbraith, 1998) reflects the poor soil fertility. The

values of total N (1.10 g kg�1) and Bray I-available P

(1.6 mg kg�1) (Khanna, 1997) were very low. Foliar N

(10.9 mg g�1) and P (0.51 mg g�1) concentrations at

age 9 years in the E. globulus monoculture (van

Winden, 2001) were below the range for adequate

nutrition for this species (Dell et al., 2001). It is

important to remember that if soil moisture or P

availability limit tree growth, the competition by

A. mearnsii may outweigh any facilitative effects

through N-fixation for E. globulus. The failure of

some mixed-species trials, which was probably the

result of the strong influence of negative interactions,

shows the importance of careful site selection and

early silviculture.

Mixed species plantations with A. mearnsii and

E. globulus may be used in a number of silvicultural

systems such as:

(1) A. mearnsii may be thinned out to release

E. globulus and sold as an early source of income

or left to decay. As the retained A. mearnsii debris

decayed it would release more N to sustain N

cyclic at a higher level.

(2) Where thinning is not economically viable,

A. mearnsii could be retained while enough well

formed E. globulus dominants in the stand would

produce a final crop. In this case A. mearnsii may

continue to fix N.

(3) Alternatively both species could be thinned so

that the best E. globulus are retained with some

A. mearnsii. This may be advantageous where

there is a market for large A. mearnsii logs.

5. Conclusions

Acacia mearnsii admixture increased E. globulus

height and diameter growth by increasing N avail-

ability through increased N cycling. The dynamics of

diameter and height growth combined to increase stem

volume and above-ground biomass production in

mixtures compared to monocultures. This shows the

benefit of selecting a species capable of fixing sig-

nificant quantities of N and with readily decomposable

litter. Acacia mearnsii also grew larger in mixture

and this appears to result from a reduction in light

D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 93

competition as indicated by the H/D. Light competi-

tion for E. globulus was reduced by canopy stratifica-

tion, illustrating the importance of selecting species

with compatible height growth patterns.

The results showed that appropriate site and species

selection and management practice (P fertiliser appli-

cation on P deficient sites) can lead to increased

growth in mixed stands. The most productive stand

in this trial was the 50E:50A mixture, which produced

1.68 times the volume and 2.09 times the above-

ground biomass of the E. globulus monoculture after

11 years. The 50E:50A mixture produced 80% of the

eucalypt stem volume in the 100E but this wood was

grown on half the number of stems.

Several important questions remain. We do not

know for how long A. mearnsii fixes N in these

mixtures. The dynamics of N-fixation with stand

age may have an important influence on silvicultural

interventions such as thinning and fertilising. It is

also not known if increases in productivity resulted

from increases in total productivity (above- and

below-ground) or a change in C allocation, or both,

a question that has implications for carbon account-

ing.

Acknowledgements

Forest and Wood Products Research and Develop-

ment Corporation provided funding for this project.

David Forrester received a scholarship from the Coop-

erative Research Centre for Greenhouse Accounting.

The experiment was established by CSIRO. DNRE

provided the site and ACIAR some financial support.

Thanks to Wenhua Xang, Dr. Marcus Schortemeyer,

Professor William Stock, Julia Dordel, Matt Forrester,

Wanda Pienkowski, Mike Connell, John Smith, Kris

Jacobsen and Mauro Davanzo for professional advice

and technical support with field and lab work. Help

with the statistical design and data analysis of parts of

this study from Robert Forrester is gratefully acknowl-

edged.

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