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

Forest Ecology and Management 193 (2004) 81–95

Growth dynamics in a mixed-species plantation of Eucalyptus globulus and Acacia mearnsii David I. Forrestera,b,*, Ju¨rgen Bauhusa,b, Partap K. Khannac,1 a

School of Resources, Environment and Society, The Australian National University, Canberra, ACT 0200, Australia b Cooperative Research Centre for Greenhouse Accounting, GPO Box 475, Canberra, ACT 2601, Australia c CSIRO 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 aboveground 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 *

Corresponding author. Tel.: þ61-2-6125-2623; fax: þ61-2-6249-0746. E-mail address: [email protected] (D.I. Forrester). 1 Present address: Institute of Soil Science and Forest Nutrition, Go¨ttingen University, Bu¨sgenweg 2, 37077 Go¨ttingen, Germany.

(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).

0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.01.024

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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 reduction 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 mixture, also known as complementarity or the competitive production principle, occurs when the interspecific 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 efficient use of site resources. Facilitation occurs when one species has a positive effect on another (Vandermeer, 1989), for example, when a nitrogen-fixing species increases the growth of the other by increasing nitrogen (N) availability. When positive interactions are dominant (facilitation 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 interactions 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 interactions 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 kg1 and Bray I-P (Bray and Kurtz, 1945) concentration was 1.6 mg kg1. 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 minimum 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 proportions: 100% E. globulus (100E), 75% E. globulus þ 25% 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

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 ha1 in the form of superphosphate 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 (Sackville Hamilton, 1994), which is the more important question in this study.

83

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 falcataria 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 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 destructive 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 biomassÞ ¼ 1:29 þ 2:168 lnðDÞ

ðAdj r 2 ¼ 0:978; P < 0:001Þ (1)

lnðstem volume underbarkÞ ¼ 8:359 þ 2:265 lnðDÞ

ðAdj r 2 ¼ 0:992; P < 0:001Þ (2)

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 8C for 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 determined simultaneously in a continuous flow system

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(Technicon1 TRAACS 800TM). N and P concentrations 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 Residual 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 increasing 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 treatment 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 significant interaction between spacing and mixture treatments.

Table 1 Stand characteristics for E. globulus and A. mearnsii monocultures after 11 years of growth Parameter

E. globulus

A. mearnsii

Height (m) Diameter (cm) Stand basal area (m2 ha1) Stand volume (m3 ha1) Stand above-ground biomass (Mg ha1)

14.1 10.0 11.9 71.3 47.1

10.5 11.2 14.1 76.3 75.4

(3.4) (3.9) (3.4) (22.2) (14.5)

(1.1) (2.9) (0.1) (1.1) (0.5)

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

3.2. Height growth The variability of stand characteristics in monocultures at age 11 years of A. mearnsii was considerably less than E. globulus as shown by the standard deviations (Table 1). Acacia mearnsii admixture significantly 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 year1 in 100E to 2.2 m year1 in 50E:50A. Eucalyptus globulus at the 3 m  3:3 m spacing were significantly taller (0.94 m; P ¼ 0:044) 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 proportion 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 year1 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.

D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 18

85

E. globulus A. mearnsii

16 11 years 14 9 years

Height (m)

12

10 5 years 8

6

3 years

4

2 years 1.3 years

2

0.7 years 0 100E

75E:25A

50E:50A:

25E:75A

100A

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.

In mixture A. mearnsii trees were significantly taller than E. globulus until age 5 years (Fig. 1). Subsequently 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 year1 (50E:50A). Similarly, A. mearnsii diameter growth increased with the proportion of E. globulus in the stand (Fig. 2). There were

significant differences between mixtures and monocultures 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 year1 (100A) to 3.1 cm year1 (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.

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D.I. Forrester et al. / Forest Ecology and Management 193 (2004) 81–95 14

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

Diameter (cm)

12 10 8 6

E. globulus

4 2

(a)

0

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

Diameter (cm)

16

12

8

A.mearnsii

4

0

0

2

4

(b)

6 Age (yr)

8

10

12

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.

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

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

E. globulus

E. globulus

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

246 221 259 183

Significance (F probability)

ns, P ¼ 0.334

A. mearnsii 335 347 290 236 P ¼ 0.031

0.021 0.157 0.47 0.615

H/D (m cm1) A. mearnsii 1.544 0.538 0.265 0.419

E. globulus 1.54 1.55 1.38 1.36 P < 0.001

A. mearnsii 0.74 0.86 0.92 1.02 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.

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87

E. globulus

100E 75E25A

400

50E50A

Stems ha-1

25E75A 300

200

100

0 1.5

4.5

(a)

7.5

10.5

13.5 13.9 16.5

Diameter class (cm) 700 600

A. mearnsii

75E25A Stems ha-1

500

50E50A 25E75A

400

100A

300 200 100 0 1.5

(b)

4.5

7.5

10.5

13.5 14.1 16.5

>18

Diameter class (cm)

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.

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 cm1) 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 cm1) than at the 3 m  3:3 m spacing (1.37 m cm1) (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 cm1) than at the 3 m  3:3 m spacing (0.82 m cm1) (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 calculated 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

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Total stand biomass (Mg ha-1)

120 100

3 years 4 years 5 years 6 years 9 years 11 years

80 60 40 20 0 100E

75E:25A

50E:50A:

25E:75A

100A

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.

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 monoculture 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 produce 50% of the volume produced in the 100E, however 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

Table 3 Above-ground stand biomass (Mg ha1) at age 11 years in the monoculture and mixtures of E. globulus and A. mearnsii Stand component

Density

E. globulus

2 m  3.3 m 3 m  3.3 m

A. mearnsii

2 m  3.3 m 3 m  3.3 m

Total

2 m  3.3 m 3 m  3.3 m

Species proportion

S.E.D.

100E

75E:25A

50E:50A

25E:75A

47.1x 47.0x

39.6x 46.1x

37.2x 38.9x

20.6x 19.8x

45.5x 38.0y

61.4x 54.2y

73.8x 63.2y

75.8x 76.7x

3.5 3.5

85.1b,c,x 84.1b,c,x

98.5d,x 93.1c,x

94.4c,d,x 88.0c,x

75.5b,x 76.7b,x

5.2 5.2

47.1a,x 47.0a,x

100A 5.4 5.4

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.

Above-ground biomass increment (Mg ha-1 yr -1)

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89

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

10

8

6

4

2

0

0

2

4

6

8

10

12

Age (yr)

Fig. 5. Mean annual increment of stand above-ground biomass (Mg ha1 year1) to age 11 years in the monocultures and mixtures of E. globulus and A. mearnsii at the 2 m  3:3 m spacing.

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. globulus 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.

Fig. 6. Annual litterfall (kg ha1 year1) 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.

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Fig. 7. Nutrient content of annual litterfall (kg ha1 year1) 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.

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 treatments 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 concentrations of E. globulus leaf litter (7.0 mg g1 in 100E) were lower than those of A. mearnsii foliage, flowers and seeds/pods (15.0 mg g1 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 g1 for A. mearnsii foliage, flowers and seeds/pods in 100A) than in E. globulus litter (0.34 mg g1 for E. globulus leaf litter in 100E).

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4. Discussion 4.1. Ecological interactions The net result of species interactions on growth indicated the dominance of positive interactions (facilitation 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 monocultures. 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. saligna 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

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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 aboveground 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 interaction 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-intolerant 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 interactions 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

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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 developed in mixtures where there was a rougher, twostoried canopy. However, since both diameter and height increased in mixtures, mechanical strengthening alone does not explain the change in the H/D. 4.1.2. Facilitation Often the primary objective of using N-fixing species 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 ha1) and the N (14 kg ha1) and P (0.58 kg ha1) that it contained was low in comparison to other Eucalyptus plantations (3.6–11.6 Mg litter ha1; 31–85 kg N ha1; 1.0–4.6 kg P ha1) (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 Lambert, 2002). This reflects the low above-ground biomass produced at the site (47.1 Mg ha1 at age 11 years) compared with the Eucalyptus plantations mentioned above (100–145 Mg ha1 at 6–8 years). Higher quantities of litter were produced in treatments that contained A. mearnsii (2.8 Mg ha1 in 100E compared to 4.7 Mg ha1 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 ha1) and 100A (54 kg ha1) treatments, respectively, than in 100E (14 kg ha1) (Fig. 7a). Similarly, stands containing 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 concentrations in fine roots and senescent foliage when compared to monocultures at age 31 and 25 months, respectively. Since leaf litterfall had not yet commenced, 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 outweighed 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

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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. Furthermore, 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 negatively skewed in mixture, the number of large E. globulus trees in the 50E:50A mixture (259 stems ha1) was only slightly greater than that in monoculture (246 stems ha1) at age 11 years. However, the mixtures may be an attractive alternative to monocultures 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 ha1 are considered, the growth at both tree and stand level follows the trends described above, with higher growth in mixtures than monocultures (data not shown). This trend is likely to continue since E. globulus in mixtures has overtopped A. mearnsii and thus should experience less competition 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 product 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 ha1) when compared to other E. globulus plantations in Australia (MAI from 8 to 45 m3 ha1 from stands aged between

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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 kg1) and Bray I-available P (1.6 mg kg1) (Khanna, 1997) were very low. Foliar N (10.9 mg g1) and P (0.51 mg g1) 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 availability 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 significant 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

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competition as indicated by the H/D. Light competition for E. globulus was reduced by canopy stratification, 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 application 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 aboveground 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 accounting.

Acknowledgements Forest and Wood Products Research and Development Corporation provided funding for this project. David Forrester received a scholarship from the Cooperative 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 acknowledged.

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