New Forest (2005) 29:117–134

DOI 10.1007/s11056-005-0250-z # Springer 2005

Growth performance and management of a mixedrainforest tree plantation

PETERD. ERSKINE1,*, DAVIDLAMB1 andGEOFFBORSCHMANN2

1Cooperative Research Centre for Tropical Rainforest Ecology and Management, School of Life

Sciences, The University of Queensland, Brisbane, Qld 4072, Australia; 2Greening Australia –

Queensland, GPO Box 9868, Brisbane, Qld 4001, Australia; *Author for correspondence (e-mail:

p.erskine@uq.edu.au; phone: +61-7-3665-1767; fax: +61-7-3365-1699)

Received Publ.: Pl. provide the received date; accepted in revised form 3 September 2003

Key words: Australia, Plantation forestry, Reforestation, Tropical forestry, Tropical rain forest

Abstract. Monoculture plantations of Pinus, Eucalyptus and Acacia have been established on

rainforest lands throughout the world. However, this type of reforestation generally supplies low

quality timber and contributes to landscape simplification. Alternatives to exotic monoculture

plantations are now beginning to gain momentum with farmers and landholders attempting to

establish a variety of rainforest trees in small plantations. When compared to the well studied

commercial species, knowledge concerning the growth and management of many of these rainforest

species is in its infancy. To help expand this limited knowledge base an experimental plantation of

16 rainforest tree species in a randomised design was established near Mt. Mee, in south-eastern

Queensland, Australia. Changes in growth, form (based on stem straightness, branch size and

branchiness), crown diameters and leaf area of each species were examined over 5 years. Patterns of

height growth were also measured monthly for 31 months. Species in this trial could be separated

into three groups based on their overall growth after 5 years and their growth patterns. Early

successional status, low timber density, high maximum photosynthetic rates and large total leaf

areas were generally correlated to rapid height growth. Several species (including Araucaria cun-

ninghamii, Elaeocarpus grandis, Flindersia brayleyana, Grevillea robusta and Khaya nyasica) had

above average form and growth, while all species in the trial had considerable potential to have

increased productivity through tree selection. As canopy closure occurred at the site between years

four and five, growth increments declined. To reduce stand competition a number of different

thinning techniques could be employed. However, simple geometric or productivity based thinnings

appear to be inappropriate management techniques for this mixed species stand as they would

either remove many of the best performing trees or nearly half the species in the trial. Alternatively,

a form based thinning would maintain the site’s diversity, increase the average form of the plan-

tation and provide some productivity benefits.

Introduction

The world’s rainforests are the source of many high-value timber species. Intheory these forests should be capable of providing a sustained yield of thesetimbers but the reality in most forests is that this does not happen (Poore et al.1989). There are a variety of reasons for this and none of them appear to beeasily soluble in the short term. A corollary of this management failure is thatthere has been an increase in the area of degraded land formerly occupied byforest. Some of this land is being reforested using exotic species such as Pinus,

Eucalyptus or Acacia. While these species are highly productive and are ex-tremely useful for rehabilitating badly degraded sites they are no substitute formany of the high-quality timber species present in the original rainforests.Their use also enhances the trend towards landscape homogeneity and biolo-gical simplification that is underway throughout the world’s forests. An im-portant task for tropical foresters is, therefore, to find ways of reforesting thesedegraded lands using more of the native tree species wherever ecological andsocio-economic conditions permit. The question is, which of the many com-mercially attractive rainforest timber species are suitable for plantations?First attempts to answer this question in Australia using native rainforest

tree species were carried out many years ago (see e.g., Cameron and Jermyn1991), but only one species (Araucaria cunninghamii) was ever subsequentlyused in a large scale plantation program. There were several reasons for this.One was that the growth rates of many of these rainforest tree species were lowin comparison with exotic softwoods such as Pinus elliotii or P. caribaea (Lamband Lawrence 1993). Another was that the market prices for native species wasrelatively low, in part a consequence of the readily available supply of thesesame timbers from the natural forests. Finally, sawmillers were never en-thusiastic about plantation grown trees when they could always obtain muchlarger logs from the natural forests.This situation changed in Australia when most of the country’s tropical

rainforests were placed on the World Heritage Register and logging in thesewas terminated in 1988. The decreased availability of rainforest timbers causedby this change has led to renewed interest in the possibility of growing some ofthe more commercially attractive native species in plantations. But for this tooccur several issues need to be resolved. These include identifying the mostpromising of these commercially attractive species to grow in even-agedplantations and developing appropriate silvicultural techniques to managesuch plantations. Initial attempts to identify candidate species used estimates oftimber value and rotation length and based the selection on economic criteria(Russell et al. 1993). However, many of these estimates were based on growthrates of species established in trials initiated over 60 years ago when the sig-nificance of establishment methods were not as well understood as they aretoday. Site preparation and weed control can make large differences to earlygrowth and it is possible that some of these assumed rotation lengths are longerthan necessary. The characteristics of the species themselves influence this es-tablishment process. Rapid canopy development helps trees capture light andshade out competing species. Differing root characteristics may also enablesome species to more rapidly acquire soil resources than others (Haggar andEwel 1995).In this paper we report on the early development of a mixed species plan-

tation of 16 high-value rainforest tree species. The species included in this trialare adapted to a variety of ecological and successional situations but our mainobjective was to evaluate the performance of the different species when grownunder favourable plantation conditions. We also attempt to address the

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following question: what are some of the management issues that becomeapparent in a plantation mixture of trees with different growth rates?

Materials and methods

Species

The species used in the trial and some of their attributes are shown in Table 1.With the exception of three species, they are all trees found in the sub-tropicalrainforests of Australia. A number of the species were found growing in nat-ural rainforest within 1 km of the study site. Most species were in the middle orat the southern end of their geographic range although Dysoxylum fraserianumreaches the northern end of its range around the study area. Of the threespecies not present in nearby forest, Flindersia brayleyana, occurs naturally inthe northern tropical forests of Australia, while the other two, Cedrela odorataand Khaya nyasica, are both well-known exotics planted widely throughout thetropics. The study species includes hardwoods and softwoods, as well asevergreen, deciduous and semi-deciduous species (most of these deciduous orsemi-deciduous species shed their leaves at the end of the dry season after July/August). Eleven of the species have compound leaves, four have simple leavesand one species (Acacia melanoxylon) has compound juvenile foliage that issubsequently replaced by phyllodes in the adult stage. The various species arerepresentative of early, mid and late successional stages of forest growth andtimber densities range from 400 to 800 kgm�3. At least one species, Acacia, iscapable of fixing nitrogen under appropriate conditions. All are recognised ashaving high value timbers. Araucaria cunninghamii was included in the trial as areference species. This is a conifer that has been widely planted in tropical andsub-tropical plantations in Australia and Papua New Guinea. At mediumquality sites it is capable of developing stands with 460m3 ha�1 at 56 years age(Dale and Johnson 1991).The seed for Araucaria came from the southern Queensland Imbil seed

orchard (Batch M104,0) of the Queensland Forest Service. The seed of all otherspecies came from unspecialised collections in natural forests or field trials insouth-eastern Queensland or northern New South Wales. The seed of the twoexotic species, Cedrela and Khaya, came from field trials of these species atImbil. In both cases the original provenance was unrecorded.No special efforts were made in the nursery to ensure that seedlings were

inoculated with particular mycorrhiza, but studies carried out several yearsafter planting revealed that all had acquired vesicular–arbuscular mycorrhiza(V. Matthews personal communication).Drane (1995) and Snell (1996) determined the maximum photosynthetic rates

of 11 species with a Li-Cor LI-6200 portable photosynthesis system (Table 1).These measurements were made on leaves from the upper north facing part ofthe plant canopies during hot and wet summer conditions in 1992 and 1993.

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Table

1.

Plantationspeciesandtheirattributes

Species

Family

Timber

density1(kgm

�3)

Successional

status2

Maximum

photosyntheticrate3

(mmolC

O2m

�2s�

1)

Phenology4

Leafmorphology

Acaciamelanoxylon

Mimosaceae

570

122.97

ESimple/Compound

Araucariacunninghamii

Araurcariaceae

560

35.45

ESimple

Argyrodendrontrifoliolatum

Sterculiacea

925

411.67

ECompound

Castanospermum

australe

Fabaceae

755

47.18

ECompound

Cedrela

odorata

Meliaceae

415

2–

DCompound

Cryptocaryaerythroxylon

Lauraceae

720

49.17

ESimple

Dysoxylum

fraserianum

Meliaceae

705

3–

ECompound

D.mollissimum

Meliaceae

640

2–

ECompound

Elaeocarpusgrandis

Elaeocarpaceae

495

213.97

ESimple

Flindersiabrayleyana

Rutaceae

575

212.47

ECompound

F.schottiana

Rutaceae

675

211.54

ECompound

Gmelinaleichhardtii

Verbenaceae

545

2–3

11.39

SD

Simple

Grevillea

robusta

Proteaceae

625

2–3

21.50

SD

Compound

Khayanyasica

Meliaceae

560

?–

ECompound

Rhodosphaerarhodanthem

aAnacardiaceae

690

313.04

ECompound

Toonaciliata

Meliaceae

450

2–

DCompound

1Timber

density:Cause

etal.(1989),Anonymous(1945),Pinkard

andBeadle(2002).

2Successionalstatus:Pioneer(1),Earlysecondary

(2),Latesecondary

(3),Mature

(4)(M

.Olsen,personalcommunication).

3Maximum

photosyntheticrates:Drane(1995)andSnell(1996)determined

withaLi-CorLI-6200PortablePhotosynthesisSystem

.4Phenology:Deciduous(D

),Evergreen

(E)orSem

i-deciduous(SD).

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Study site

The study site is an area of pasture within a dairy farm near Mt. Mee about70 km north of Brisbane (at latitude S278050, longitude E1528450). The site hasan altitude of 500m and was formerly occupied by sub-tropical rainforest buthas been cleared for perhaps 100 years. The climate in the area is sub-tropicalwith an average annual rainfall of 1514 (±246)mm most of which falls be-tween December and April. The rainfall of the driest month (August) averages41mm. The temperature at the nearest climatological station (Samford, 25 kmsouth and at altitude 80m) varies between a July mean monthly minimum of68C and a February mean monthly maximum of 308C. Because of the altitudedifference minimum temperatures at the study site are likely to be slightlycooler. The underlying geology of the site is basalt and the soils are pre-dominantly kraznozems. Previous work by Lamb and Borschmann (1998)suggested that the fertility levels at the site are adequate for plant growth andthat there are no obvious deficiencies present. The site has a gentle slope ofabout 58 and is exposed to sometimes strong south-easterly and westerly winds.The pasture in which the trees were planted is dominated by Kikuyu grass(Pennisetum clandestinum) and has been fertilised and managed for dairyproduction for many years.Planting was carried out in June 1990 following a late wet season. Planting

holes for each tree were prepared using a special auger that loosened the soilbut left a roughened surface on the edge of the hole. Once planted the treeswere surrounded by cylindrical plastic shelters 100 cm tall and 25 cm diameterand having an internal collar about 5 cm deep at ground level. These cylindershave been previously shown to provide early growth advantages to somerainforest tree seedlings by maintaining a high atmospheric water content inthe vicinity of the seedling leaves (Applegate and Bragg 1989). These plasticshelters were maintained until the seedlings emerged from the open tops afterwhich they were removed. A few of the newly planted seedlings died not longafter planting. Such seedlings were replaced within a month but were notincluded in subsequent analyses. Besides being established to evaluate the earlygrowth of the various species it was intended that the trial would also be usedto test a plantation growth model. For this reason the 16 species in the trialwere planted in a mixture. Every species was represented by a single individualin a plot of 16 trees (four rows of four trees each) spaced 3m apart (1100 treesper ha); the trial contained 28 such plots. A row of Araucaria was planted as abuffer row around the outside of the 28 plots.All trees were given 150 g of diammonium phosphate at the time of estab-

lishment. A further 80 g per tree were applied in each of the next 4 years. Weedcontrol was maintained by regular applications of glyphosate in a radius of90 cm around each tree and by mowing between rows. This weed control wasmaintained until canopy closure shaded out the original ground cover species.Rainfall in the wet season of the planting year was above average (total

rainfall in 1990 was 1802mm) and late. However, rainfall in the year following

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planting was much lower than average (1144mm) so trees were watered once inAugust 1991 when they were 13 months old. Annual rainfall from 1992 to 1995averaged 1350mm.

Growth

Growth measurements were made on all trees every 6 months for the first 4years and then annually (1995–2002). Measurements included height anddiameter at 10 cm above ground and subsequently at 130 cm breast height(DBH) when the trees had grown taller than 150 cm. Conical volume of thetrees was calculated using an estimate of the stem cross sectional area based onmeasures at 130 cm, total height and an empirical form factor of 0.33. Crownradius was measured in four directions at each measurement occasion. Theform of the various species was rated using a scale of 1 (very poor) to 10(excellent) based primarily on stem straightness, branch size and branchiness;in addition the number of leading shoots were tallied.After December 1991 monthly measures of tree height growth were also

made on a subsample of 10 of the faster growing individual trees of eachspecies to observe whether there were differences between the species in growthpatterns. This monitoring was maintained for 31 months.

Leaf areas

The leaf area of each species was assessed at 4 years age by determining therelationship between branch diameter and leaf area on about 30 branchescollected from 4 to 6 individuals of each species. Branches were collectedrandomly from throughout the tree crown, although, in the case of taller trees,most larger branches were necessarily from lower crown positions. Samplingwas carried out in May, well before leaf shed of the deciduous and semi-deciduous species, which mostly occurs by August. Linear relationships wereestablished between branch diameters and total branch leaf areas by strippingall of the leaves from the branches, measuring the leaf fresh weight and con-verting this to a branch leaf area using the specific leaf area for each species.Specific leaf area was measured for a 50–200 g subsample of the stripped leavesfrom each branch.This procedure could not be used for the conifer Araucaria because of its

small, scale-like leaves that have a spiral phyllotaxis and are crowded aroundthe branchlet. In this case, leaf area was calculated by spraying hairspray over abranchlet to act as an adhesive and then coating this in a single layer of fineglass beads. The leaf area could then be calculated knowing the branchletweight before and after adding the glass beads (and hence the weight of thebeads needed to fully coat the branchlet) plus the diameters and weights ofthe beads. This procedure estimates the total leaf area (i.e., top and bottom

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surfaces) and this estimate was subsequently halved to provide a leaf surfacearea comparable to the single-sided leaf area estimate of the other species.Total leaf area was then calculated by measuring the diameter of all branches

on 7–11 trees of each species and summing the derived branch leaf areas (theleaf area of the leading tip of the tree was assumed to be similar to that of abranch of comparable size and was treated accordingly). A separate biomassstudy using trees from an adjoining trial allowed further sample trees of F.brayleyana and Araucaria to be included in this investigation. Species-specificrelationships were then found between total leaf area and tree stem girth.This procedure was followed for most species but could not be used for

Cryptocarya, F. schottiana, Khaya and Toona because they were either toosmall or had too few branches to destructively sample. In these cases all leaveson each of five trees were counted and converted to a total leaf area by mul-tiplying this number by the area of an average leaf. The average leaf wasestimated after collecting and quantifying the size of 20 leaves from each ofthese species.

Results

Height growth

The height growth of all species over the 5-year period of this study is illu-strated in Figure 1. The tallest trees after 5 years were Elaeocarpus, whichaveraged 9m in height. Other fast-growing species included Acacia, Cedrela,Grevillea – all of which reached 7m or more (Table 2). These four species werealso the fastest growing when growth was expressed in terms of diameters orvolumes. Four other species were intermediate in growth, reaching more than5m in height in 5 years. This group included Araucaria, the only native rain-forest species currently used in commercial plantations, F. brayleyana, Khayaand Rhodosphaera.

Cryptocarya was the slowest-growing species in this trial. A number of in-dividuals died during the study period; however, some of the surviving seed-lings of this species have recently begun to show an improvement in vigour andgrowth rate. This poor early performance may have been due to photo-inhibition that is now being reduced through shading by surrounding treesof other species. Another one of the poorer performing species was Toonaciliata, but in this case the cause was insect damage. Like many members ofthe Meliaceae, Toona is susceptible to shoot borers. In this case, the insectresponsible was Hypsipyla robusta, which appeared in the first summer afterplanting and has reappeared every summer since then. Until that time Toonawas growing very fast; subsequently, each summer it has lost most of its leadingshoots. Only a few trees have died, but overall growth has been poor. Insectsalso damaged Cedrela (another member of the Meliaceae) in the secondgrowing season of the study. However, the borer damage stimulated axillary

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Figure 1. Height growth of each species to the age of 5 years.

Table 2. Average tree growth parameters after 5 years. Values in brackets are standard deviations

Species Height (m) DBH (cm) Conical volume (m3)

All Best 5

Acacia melanoxylon 8.3 (1.8) 10.3 (0.9) 16.2 (3.8) 0.081 (0.034)

Araucaria cunninghamii 5.2 (1.2) 6.6 (0.4) 9.2 (1.7) 0.018 (0.005)

Argyrodendron trifoliolatum 3.3 (1.3) 5.0 (0.5) 4.5 (2.2) 0.005 (0.002)

Castanospermum australe 3.8 (1.0) 5.3 (0.3) 4.7 (1.9) 0.005 (0.003)

Cedrela odorata 7.3 (1.2) 8.8 (0.3) 14.6 (2.9) 0.073 (0.020)

Cryptocarya erythroxylon 2.2 (1.0) 2.7 (0.8) 2.6 (1.6) 0.001 (0.001)

Dysoxylum fraserianum 3.1 (0.7) 4.1 (0.3) 5.6 (6.2) 0.004 (0.001)

D. mollissimum 4.3 (0.6) 5.2 (0.2) 6.7 (1.2) 0.010 (0.002)

Elaeocarpus grandis 9.0 (1.0) 10.4 (0.3) 14.6 (2.0) 0.071 (0.019)

Flindersia brayleyana 6.0 (0.6) 6.6 (0.4) 9.6 (1.2) 0.020 (0.005)

F. schottiana 3.7 (1.2) 5.5 (0.5) 4.9 (1.7) 0.004 (0.003)

Gmelina leichhardtii 4.2 (0.7) 5.1 (0.3) 7.3 (1.7) 0.011 (0.003)

Grevillea robusta 7.6 (1.6) 8.6 (0.8) 14.4 (3.4) 0.060 (0.016)

Khaya nyasica 5.2 (1.2) 6.5 (0.7) 7.9 (1.6) 0.019 (0.005)

Rhodosphaera rhodanthema 6.3 (0.8) 7.2 (0.3) 13.7 (1.7) 0.046 (0.010)

Toona ciliata 2.8 (1.3) 4.3 (0.4) 5.5 (2.4) 0.006 (0.003)

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growth and many individuals developed multiple leaders. Subsequent insectproblems have been slight. Other members of the Meliaceae in this trial(Dysoxylum and Khaya) appeared to be unaffected byHypsipyla or other insectproblems.Most of the slower-growing species showed no obvious signs of ill-health.

The only exception was Gmelina. Many trees of this species had significantlevels of herbivory and suffered premature leaf drop from the upper crown. Thegrowth rate of Gmelina was initially rapid, but declined in recent years as thepremature leaf drop problem worsened. No other major insect problems werefound except that some unidentified borers were noted in several Acacia stems.The performance of the best five trees of each tree species is also given in

Table 2 and shows that these averaged 1–2m faster height growth than theoverall mean height over the measurement period. That is, there is considerablescope for productivity increases through tree selection in all species.The mean height growth of the plantation trees (excluding Toona because of

the shoot borer) was also compared to the species attributes listed in Table 1.Height growth rate was negatively correlated with wood density (height at 5years=12.82 – 0.01 (wood density), n=14, r2=0.487), successional status(height at 5 years=9.44 – 1.57 (successional rank), n=14, r2=0.452)and positively correlated to maximum photosynthetic rates (height at 5years=0.27(Psat)+1.99, n=11, r2=0.447). This suggests that species withlow timber densities, early successional status and/or high photosynthetic rateswere generally the faster growing species in the trial.

Tree form and DBH size class distribution

The survival, tree form and the number of leading shoots for each species areshown in Table 3. Of the faster growing trees, Elaeocarpus was clearly superiorwith a high survival rate, a form ranking of 9.8 (out of 10) and an average of1.1 leaders per tree. Grevillea also ranked highly. Several of the remaining fastgrowing species had form ratings of 7 or less and a tendency for multipleleaders indicating the need for improvement through some kind of selectionprogram. Araucaria had a high form rating reflecting both the good naturalhabit of this species and the fact that the seed selected for this trial originatedfrom a Queensland Forest Service seed orchard. Cryptocarya had an extremelylow survival rate (32%) after 5 years while all other species had a survival rateabove 80% (Table 3).Figure 2 illustrates the DBH size class distribution of the eight slower verses

the eight faster growing species at the age of 5 years. For the slower growingspecies (Figure 2(a)) the number of individuals in each size class increases up tothe <8 cm DBH size class, after which there is rapid decline with only oneindividual (Gmelina) greater than 10 cm DBH. In the faster growing species theDBH distribution is bimodal with Araucaria, Khaya, Rhodosphaera and F.brayleyana comprising most of the individuals in the lower size classes (<6 to

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<14 cm) while Elaeocarpus, Acacia, Cedrela and Grevillea are generally in thelarger DBH classes (Figure 2(b)).Individuals without acceptable form (�7) are spread across most size classes

but account for nearly 50% of the individuals in certain size classes. For treeswith a DBH <2 cm, Toona and D. fraserianum make up most of the in-dividuals with poor form while Acacia and Cedrela were the main species withpoor form in the 14–16 cm and 16–18 cm DBH size classes.

Growth patterns

The various species differed substantially in terms of monthly height growthbut three broad categories can be recognised (Table 4). Several species wereable to grow very rapidly (more than 18 cm per month) during favourableconditions and rarely had periods of slow growth (e.g., less than 6 cm permonth). These species included Elaeocarpus, Acacia, Cedrela and Grevillea(Group 1). Most of these are representative of early rather than late succes-sional stages. Other species, including Cryptocarya and D. fraserianum, had fewsuch growth spurts and rarely grew faster than 6 cm per month, irrespective ofseason (Group 3). These tended to be later successional species. Between thesetwo extremes were species such as Araucaria, Khaya, Rhodosphaera and F.brayleyana which were not able to match the overall vigour of the Group 1species but maintained steady growth throughout the 31 month period and,like Group 1 seldom grew slower than 6 cm per month (Group 2). Additionally,

Table 3. Survival, number of leading shoots, form and the proportion of trees that

have acceptable form (>7 out of 10 form rating) after 5 years growth

Species Survival (%) Mean Form rating >7 (%)

Leaders Form

Acacia melanoxylon 86 1.5 6.8 37

Araucaria cunninghamii 96 1.0 9.6 96

Argyrodendron trifoliolatum 82 1.3 8.5 86

Castanospermum australe 96 1.5 7.8 64

Cedrela odorata 96 1.7 6.8 33

Cryptocarya erythroxylon 32 1.0 9.9 100

Dysoxylum fraserianum 89 2.2 6.4 22

D. mollissimum 100 1.1 7.9 75

Elaeocarpus grandis 93 1.1 9.8 96

Flindersia brayleyana 96 1.3 8.2 82

F. schottiana 100 1.1 9.3 93

Gmelina leichhardtii 93 1.2 9.4 100

Grevillea robusta 100 1.2 8.6 81

Khaya nyasica 89 1.4 9.1 100

Rhodosphaera rhodanthema 100 2.1 7.1 39

Toona ciliata 93 1.5 6.8 25

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species such as Dysoxylum mollissimum, Gmelina, Castanospermum and Toonawere capable of occasional periods of faster growth but were mostly slow-growing (less than 6 cm per month) and therefore in Group 3. In the case ofToona this pattern was largely a consequence of the insect damage. Withoutthis damage Toona would have almost certainly been classed in one of thefaster growing groups.

Crown development

The rate of lateral expansion of the crown is an indication of the capacity of aspecies to capture site resources and exclude weed competition. The first speciesto have a crown diameter >3m and thereby achieve canopy closure was

Figure 2. DBH size class distribution for (a) the eight slower growing species (Argyrodendron,

Castanospermum, F. schottiana, Cryptocarya, D. fraserianum, D. mollissimum, Gmelina, and Toona)

and (b) the eight faster growing species (Acacia, Araucaria, Elaeocarpus, Cedrela, Grevillea, F.

brayleyana, Khaya and Rhodosphaera). The darkened portions of the bars represent the number of

individuals with unacceptable form (<7).

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Elaeocarpus (at just over 3 years of age), but by 5 years age, eight of the 16species had crowns of this size (Table 5). Most of these were species able togrow for much of the year and were in Groups 1 and 2 (Table 4). While fastcrown diameter growth was generally accompanied by rapid height growth,this was not always the case and height: crown diameter ratios ranged from1.18 to 2.32 with taller species having both higher and lower ratios.The mean leaf area of each species at the age of 5 years was calculated using

regression equations based on the stem basal area of trees in the trial derived at4 years. The only species for which a significant relationship could not beobtained was Toona. The estimated mean leaf areas of all other species areshown in Table 5. Elaeocarpus and Acacia had the largest leaf areas, both ofwhich were >100m2. Most other species had leaf areas considerably less thanthese values. Across all species there was a good correlation between leaf areaand mean height growth at 5 years (n=15, r2=0.690, p<0.01). Since crowndiameters were measured for each tree it was also possible to calculate a leafarea index (LAI) for individuals of each species. These are also given in Table 4and show mean LAI values mostly <10, although Acacia had an LAI of 12.9.LAI and height growth were correlated (n=15, r2=0.298, p<0.05), but notas strongly as total leaf area and height growth.

Table 4. Species groupings according to rates of monthly height growth

between 17 and 48 months. Data are the number of months (out of 31) in

which a species achieved a particular growth rate (cm month�1)

Species groupings Height growth categories (cmmonth�1)

<6.0 >6.0 >12.0 >18.0

Group 1

Acacia melanoxylon 5 26 15 8

Cedrela odorata 10 21 13 8

Elaeocarpus grandis 8 23 17 14

Grevillea robusta 8 23 14 8

Group 2

Araucaria cunninghamii 5 26 13 3

Flindersia brayleyana 9 22 14 4

Khaya nyasica 9 22 7 4

Rhodosphaera rhodanthema 8 23 8 4

Group 3

Argyrodendron trifoliolatum 14 17 8 0

Castanospermum australe 18 13 7 4

Cryptocarya erythroxylon 26 5 0 0

Dysoxylum fraserianum 23 8 2 0

D. mollissimum 16 15 5 1

Gmelina leichhardtii 20 11 5 2

Flindersia schottiana 20 11 1 0

Toona ciliata 22 9 4 3

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Discussion

Species performance

Five years age is an early stage at which to assess the performance of speciesthat might be expected to grow in a plantation for 40 years or more. However,it is already clear that three groups of species can be identified. Tree species ingroup 1 were those that grew most rapidly in height and DBH over the first 5years of the trial and could be expected to rapidly outcompete grasses andweeds. They included Elaeocarpus, Grevillea, Acacia and Cedrela. Of these,Elaeocarpus and Grevillea also had good form and appeared to be unaffectedby insect problems. The growth of Acacia was rapid but poor form and theoccurrence of stem borers make the species much less attractive. Acacia mel-anoxylon is distributed over a very wide latitudinal range from 168S to 438S(Boland et al. 1984) and is regarded as an important timber species at higherlatitudes. There is also evidence that the southern provenances are geneticallydistinct from the sub-tropical provenances from which the seed for the trees inthe current trial originated (Playford et al. 1993). Trials in Africa have foundthat A. melanoxylon will grow well in many environments but will only producequality timber in certain environments using seed of particular provenances(Harrison 1975; de Zwaan 1982). Notwithstanding the potential for tree im-provement through provenance selection, a recent review (Pinkard and Beadle2002) suggests that A. melanoxylon requires a substantial amount of inter-vention (including form and lift pruning) to grow well in plantations. Thecurrent trial supports this general conclusion. Cedrela had few significant insect

Table 5. Mean leaf area, LAI and crown diameter for all species after 5 years

Species Leaf area

(m2)

LAI

(m2m�2)Crown

diameter (cm)

Acacia melanoxylon 127.1 12.9 356

Araucaria cunninghamii 47.7 6.0 318

Argyrodendron trifoliolatum 22.2 6.1 216

Castanospermum australe 9.4 4.9 158

Cedrela odorata 94.4 8.9 368

Cryptocarya erythroxylon 6.8 6.1 118

Dysoxylum fraserianum 34.3 7.0 250

D. mollissimum 80.5 7.7 366

Elaeocarpus grandis 224.6 8.7 574

Flindersia brayleyana 29.8 3.4 334

F. schottiana 6.6 1.6 232

Gmelina leichhardtii 19.8 4.0 250

Grevillea robusta 77.7 5.9 410

Khaya nyasica 13.9 3.5 226

Rhodosphaera rhodanthema 93.2 8.9 366

Toona ciliata NA NA 142

NA: not available.

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problems but did have the second poorest form (after D. fraserianum) of all thespecies in the trial. Newton et al. (1995) noted large differences in the form ofCedrela between various provenances and such differences would have to beexplored further for Cedrela to become acceptable at the present site.All of the group 1 species had heights of 7m or more after 5 years. Group 2

consisted of species with heights exceeding 5m meters after 5 years (this in-cluded Araucaria, F. brayleyana, Khaya and Rhodosphaera). Araucaria is al-ready recognised as an important plantation species and was included in thetrial as a reference species. Its intermediate ranking, in terms of height growth,therefore suggests that some of the other species in this trial may be at least asattractive as plantation species as Araucaria. However, Araucaria does haveseveral important advantages that compensate for the slower initial growth.One is its very good form, including a strong apical dominance; another is itsexceptional ability to maintain vigorous growth beyond 30 years age whenmany other plantation species are beginning to suffer declining current annualincrements (Dale and Johnson 1991). More time is needed to determine whe-ther the faster initial growth and higher timber prices of the other species willoutweigh these considerable advantages. Of the remaining species in group 2,F. brayleyana is also recognised as a promising plantation species with highlyregarded timber. Rhodosphaera grew vigorously but its poor form, denselybranched crown, and multiple leading shoots may outweigh its height growthadvantage.Group 3 includes those species with a mean annual height growth of <1m.

This included Toona (because of the well-known shoot borer problem) andCryptocarya. In both cases it may be possible that these species would benefitfrom some form of temporary nurse crop. There is evidence that suggests thefrequency of insect attack on Toona is reduced when the trees are shaded(Keenan et al., 1995), while at least some of the Cryptocarya trees in the trialappear to have recently benefited from the shade provided by neighbouringtrees. More time is needed to evaluate the performance of these species inplantation mixtures. Others in this group, such as Gmelina, Castanospermumand Flindersia schottiana had high rates of survival under the trial conditions,but had subsequent insect problems or slow growth rates and might be difficultto establish in routine plantations.It is noteworthy that all species in the trial generally grew faster than in the

various previous trials in tropical and sub-tropical Queensland summarised byCameron and Jermyn (1991), imply that the attempt in the current study toprovide optimum growing conditions succeeded, notwithstanding the dryweather conditions sometimes experienced during the trial period.

Factors contributing to the differences in species performance

There are a large number of potential factors able to contribute to the differ-ences in species performance. In a natural forest the particular assemblage of

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attributes results in a species being best suited to one or another ecologicalcircumstance (e.g. light-demanding early, successional-species better adaptedto larger gaps or more shade-tolerant, later-successional species better adaptedto smaller gaps). The environmental conditions in this plantation differed fromthose of a natural forest and most likely approximate those found in a largegap. It is not surprising, therefore, that species from earlier successional stagesgrew faster than those from later stages adapted to smaller gaps or moresheltered conditions. Likewise, it is widely observed that species with lowtimber densities grow faster than species with high timber densities (Whitmore1984) and the results from this study support this generalisation. But, bythemselves, such statements are not useful in explaining the reasons for thedifferences in species performance in this trial.A fuller explanation of these patterns will require a more detailed analysis of

the architecture of these canopies. It has been suggested, for example, that inhigh-light environments any multi-layered canopy provides more carbon perunit of ground area than a mono-layered canopy (Kuppers 1989). This re-lationship did not appear to apply in the present study. The mean LAI valuesof each species were not as well related to growth in the present study as wasoverall leaf area. It has also been suggested that compound leaves are beneficialin sub-tropical forests subject to marked seasonal dry periods and for speciesneeding to achieve rapid vertical growth because the necessity to develophigher order branches is avoided and the costs of leaf shedding are low(Givnish 1988). This site is subject to a seasonal dry period but the fastestgrowing species (Eleaocarpus) had simple leaves. Under the conditions of thetrial this species also produced large, and presumably costly, near-horizontalside branches. Such branches are less likely to develop in natural forest con-ditions or where closer spacing reduces the amount of side light present.

Management issues

Initial inspection of Figure 1 suggests that over 5 years each species had re-latively constant growth rates, with the exception of Toona and Gmelina (whichsuffered insect attacks and leaf drop). However, the annual stand growth in-crement actually experienced a 30% reduction between years 4 and 5. Themean height increment declined from 1.25 to 0.9m per year, while the meanDBH increment decreased from 2.4 to 1.7 cm per year over this timeframe.Rainfall totals over these two years were similar, 1330mm in 1994 and1353mm in 1995, suggesting that the decline in growth was not related torainfall. It is more likely that competition for light and soil resources, due tocanopy closure and more extensive root systems, had become more intenseover this latter period. Silvicultural systems for timber production encouragethinning when growth rates start to decline so that a stand does not stagnate.In a plantation mixture, such as the one in this study, there are many optionson how to thin but as most mixed plantations will probably be managed by

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farmers it is appropriate to assess which method they might readily use. Thesimplest form of thinning is ‘geometric’ (or mechanical) which involves theremoval of trees according to a predetermined spacing or thinning pattern(Shepherd 1986). With the variable growth rates in this plantation, any type ofindiscriminant geometric thinning would probably remove many of the bestperforming trees and reduce the value of the stand. Alternatively, a simple lowthinning (Smith et al. 1997) would remove the suppressed and slower growingtrees and leave the trees (and species) that have had superior growth. Thus,trees below a certain size could be removed during thinning operations toprovide better growth conditions for the larger trees. If all stems smaller than10 cm DBH (Figure 2(a)) were removed by thinning operations, seven species(Argyrodendron, Castanospermum, Flindersia schottiana, Cryptocarya, D. fra-serianum, D. mollissimum, and Toona) would be removed entirely, with only asingle individual of an eighth species (Gmelina) remaining in the plantation.Therefore, a low thinning to increase productivity would virtually halve thediversity of the site.Shepherd (1986) suggested that a thinning should not only accelerate dia-

meter increment but also improve the average form of the trees that remain.This logic suggests that a form-based thinning, perhaps removing trees acrossall DBH size classes with unacceptable form (<7, Figure 2), could be em-ployed. This type of thinning would remove 36% of the basal area, comparedto a 25% reduction with a low thinning of stems under 10 cm DBH, but willsignificantly improve the average form of the stand and be more likely tostimulate growth of the remaining plantation trees. Several species, includingA. melanoxylon, C. odorata, D. fraserianum, Rhodosphaera and Toona, wouldhave significantly fewer individuals in the plantation following this formthinning (see Table 3), but the overall diversity of the site would not change.Although the methods for grading tree form are relatively subjective, mostfarmers readily recognise extremely branchy or crooked trees and could re-move them prior to canopy closure to reduce stand competition. Thus, aninitial form-based thinning in a farm forestry context would be far less pre-scriptive than other thinning methods, relatively easy to apply, and providesome productivity improvements.

Conclusions

The ever-increasing calls to use native species in mixtures for forestry activities(Haggar et al. 1998; Hartley 2002; Stier and Siebert 2002) is unlikely to becomewidespread in industrial plantations unless economic returns for non-timberproduction related services such as biodiversity and carbon sequestration canbe realised. Presently, most mixed rainforest species plantations are beinggrown by small landholders in the tropics for a diversity of benefits notachieved by exotic monoculture plantations (Harrison and Herbohn 2001).Many of these farm forestry systems will continue to be established with seed

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stock which has not undergone any sort of selection process, but this studysuggests that adequate weed and fertiliser treatments could result in reasonablegrowth rates for a range species. Furthermore, simple geometric or low thin-ning operations to release competition or maximise productivity appear to beincompatible with the non-production orientated goals of many of theseplantings. An effective initial management tool for a mixed rainforest speciesplantation that is starting to stagnate is a selective thinning based on tree formrather than size. After this thinning, a series of tradeoffs between productionand diversity will have to be balanced by individual farmers as their treesmature.

Acknowledgements

The authors would like to thank: Dave Cameron for his contribution to thedesign of the trial; Don and Audrey Pickering for allowing the trial on theirland; Greening Australia volunteers for planting and maintaining the trial;David Doley for valuable discussions concerning stand management; and allthe students from the University of Queensland who have helped monitor andmeasure the trial over the years.

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