47Tomy Listyanto, Kevin Glencross, J. Doland Nichols, Lesley Schoer and Chris Harwood

Australian Forestry 2010 Vol. 73 No. 1 pp. 47–52

Performance of eight eucalypt species and interspecific hybrid combinations at three sites in northern New South Wales, Australia

Tomy Listyanto1,2, Kevin Glencross1,4, J. Doland Nichols1,5, Lesley Schoer3 and Chris Harwood4,6

1Sustainable Forestry Program, School of Environmental Science and Management, Southern Cross University, Lismore, NSW 2480, Australia

2Faculty of Forestry, Gadjah Mada University, Jl. Agro. No. 1 Bulaksumur, Depok Sleman, Jogjakarta 55281, Indonesia 3Lesley Schoer & Associates Pty Limited, PO Box 477, Coffs Harbour, NSW 2450, Australia

4CRC Forestry, Private Bag 12, Hobart 7001, Australia 5Email: doland.nichols@scu.edu.au

6CSIRO Sustainable Ecosystems, Private Bag 12, Hobart 7001, Australia

Revised manuscript received 10 December 2009

Summary

Eight eucalypt taxa, represented by six seedlots and nine clones (two seedlots of Corymbia citriodora subsp. variegata (CCV), one seedlot each of Eucalyptus cloeziana, E. dunnii and E. pilularis, one seedlot and two clones of E. grandis, one clone of E. saligna, three clones of the interspecific hybrid E. grandis × E. camaldulensis and five clones of the hybrid E. urophylla × E. grandis) were tested in taxa trials on three sites in northern New South Wales, Australia.

Significant differences between sites and between taxa in survival, growth, stem form and attack by stem-boring insect larvae were demonstrated. All taxa grew best on the Convery site that had the highest mean annual rainfall, 1400 mm, on a Red Ferrosol soil derived from volcanic parent material; E. dunnii and E. pilularis grew to a mean dominant height of 17 m in 6 y, E. cloeziana to 15 m, C. citriodora subsp. variegata and E. saligna to 14 m. Fewer than 2% of trees of all taxa at this site were attacked by insects. The other two sites had Brown Dermosol soils and were drier, the Muller site receiving 1200 mm y–1 and the Ironbolt site 1100 mm y–1. Height and diameter growth of all taxa was slower at these sites, with E. pilularis and E. cloeziana showing the greatest reduction in growth while E. grandis × E. camaldulensis showed the least reduction. Overall survival at age 6 y was acceptable at all three sites, being highest at Convery site (86%) followed by Muller (77%) and Ironbolt (74%). While C. citriodora subsp. variegata and E. cloeziana had minimal damage at Mueller and Ironbolt, the other taxa had from 9% to 100% of stems affected by borers, with E. grandis, the E. saligna clone and E. urophylla × E. grandis proving particularly susceptible on these sites.

Keywords: plantations; species trials; growth; survival; stem form; insect pests; eucalypts; New South Wales

Introduction

The area of subtropical eucalypt plantation estate in north-eastern New South Wales (NSW) and south-eastern Queensland was about 115 000 ha in 2008 (Nichols et al. 2008). Planting commenced

in the 1960s and 1970s, when 20 700 ha were established around Coffs Harbour (NSW) comprising approximately equal areas of Eucalyptus grandis and E. pilularis. State Forests NSW then planted 26 500 ha in the period 1994–2007; 40% of this area was established to E. dunnii, 40% to Corymbia citriodora subsp. variegata and 15% to E. pilularis, with peak plantings taking place during 1997–2001. Forest Plantations Queensland has established a further 10 000 ha in south-eastern Queensland, mostly since 2001. The remainder of the estate has been planted by private companies.

The decision as to which species or provenance to plant is critical in plantation forestry. In Australia, subtropical timber plantation growers in particular have been faced with challenges selecting species because of site variability and a number of significant pests and diseases (Dickinson et al. 2004; Lee 2007). Trials that test a wide range of taxa at common locations using replicated treatment plots of small dimensions can help to inform decisions about species and provenance selection by rapidly providing comparisons of early growth, survival, health and stem form. Such trials are often described as species elimination trials (Eldridge et al. 1993). They are commonly followed by further comparison of the most promising candidate taxa using trials with larger plots. These are grown through a typical rotation without competitive effects between adjacent plots confounding comparisons.

Considerable research has been done to test candidate eucalypt taxa suitable for plantation forestry in subtropical Australia (Johnson and Stanton 1993; Lee et al. 2001, 2009). In trials testing 60 taxa across five sites in south-eastern Queensland, Lee et al. (2009) found that C. citriodora subsp. variegata was the most promising taxon at age 6–8 y after planting at both high-rainfall and low-rainfall sites, displaying relatively good growth and excellent resistance to attack by stem-boring insects. Eucalyptus grandis and E. longirostrata also displayed above-average early growth though they were highly susceptible to borer attack and were showing serious drought stress symptoms and mortality due to drought by age 6–8 y.

48 Performance of eucalypt species and hybrids in northern NSW

Australian Forestry 2010 Vol. 73 No. 1 pp. 47–52

Precise prescriptions for species selections have yet to be defined across the highly variable landscapes of the sub-tropical region. Of particular concern are periodic dry periods, which can lead to trees being stressed and therefore more susceptible to damage by boring insects (Lawson 2003b). Even species that demonstrate low or moderate susceptibility during ‘normal’ years will have heightened susceptibility during stress periods. Here we compare the performance of eight eucalypt taxa that were tested at three sites representative of environments available to plantation forestry in northern NSW. Survival, growth, stem form and susceptibility to attack by boring insects were assessed 6 y after planting to identify taxa with high survival, good early growth rate and stem form and low susceptibility to major insect pests.

Methods

Trial sites

This trial was established in 2001 on three sites in north-eastern NSW (Fig. 1), in plantation blocks belonging to Forest Enterprises Australia. The Convery site is about 20 km north-east of Casino on a sloping plateau (15°) and exposed western to south-western aspect at 450–500 m above sea level (asl) in the Mackellor Range.

Annual rainfall is about 1400 mm (Table 1). The soils are derived from Tertiary basalt, are well structured and referred to as Kraznosems (Morand 1994) or Red Ferrosols (Isbell 2002). This soil type has a good water-holding capacity and is well drained, with relatively high organic matter content and nutrient status and thus potentially supports successful plantation establishment (Gerrand et al. 2003).

The Muller site is about 20 km north-west of Kyogle in the Roseberry Creek locality. Annual rainfall is around 1200 mm and

altitude 220 m asl. The soils are derived from Tertiary volcanics and are commonly referred to as ‘chocolate’ soils derived from basalt. They have good water-holding capacity, organic matter levels and nutrient status. This soil is classified as a Brown Dermosol (Morand 1994; Isbell 2002). The trial is on an upper bench, with a slope of 7° and a southern to south-eastern aspect and sheltered by an escarpment from northerly and north-easterly winds.

The Ironbolt site is north-west of Casino on sloping hills near Dyrabba; annual rainfall is about 1100 mm and altitude 200 m asl. The soils are volcanic and range from chocolate to red with some sedimentary sandstone outcrops nearby. These soils are also classified as Brown Dermosols. The trial is located on the upper slope (15°) with an exposed eastern to southern aspect.

Taxa evaluated

Eight eucalypt taxa, either pure species or interspecific hybrids, each represented by one or more clones and or seedlots, were tested in the field trial (Table 2). The 17 genetic treatments tested included natural provenance seedlots of some species, and also seed orchard seedlots and clones that were commercially available for planting at the time, although they had not then been tested in the target environments in northern NSW. One seedlot of E. grandis from a South African seed orchard and two E. grandis clones were tested, as well as five hybrid clones of E. grandis × E. urophylla and three hybrid clones of E. grandis × E. camaldulensis. These clones were selected and tested in Brazil, and introduced to Australia in tissue culture (D.A. Kleinig, pers. comm. 2004). Also included were one clone of E. saligna, a seedlot of E. dunnii from a South African seed orchard, natural provenance seedlots of E. pilularis and E. cloeziana, and two natural provenance seedlots of spotted gum, Corymbia citriodora subsp. variegata. Three nurseries supplied the planting material (Table 2). Average heights of the different taxa 13 days after planting varied from 15 to 53 cm.

The treatments were tested in randomised complete block designs. At each trial site there were ten replicates, each with one five-tree row plot of each treatment. Replicates were set out as single rows

28°3

0'S

28°4

5'S

153°E 153°15'E 153°30'E

Mullers

Ironbolt

Convery

NightcapNational Park

Bangalow

Lismore

Casino Ballina

Kyogle

Kilometres0 6 12 18 24

N

AUSTRALIAStudyarea

Figure 1. Location of three trial sites in north-eastern New South Wales

Table 1. Yearly rainfall and long-term means (mm) at Bureau of Meorology sites nearest to each of the three trials. (Blank = rainfall records not available for this year.)

Year Site

Ironbolt(Casino airport)

Converys (Lillian)

Mullers(Grevillea)

2002 0559 08732003 0718 1339 09662004 1065 15462005 0883 1182 08582006 1292 12732007 0872 1159 10672008 1228 2161 1562Mean 1096 1400 1209

49Tomy Listyanto, Kevin Glencross, J. Doland Nichols, Lesley Schoer and Chris Harwood

Australian Forestry 2010 Vol. 73 No. 1 pp. 47–52

along the contour and plots were randomised within each replicate. Hence each treatment was represented by 50 trees at each site.

The trials were located within operational plantations of E. pilularis at Convery, E. grandis at Muller and C. citriodora subsp. variegata and E. grandis at Ironbolt, using standard site preparation that involved deep ripping of planting rows and mounding along the contours. The trees were planted at 2-m centres within rows and nominal spacing between rows was 4 m, resulting in an initial stocking of 1250 stems ha–1. Company records indicate that no fertiliser was applied, although this is not certain. Standard weed control practices used for the operational plantings were implemented, including the intra-row application of herbicide and the slashing of inter-row areas. The last use of herbicide was at about 18 mo after planting.

Examination of rainfall records at meteorological stations close to the three trial sites showed that annual rainfall for most years during the period 2001–2007 was below the long-term average (Table 1). The year 2005 appears to have been unusually dry at all three sites. Measurements and assessments were carried out in 2007, 6 y after establishment. The diameter of tree stems over bark (dbh) was measured on all surviving trees at breast height (1.3 m). Multi-stemmed trees had all stem diameters measured and recorded, but the diameter of the largest stem was used in the calculation of diameter means for a given species. The form of stems was assessed by assigning each tree to one of five stem straightness classes: 1 = multi-stemmed, 2 = poor straightness, 3 = fair straightness, 4 = only a slight bend and 5 = completely straight. Damage resulting from attack by stem-boring insect larvae was assessed by recording presence or absence of visible damage: 0 = no damage, 1 = damage. An estimate of mean dominant height for some of the taxa at each site was obtained by measuring the heights of five dominant trees of each sampled taxon that were felled for studies of wood quality.

Plot means were calculated for percentage survival, dbh, straight-ness score and percentage of trees with borer attack. Analysis of variance of each trait was carried out on plot mean values using

Genstat Release 11 with the following linear mixed model:

Y = µ + SITE + TAXON + SITE.TAXON + SITE.REPL + ERROR, where µ is overall mean, SITE is a fixed site effect, TAXON is a fixed effect, TAXON.SITE is the fixed taxon × site interaction, SITE.REPL is a random term for replicate within sites and ERROR is the vector of residual errors. Plot variance of dbh was also analysed using the same model to test whether taxa differed significantly in the tree-to-tree variability of their growth (Williams et al. 2002). Wald chi-square tests were used to test the significance of fixed effects in the model; residual degrees of freedom were very large, relative to denominator degrees of freedom, for all Wald tests. A critical difference (t0.05 × mean standard error of the difference of taxon × site comparisons) was calculated for comparing taxa performance at individual sites; this single value is approximate only, because the different taxa were represented by different numbers of genetic treatments, ranging from one seedlot or clone (E. cloeziana, E. dunnii, E. pilularis and E. saligna) to five clones (E. urophylla × E. grandis).

Results

Wald chi-square tests indicated significant differences between sites (P < 0.001) and between taxa (P < 0.001) for survival, dbh, stem form score and borer attack. Site × taxon interaction was statistically significant (P < 0.001) for all traits. Given the high levels of significance and the large denominator degrees of freedom in the tests, the significance of taxon differences and site × taxon interactions can be accepted with confidence (Menard 1995).

Overall survival to age 6 y was acceptable at all three sites. It was highest at the Convery site (86%) followed by Muller and Ironbolt (77% and 74%, respectively) (Fig. 2). At Convery, the taxon with the poorest survival (74%) was E. pilularis. E. saligna had 100% survival, with seven other taxa having more than 90% survival there. At the Muller site, survival was notably poorer for both

Table 2. Seedlots, clones and taxa tested at three locations in northern New South Wales

Code* Taxon Seedlot / Provenance / Clone No. Supplier

cloez Eucalyptus cloeziana Natural provenance (identity not known) QFRI/Mogill Tree Farm dun E. dunnii Safcol clonal seed orchard, South Africa Minyon/Payne gra-so E. grandis Safcol clonal seed orchard, South Africa Minyon/Payne gra-50 E. grandis Clone 5150 Yuruga/Dendrotech gra-52 E. grandis Clone 5152 Yuruga/Dendrotech gc-11 E. grandis × E. camaldulensis Clone 11 Yuruga/Dendrotech gc12 E. grandis × E. camaldulensis Clone 12 Yuruga/Dendrotech gc-20 E. grandis × E. camaldulensis Clone 20 Yuruga/Dendrotech pil E. pilularis Whian Whian elite, NSW Minyon/Payne sal-56 E. saligna Clone 5156 Yuruga/Dendrotech ug-57 E. urophylla × E. grandis Clone 5157 Yuruga/Dendrotech ug-58 E. urophylla × E. grandis Clone 5158 Yuruga/Dendrotech ug-59 E. urophylla × E. grandis Clone 5159 Yuruga/Dendrotech ug-60 E. urophylla × E. grandis Clone 5160 Yuruga/Dendrotech ug-61 E. urophylla × E. grandis Clone 5161 Yuruga/Dendrotech var-r Corymbia citriodora subsp. variegata Richmond Range, NSW Minyon/Payne var-w C. citriodora subsp. variegata Woondum, Queensland Minyon/Payne

*The code is an abbreviation of the taxon identity, as used in Figures 2–6, followed in some cases by an identity for the individual clone or seedlot

50 Performance of eucalypt species and hybrids in northern NSW

Australian Forestry 2010 Vol. 73 No. 1 pp. 47–52

E. pilularis and E. grandis than for the other taxa. At Ironbolt no species had more than 90% survival; E. grandis had only 45%.

Dominant tree height of those taxa and individual clones that were sampled for wood properties (Fig. 3), although not statistically analysed, displayed trends that were very similar to those for dbh (Fig. 4). Dominant height was greatest at the Convery site, being in the range 17–20 m for all sampled taxa. Eucalyptus cloeziana and E. pilularis showed the greatest reduction in dominant tree

height at the other two sites, having mean dominant heights of only 9–13 m at these sites.

The largest site mean dbh was recorded at Convery (14.6 cm) followed by Muller (11.1 cm) and Ironbolt (9.7 cm). Eucalyptus dunnii had the largest mean dbh, averaging 14.1 cm across all sites at age 6 y (Fig. 4). Eucalyptus pilularis displayed excellent growth at Convery (mean dbh 17.9 cm) and much poorer growth at the other two sites (dbh < 10 cm). Several other taxa including E. cloeziana, E. dunnii and C. citriodora subsp. variegata also showed poorer performance at Muller and Ironbolt than at Convery, although the difference between sites was less pronounced for these taxa. This can be contrasted with the performance of the E. grandis × E. camaldulensis clones, which did not respond to the better site quality at Convery, having dbh close to 10.5 cm at all three sites.

Analysis of plot variance of dbh showed that taxa differed significantly in the tree-to-tree variability of dbh. The clonal taxa E. urophylla × grandis and E. camaldulensis × grandis had significantly lower dbh variance. The average coefficient of variation (standard deviation of plot dbh / mean dbh) exceeded 30% for the seed-based taxa E. cloeziana, E. pilularis and C. citriodora subsp. variegata, whereas it was only 17% for the E. grandis × E. camaldulensis clones and 16% for the E. grandis × E. urophylla clones.

Stem form was superior for most taxa at the Convery site, where growth was fastest (Fig. 5). Eucalyptus grandis and E. pilularis had poorer stem form than the other taxa at all three sites, whereas E. saligna had the best stem form at all sites.

Borers observed were predominantly the larvae of the giant wood moth, Endoxyla cinerea (Lepidoptera: Cossidae) but also included Phoracantha species (Coleoptera: Cerambycidae). Convery, the site where growth was most rapid and survival was best, had the lowest mean incidence of stem borer attack, with 22% of trees affected (Fig. 6). The other two sites where growth was slower had much higher levels of damage, with 54% and 59% of trees overall showing damage at Muller and Ironbolt, respectively. The level of borer attack varied markedly between taxa, ranging from 0 to 100%. All five E. urophylla × E. grandis clones were highly susceptible to borer attack, as were the three E. camaldulensis × E. grandis clones and the E. grandis seedlot and clones. Levels

0

20

40

60

80

100

120

cloez dun gra gc pil sal ug var

Taxon

Sur

viva

l (%

)

Convery Ironbolt Muller

Critical difference for comparing site × taxon combinations (P = 0.05)

Figure 2. Survival of eight taxa to age 6 y. Taxa abbreviations are explained in Table 2.

0

5

10

15

20

25

cloez dun gra50 gc11 pil sal ug57 var R

Mea

n do

min

ant h

eigh

t (m

)

Taxon

Convery Ironbolt Muller

Figure 3. Dominant mean height for sampled taxa at age 6 y. Taxa abbreviations are explained in Table 2.

Figure 4. Mean stem diameter at breast height for eight taxa at age 6 y. Taxa abbreviations are explained in Table 2.

Figure 5. Mean stem form score for eight taxa at age 6 y. Taxa abbreviations are explained in Table 2.

02468

101214161820

cloez dun gra gc pil sal ug var

Mea

n db

h (c

m)

Critical difference for comparing site × taxon combinations (P = 0.05)

Taxon

Convery Ironbolt Muller

cloez dun gra gc pil sal ug var

Taxon

Convery Ironbolt Muller

0

1

2

3

4

5

Mea

n st

em fo

rm s

core Critical difference for comparing taxon × site combinations (P = 0.05)

51Tomy Listyanto, Kevin Glencross, J. Doland Nichols, Lesley Schoer and Chris Harwood

Australian Forestry 2010 Vol. 73 No. 1 pp. 47–52

of borer attack on E. pilularis and E. dunnii varied markedly between the three sites, with lower levels of attack at Convery, while E. cloeziana and C. citriodora subsp. variegata displayed little or no attack by borers at any of the sites.

Discussion

These experiments were similar in design to species elimination trials, which aim to rank many taxa using replications of small line plots, to eliminate poorly adapted taxa and identify the best performers for further testing in trials with larger plots, from which estimates of productivity may be obtained (Eldridge et al. 1993). Rankings for growth tend to become unreliable after three or so years because of competition between adjacent line-plots; slower-growing taxa tend to be suppressed, accentuating growth differences. In these trials, survival was generally high and mean dominant height at age 6 y differed by no more than 3–4 m among eight assessed taxa at each of the three sites (Fig. 3). There were ten replicates per site, providing a range of competitive environments for ten plots of each taxon. Therefore the growth rankings at age 6 y probably remain fairly accurate, although they must be considered with some caution. Borer damage can be more reliably assessed at ages beyond 4 y, so the 6-y assessment of survival, growth and borer damage presented here is considered to provide a reasonable indication of overall performance rankings of the taxa. However, the trials tested some taxa more rigorously than others. For example, testing one clone of E. saligna gave a much more tentative conclusion about the performance of this species than was obtained for C. citriodora subsp. variegata by testing two natural provenance seedlots. Differences in performance within each taxon are of interest, but it is difficult to draw precise conclusions about their significance because of the relatively small plot size and the heterogenous nature of the material (clones and seedlings) under test. As might be expected from their genetic uniformity, clones displayed lower within-plot variance in individual-tree growth than did seedling treatments. The two provenances of C. citriodora subsp. variegata tested (Woondum, Queensland and Richmond Range, NSW) differed little in their performance. In south-eastern Queensland, the Woondum provenance has been found to be superior in terms of its growth and resistance to attack by the fungal pathogen Quambalaria (Lee 2007), which did not affect trials being reported here. Some of the taxa tested had been selected under different

climate and soil conditions in countries such as South Africa and Brazil, where the giant wood moth is absent. Natural provenance seedlots of E. dunnii and E. grandis, and clones of E. grandis, E. grandis × E. camaldulensis and E. urophylla × E. grandis selected from trials in northern NSW, might therefore have performed better than the imported material tested in these trials. However, in taxa trials in south-eastern Queensland, Lee et al. (2009) found that E. cloeziana from a South African seed orchard displayed performance very similar to that of natural-provenance seedlots of this species.

From the results of these trials, some preliminary recommenda-tions on suitability of different taxa can be made. Considering diameter growth, E. dunnii was close to top-ranked at all three sites, although E. pilularis grew slightly faster at the Convery site. The poor stem form and much poorer diameter growth of E. pilularis at Ironbolt and Muller accord with earlier findings (Florence 1996) that this species should be planted only on high-quality sites. The growth of the E. grandis × E. camaldulensis clones was poor at all three sites, making this hybrid combination unattractive for further testing or operational planting, taking into account its relatively poor stem form and susceptibility to stem borer attack. There was relatively little in either the growth or stem straightness to separate the remaining five taxa at all three sites.

For growers of solid-wood products, damage to logs by stem-boring insect larvae resulting in increased stem breakage and reduced wood value is an important consideration. Lawson (2003a,b) considered levels of over 15% of trees attacked to be ‘very high’ in this context. On this basis, E. grandis and E. urophylla × E. grandis clones would be ruled out for solid-wood products at all three sites, while E. dunnii, E. pilularis, the E. grandis × E. camaldulensis clones and the E. saligna clone would be ruled out at the Ironbolt and Muller sites unless effective control measures can be implemented to reduce borer attack. Corymbia citriodora ssp. variegata had very low presence of borers, consistent with the findings of Lawson (2003a) who found this species to display low levels of attack in a study of 25 taxa at 2.5–3 y of age in south-eastern Queensland. Subsequent assessments of five Queensland trials by Lee et al. (2009) confirmed the very low levels of borer attack on C. citriodora subsp. variegata to age 6–8 y, and the very high levels of borer attack on E. grandis and E. urophylla and their hybrids.

The deeper soil and higher rainfall at Convery may result in lower tree stress during dry periods, such as occurred during this trial, and may have contributed to the lower incidence of borer attack in susceptible taxa at that site. The good growth of many of the tested taxa and lower levels of borer attack at Convery make species selection less critical on high-quality sites such as this. The Convery site is not typical of the large blocks of land now available for plantation forestry in subtropical NSW; on the basis of affordability Ironbolt and Muller are more typical of the land likely to be used for plantations in the region.

At the Ironbolt and Muller sites the two provenances of spotted gum (C. citriodora subsp. variegata) showed high survival, good growth, good stem form and low incidence of borer attack, enabling them to be clearly recommended. In the lower parts of the landscape in this region, however, frost is common. Since spotted gum is frost-sensitive when young (Lee 2007) it cannot be recommended for frost-prone lower slopes and valley-bottoms.

Figure 6. Incidence of trees with borer damage for eight taxa. Taxa abbreviations are explained in Table 2.

cloez dun gra gc pil sal ug var

Taxon

Convery Ironbolt Muller

Critical difference for comparing taxon × site combinations (P = 0.05)

0

20

40

60

80

100

Frac

tion

of tr

ees

with

bor

er d

amag

e (%

)

52 Performance of eucalypt species and hybrids in northern NSW

Australian Forestry 2010 Vol. 73 No. 1 pp. 47–52

Our results are generally consistent with those of Lee et al. (2009) who concluded that although a number of Symphyomrtus species and hybrids can grow well in good years, suitable climatic conditions are not guaranteed because of the high year-to-year climatic variability of this subtropical region, and Symphyomyrtus were highly susceptible to boring insects. In our trials, the Symphyomyrtus species and taxa (E. dunnii, E. grandis, E. saligna and the hybrids of E. grandis) grew relatively well but proved much more susceptible to stem borers than did the Corymbia species C. citriodora subsp. variegata and E. cloeziana (subgenus Idiogenes). Results from our study support the general pattern of planting now followed in the region. E. pilularis can be successfully planted only on high-rainfall, well-drained, fertile coastal sites. Inland from such sites, upper slopes are planted to C. citriodora ssp. variegata and lower frost-susceptible slopes to E. dunnii. The relatively rapid growth that is characteristic of E. dunnii was recorded at all three sites, although only at the Covery site was there a low incidence (2%) of borer attack. Eucalyptus dunnii is frost-resistant and for that reason is often planted in lower topographic positions. Typically those sites are on cracking, shrink–swell soils (Vertosols), periodically susceptible to waterlogging and to outbreaks of the Creiis psyllid (Angel 2007). The need to find taxa that are able to tolerate frost and water-logging-prone lower-slope sites, which are widespread in the region, warrants further trials testing a range of taxa that might prove suitable for these conditions.

Conclusion

In this trial we confirmed the rapid growth of E. dunnii at all three trial sites in northern NSW, as well as the rapid growth of E. pilularis on a high-quality site and the adaptability of C. citriodora ssp. variegata to sites of lower quality. Attack by stem-boring insects ruled out most taxa for solid-wood production, particularly on the sites of lower quality. Thus C. citriodora subsp. variegata would be the first choice for planting on lower-quality sites not exposed to severe frost, with E. cloeziana worthy of consideration as well. There is still a clear need to test a range of taxa for planting on the challenging lower-slope sites available for plantation forestry in this subtropical region.

Acknowledgements

We thank Forest Enterprises Australia for providing access to the trials and details of their establishment and management. John Grant provided information on soils at the three sites, and David Kleining provided taxa details. We also thank AusAID for its support of Tomy Listyanto’s studies at Southern Cross University.

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