Preliminary Results of a Corymbia maculata (syn. Eucalyptus maculata) (Spotted Gum) Provenance Trial using Street Tree Selection criteria

Sarah Bone, The University of Melbourne, Burnley College

Summary

Eight Spotted Gum types (6 provenances of Corymbia maculata and one each of the species Corymbia variegata and Corymbia henryi) were assessed in a field-grown provenance trial undertaken on the grounds of Burnley College, Melbourne Victoria.  Differences in growth rate, stem structure, tree form and health of the provenances were recorded over an eight-month period (Nov 2000 – July 2001) from the planting of glasshouse-raised four-month-old seedlings to harvest at twelve months of age.  While preliminary data analysis has identified significant differences in provenances for measures of survival, growth rate and stress tolerance (p £ 0.05) significant differences in stem structure and other measures of tree form have not been recorded.  This paper summarises methods used to assess and measure growth traits of the 160 trees studied and presents a preliminary view of the results (project completion date March 2002).

Introduction

Urban tree improvement through a selection of superior ecotypes (within tree species) is an under-utilised tool in Australian amenity horticulture. Our native trees have received relatively little attention in street tree improvement programs to date.  With the growing trend of municipalities selecting alternatives to eucalypts for street plantings, the need for reliable data on the performance of our eucalypts and the identification and selection of ecotypes better suited to urban situations is now greater than ever. Corymbia maculata (formerly Eucalyptus maculata Hook.) has been the subject of a number of provenance studies over the years (Larsen, 1965; Andrew 1969; Darrow 1985; Forestry Commission of NSW 1985; Tibbits 1999; Mazanec 1999) none of which have compared ecotypes using horticultural criteria.  The provenance trial described in this paper was established in order to identify and recommend superior ecotypes (if any) of the Spotted Gum group using urban tree selection criteria.

Materials and Method

Eight Spotted Gum provenance seed lots were purchased from the Australian Tree Seed Centre – a division of CSIRO following an assessment of a total of ten natural forest stands of C. maculata in NSW and Victoria in June -July 2000.  From available seed lots, eight ecotypes were selected (listed in Figure 1) including one each of two other Spotted Gum species previously known as Corymbia maculata – Corymbia henryi and Corymbia variegata.

Trial Design & Establishment

Germination was undertaken in August 2000 and seedlings were raised in a glasshouse for a period of 4 months before field planting.  Of an average of 120 plants reared from each provenance 30 relatively uniform (in height) seedlings were selected. To offset any tendency to pick the tall poppies (although this was carefully considered during tagging of plants) of the 30 selected, the ten largest specimens were removed to arrive at the final 20 for planting.

An 8 x 35m plot of sandy loam soil in the grounds of Burnley College was cultivated and covered with a weed mat prior to planting. 160 trees (20 replicates of each provenance) were planted at 1 x 1.3m spacing within the plot.

Trees were planted following a randomised block design consisting of four blocks of forty trees.  Each block contained five replicates of each provenance, occurring as a randomly planted row.  A buffer strip was planted on the northern and southern perimeters of the plot but not on the eastern or western plot boundaries due to space restrictions.

The trees remained in the ground for a period of eight months.  After an establishment phase of three months, measurements commenced and continued until mid-July 2001 when trees were harvested.

Measurements

Trees were assessed on numerous traits of growth and early structural development – categorised as either growth rate, stem form, tree form or health and stress response (See Table 1). Both numerical and categorical data were collected.  The trees were subjected to several biotic and abiotic stresses during the course of the trial.  These included insect grazing and bird damage during the establishment phase, the presence of a fungal pathogen as a secondary stress (isolated yet not identified), cold temperature injury and soil nutrient imbalances. Originally planned to run until the end of August, the trial was harvested in mid-July 2001 due to declining growth rates and large levels of dieback and foliage distortion exhibited.  The unexpected stresses did however allow for further comparison of the performance of the eight provenances prompting additional measures of health and structure to be recorded.

Table 1.  Growth and development traits recorded in Spotted Gum provenance trial

Data Analysis

Only preliminary data analysis has been conducted to date.  Two-way analysis of variance has been undertaken to establish a significant difference (p £ 0.05) between provenances and the necessity for further analysis (log transformation of data, pair-wise comparisons and least significant difference tests).  Categorical data appearing mostly as percentages were subject to arcsine square root transformation prior to initial statistical analysis. Three deaths were recorded during the trial and were removed from the data set prior to analysis. Tables 2-5 display the means for each provenance and the relative ranking of each (if applicable). P-values that have been generated in the analysis of variance are given for most of the assessments reported in this paper.  Readers are warned to view results with caution as neither variation within the provenances nor variation between blocks (accounting for site differences and nuisance variables) have been taken considered in this early stage of data analysis.

Results and Discussion

Growth Rate

Table 2. Growth rate. Preliminary results

Although the findings recorded in Table 2 are at a preliminary stage of statistical analysis, the most vigorous provenances are clearly identifiable.  Significant differences between provenances have been found for stem fresh weight (P < 0.05:P = 0.005), fresh weight of roots (P < 0.05:P = 0.000), stem diameter 1 (P < 0.05:P = 0.006), height (P < 0.05:P = 0.000), lignotuber diameter (P < 0.05:P = 0.004) and the number of lateral branches present (P < 0.05:P = 0.001).  Provenances 4 (C. maculata Bodalla SF site 2), 8 (C. maculata Wingello) and 6 (C. variegata – Richmond Range S F) appear to exhibit stronger rates of growth than the other provenances.  These three ecotypes are ranked between first and fourth for every measure.  Conversely, provenance number 3 (C. maculata Mumbula SF) is ranked seventh or eighth for 87.5% of measurements – demonstrating its weakness in comparison to the other ecotypes.

Of the 160 trees grown only three deaths were recorded all belonging to provenance number 1 (C. maculata Bodalla SF site 2) with 15% of the population not surviving, differing significantly from the other ecotypes (P < 0.05:P = 0.032). A number of low values were however recorded for fresh weights, heights and stem diameters suggesting that trees were struggling to survive in provenances 3 (C. maculata Mumbula SF) and 7 (C. maculata Curryall SF) also.   Provenance 4 (C. maculata Bodalla SF site 1) ranked highest for mean condition rating closely followed by Provenance 8 (C. maculata Wingello), with mean values of 4.05 and 3.8 respectively. Apical death was recorded as significantly different between provenances (P < 0.05:P = 0.001) with 90% of C. henryi (Provenance 5) specimens injured.   Significant differences were also found between provenances (P < 0.05: P = 0.019) for a change in leader as identified in the stem but were not recorded for foliage density, die back, or limb drop.

Stem structure

The proportion of single-stemmed trees was significant (P < 0.05:P = 0.000) with Provenance 6 (Corymbia variegata) recorded as a single-stemmed tree 80% of the time.  Additionally, the same provenance was ranked above average to excellent (score 4 or above on a scale of 1 –6) in terms of stem straightness for 75% of assessments – 35% of the trees were recorded as exhibiting excellent stem form (6/6).  Significant differences were not recorded between provenances for percentages of bifurcated or multi-stemmed specimens, additional forking of stems or shared dominance in forked or multi-stemmed trees.

Tree Form

Most measures of tree form did not reveal significant differences between provenances in the two-way analysis of variance. This can most likely be attributed to the nature of the data (categorical) and its method of collection (subjective assessment).  The only differences in measures of tree form applied to trees with an upright growth habit (P < 0.05:P = 0.000). Corymbia variegata exhibited a strongly upright form in 90% of situations and specimens of Corymbia maculata (Provenance 8 Wingello) were 80% upright in growth habit. Similarly, there were no weeping trees recorded in provenances 6 (C. variegata) and 3 (C. maculata Mumbula SF).  Data concerning the radial distribution of branches, canopy balance and mean angle of branch attachment did not uncover any significant differences between Spotted Gum ecotypes.

Conclusion

While preliminary results regarding stem structure and tree form do not suggest that variation between provenances is significant, promising results for vigour (tree growth) and vitality (tree health) have emerged.   Corymbia maculata provenances 4 (Bodalla SF site 2) and 8 (Wingello), and the species Corymbia variegata (provenance 6) have consistently ranked higher than other provenances in statistically significant traits measured.  Conversely, provenances 3 (Corymbia maculata Mumbula SF) and 7 (Corymbia maculata Curryall SF) have been the poorest performers in many aspects of this trial. Whether these patterns of performance will remain fixed after complete statistical analysis of data is currently unknown.  It is hoped however that by the completion date of this research project (March 2002) superior ecotypes of the Spotted Gum group can be recommended to the tree grower industry.

References

• Andrew, I. A. 1969 Inter-provenance variation of Eucalyptus maculata Hook. grown in Zambia, Australian Forestry: 192-202.
• Darrow, K. 1985 Provenance trials of Eucalyptus maculata in South Africa: tenth year results, South African Forestry Journal (133): 12-17.
• Forestry Commission of NSW 1985 Notes on the silviculture of major NSW forest types, Sydney: Forestry Commission of NSW.
• Larsen, E. 1965 A study of the variability of Eucalyptus maculata Hook. and Eucalyptus citriodora Hook., Canberra: Commonwealth of  Australia Department of National Development, Forestry and Timber Bureau, Forest  Research Institute.
• Mazanec, R. 1999 Thirteen year results from a Spotted Gum provenance trial in the Wellington catchment of Western Australia Australian Forestry: 62 (4) 315-319.
• Tibbits, J. 1999  An evaluation of the breeding potential of spotted gum for timber, Thesis (Bachelor of App. Sc Hons.), Creswick School of Forestry, Institute of Land and Food Resources, University of Melbourne.