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Blue Gum for Farm Forestry

Note Number: AG0851
Published: September 2000
Updated: February 2009

This Agriculture Note describes the characteristics, site requirements and wood quality of blue gum (Eucalyptus globulus) as a farm forestry species.

Introduction

 

Figures 1. Blue Gum Forest	Figure 2. Blue Gum Forest

Blue gum, (Eucalyptus globulus), is the most commonly grown plantation eucalypt in higher rainfall areas in Australia and overseas. This is due to its many favourable characteristics, especially its fast growth rate. The name globulus is Latin for ball or sphere and refers to the shape of the operculum or fruit cap.

Natural occurrence

 

There are four subspecies of Blue gum, all with similar general appearance but with slightly varying buds and fruit. The main subspecies grown is the Southern or Tasmanian Blue Gum (E. globulus ssp globulus) occurs mainly along the east coast of Tasmania, generally within 20 km of the sea, but can be found up to 60-70 km inland. It is also found on Flinders and King Islands. On the mainland it is restricted to South Gippsland and the Otways. The other subspecies occasionally grown for timber is the Victorian Blue Gum or Eurabbie (E globulus ssp bicostata) which is found in North East Victoria and on the south west slopes of NSW.

Provenances

 

There are many provenances of blue gum, each coming from different areas of its natural distribution. Most work on tree breeding in this species has concentrated on improving growth rates and pulp fibre characteristics, such as basic density and pulp yield of the Tasmanian Blue Gum. Trials undertaken in north east Victoria and Tasmania have revealed that provenances from the Otways, Jeeralang and Kuark areas have higher growth rates under plantation conditions, than many others tested. Flinders Island provenance is suggested to be more insect resistant than many others and provenances from the higher altitudes have greater frost tolerance than those from lower altitudes. There is some evidence, based on a very small sample across two sites, that the King Island provenance had a lower level of growth strain than Jeeralang and south east Tasmania provenances. Further work on tree improvement through selection and breeding is beginning to focus on sawing traits and showing some promise with respect to wood property traits.

Site requirements

 

Blue gum requires sites with an absolute minimum annual rainfall of 700 mm/year (unless reliable irrigation is available) and soils with a good water holding capacity. It prefers cool moist valleys with loamy to clay loam soils and can grow under conditions of moderate exposure. It has only a limited ability to tolerate dry conditions. As long as water is available it will grow well under high temperatures, but as with most eucalypts, they do not tolerate poor drainage. Seedlings are also only moderately frost tolerant, and leaf scorch has been observed at temperatures below -5 oC.

Growth rates

 

Blue gum reaches heights of 70 m tall and diameters of 2 m at breast height when grown on the best sites with over 1500 mm rainfall, mild summers and over 2 meters of topsoil. Growth rates can reach 35 - 40 m 3/ha/yr on these good sites, but are more commonly between 15 - 25 m 3/ha/yr.

Management

 

Blue gum generally has good straight form, even when open grown. If clearwood is being grown, regular pruning will be necessary to keep the knotty core of the log to a minimum 15 cm in diameter. The susceptibility of the species to dry conditions means thinning of Blue Gum plantations grown for clearwood production is imperative where they are planted at high densities. Under good growing conditions, sawlogs can be produced in 20 years.

 

Where Blue Gum is grown for woodchip, dense plantings of around 1000 stems/ha are established to get maximum wood volume from the area. Generally no thinning or pruning is undertaken and harvest is within 10 - 15 years.

Blue gum responds well to fertiliser application, particularly phosphorus, nitrogen and potassium. Increases in growth rate of 5 m 3/ha/yr have been measured as a result of fertiliser addition to previously unfertilised country in NE Victoria. Even in previously fertilised pasture paddocks, responses to fertiliser addition at planting have still been observed. These responses are only evident where good weed control is also undertaken during early growth. The micronutrient, boron, has been found to affect tree form significantly where deficiencies occur, particularly on ex-pasture sites, causing distorted and stunted growth. In coastal areas, zinc and copper responses have also been observed.

 

The species grows well from the cut stump after harvesting. This ability to coppice can reduce the need for re-establishing plantations after early harvest. Harvesting should be to a height of 10 cm above the ground. If trees are cut too high, valuable log is left on the stump and coppice shoots are more vulnerable to wind damage. The coppicing ability of trees cut at the level of the root collar is poor. Thin the coppice regrowth to one main leader per stump at 1 2 years. The ability to reshoot well gradually declines with each coppice. The future release of improved genetic material by breeders is likely to lead to replacing trees after one coppice harvest with superior stock.

Pest and disease risks

 

Blue gum is susceptible to a number of pests including autumn gum moth larvae, sawflies, and grasshoppers. It is the juvenile foliage that is most susceptible. Healthy, fast growing trees are the best insurance against attacks from these pests. Generally pest outbreaks in plantations are not as common as seems to occur in more isolated or exposed trees planted as windbreaks or in gardens. Drought stressed trees are susceptible to wood borer attack. Blue gum is moderately resistant to the Phytophthora root rot ( Phytophtora cinnamomi spp )fungus. Leaf diseases such as Crinkle Leaf Disease (Mycosphaerella Spp.) and Corky Leaf Spot (Aulographina eucalypti) can affect Blue Gum leaves, but do little long term damage. In the early growth stages, seedlings are susceptible to browsing by rabbits, hares and wallabies, but are relatively unpalatable to stock.

Uses

 

The timber from Blue Gum has a wide range of uses, from pulp to sawn products. It is considered as the worlds top-quality pulping species due to its light colour and high fibre yield. The sawn timber is useful for tool handles, furniture, panelling, veneering, fencing, posts, poles, sleepers, heavy constructions, beams, building frames and internal flooring. It is currently being sawn in mills in Victoria for flooring, parquetry and furniture as well as structural timber.

Wood quality

Figure 3. Blue gum timber used in flooring	Figure 4. Wood grain of blue gum

The heartwood is pale brown, sometimes with a pinkish tinge. The sapwood is paler but not always distinguished from the heartwood, and can be up to 50 mm wide. The texture is medium and relatively even with the grain often interlocked and growth rings prominent on the end sections.

The green density is between 1100 and 1200 kg/m 3 of native forest trees, with the air-dry density at 12% moisture being about 900 kg/m 3. Fast grown young plantation timber will have a lower density than this. The heartwood is only moderately durable in the ground (Class 3) and hard. The sapwood is susceptible to the lyctid borer.

 

The fibre of Blue Gum is short with high strength, making it a sought-after species for pulpwood for tissue and paper production. Most processors are prepared to pay a premium for plantation-grown blue gum pulpwood compared to mixed native forest pulpwood. Freedom from fire damage and associated charcoal, low levels of gum and lack of fungal disease or termite mud are also important for quality pulpwood.

 

The slow self-pruning in plantation-grown blue gum, particularly of dead branches, can have a large affect on the presence of knots in the timber, and therefore the quality of a sawn product. The need to plant either very densely to minimise branch size and therefore the pruning required, or to plant at wider spacing and mechanically prune all branches, must not be underestimated in the production of quality sawn timber.

Drying characteristics

 

Wood from Blue Gum is difficult to dry successfully, particularly for back-sawn products, with problems of surface and internal checking. Successful drying without defects can be achieved by using careful drying regimes. This includes initial slow air drying in enclosed sheds or plastic wrap, to a moisture content below 30% (through the whole board). Once this is achieved, boards can be dried more rapidly in kilns with reconditioning.

Summary

Blue gum is an excellent species to grow in high rainfall sites with moderate to deep topsoils. It is mainly grown for pulpwood, requiring close access to markets. If grown for sawn timber, the wood from plantation grown trees will need to be carefully sawn and dried to minimise processing defects.

References

Bootle, K.R. (2004). Wood in Australia. Types, properties and uses.2nd edition. McGraw-Hill Book Co, Sydney.

Borschman, R. (May 1996). Report on the Monitoring and Evaluation of Farm Forestry Sites in North East Victoria. North East Farm Forestry Program, Centre for Forest Tree Technology.

Costermans, L. (1994). Native Trees and Shrubs of South Eastern Australia. Lansdowne Publishing Pty Ltd, Sydney.

Costermans,L (1998) Trees of Victoria and Adjoining Areas. Costermans Publishing, Frankston.

Hall, N., Johnston, R.D., and Chippendale, G.M. (1975). Forest Trees of Australia, Australian Department of Agriculture, Forestry and Timber Bureau, Australian Government Publishing Service, Canberra.

Hillis, W.E. & Brown, A.G., Ed. (1984). Eucalypts for Wood Production , CSIRO Australia.

Phelan, T. & Higgins, I. (Nov 1996). Timber Growing Opportunities in North Central Victoria. Centre for Land Protection Research, Technical Report No 38. Department of Natural Resources and Environment, Victoria.

Washusen, R. (1995). Farm Forestry Species for the Benalla Landcare Farm Forestry Group. Benalla Landcare and Farm Forestry Group, Benalla.

Acknowledgements

This Agnote was developed by Philippa Noble, September 2000.

It was Reviewed by:
Philipa Noble, Farm Services Victoria Farm Forestry. January 2008.