Sugar gum for farm forestry

Note Number: AG0892
September, 2000

This Agriculture Note describes the characteristics, site requirements and wood quality of sugar gum as a farm forestry species.

Figure 1. Sugar gum woodlot near Dimboola, Victoria

Sugar gum (Eucalyptus cladocalyx) is a medium to large eucalypt that has been widely planted in western Victoria since the 1870s. It is a common sight along roadsides and fence-lines, where it has been planted to provide firewood, poles and shelter.

Sugar gum has a hard, durable and attractive pale timber suited to a range of products. Its favourable sawing and drying characteristics, combined with tolerance to adverse conditions, make it a suitable species for farm forestry.

This species is characterised by its colourful, smooth ‘gum’ bark. Plates of old bark are periodically shed, leaving irregular patches of orange, yellow and gray. Its leaves are dark green and glossy on the top and pale green below. The foliage is typically clustered at the ends of long, upright branches. The fruit is ribbed and barrel-shaped with a small opening at the top. They grow on leafless branches inside the crown and are shed in abundance.

Natural occurrence and species description

Figure 2. Natural occurrence of sugar gum

Sugar gum comes from a few distinct locations in South Australia (Kangaroo Island, Eyre Peninsula and southern Flinders Ranges), where rainfall ranges from 380-650 mm per year(see Figure 2). In the high rainfall areas of the Flinders Ranges trees may attain heights of 35 m and widths of 1.5 m. On poorer sites, heights range from 8-15 m with diameters to 40 cm.

In their natural range, sugar gum grows on poor, often shallow soils.

Provenance

No formal breeding work has been done on sugar gum. However, after a 100+ year history of cultivation it is clear that there is considerable variability in form between different collection locations. Investigations have commenced to determine which provenances (seed-sources) produce the straightest, fastest growing timber trees.

It is known that trees from the southern Flinders Ranges grow larger and straighter than from the Eyre Peninsula and Kangaroo Island. In Western Australian plantings, the Port Lincoln provenance has proven to be the most drought-tolerant. In South Australian trials, the Wanilla provenance is proving to be very poor for timber production. Seed from western Victorian plantations has performed well in Victorian provenance trials. Seed from the Wail plantation, derived from the Wirrabara provenance, is probably superior to most other collections, as the trees have undergone some selection since the first plantations were established.

A genetic defect causing multiple stem form and low bushy growth has been popularised in the horticultural trade. This variety is not suited for timber production but together with its aesthetic value, may be useful for shade and shelter.

If collecting seed to grow for timber production, collect it from preferred sources exhibiting the tallest, straightest growth. These traits will tend to be maintained in the new planting. An opportune time to collect large quantities of seed is when trees are being harvested from belts of woodlots.

Site requirements

Sugar gum is often selected for farm forestry on infertile sites because it typically performs well, where few other species do. It is tolerant of drought and exposed conditions. Sugar gum will tolerate low to moderate salinity but is intolerant of waterlogging. Avoid planting too early in the season if there is a likelihood of post planting frosts, as it is susceptible to frost damage in the first year.

Growth rates

In a 450 mm rainfall zone a saleable firewood crop can generally be harvested after 15 years, while sawlogs may be produced by age 35. Recently, Melbourne University released a report on growth measurements of several tree species in the Wimmera region. This report includes growth information for sugar gum planted in state forest reserves, community plantings and school blocks from the 1940s. The report estimated growth per year, expressed as basal area (m2/ha, measured at 1.3 m height), to be 0.27-0.63 m2/ha/yr. These growth rates, and the measurements presented in Table 1, suggest that sugar gum could be grown for sawlog production on a 25-35 year rotation length.

Table 1 highlights the environment’s impact on growth, with areas of higher rainfall showing higher MAI. The above growth rates refer to growth in the main stem. Overall growth (including side branches harvested for firewood) could perhaps double these figures.

Table 1. Sugar gum growth rates from established plantations (excluding branch volume)

In other yield information collected from firewood cutting operations on the basalt plains of western Victoria, the MAI was estimated in 11 various-aged plantations; both coppiced and previously uncut stands. These yields suggest a MAI of stacked merchantable firewood of around 16 m3/ha/yr. A wood density of 0.85 tonnes per cubic metre was assumed when estimating volume. This higher growth rate can be attributed to a higher stocking rate (averaging around 1200-1400 stems/ha), greater rainfall and inclusion of side branches in volume estimates. With improved management and use of select seed it is expected that higher growth rates will be achieved.

Managing for specialty timber production

To improve wood quality trees should be regularly pruned to produce ‘clearwood’ - timber that is free from knots (see Agriculture Note AG0773: Eucalypt stem pruning). If contract pruners are not used, it is recommended that the landholder only commits to small annual plantings of 1 to 2 hectares. This will spread the pruning requirement out over a number of years.

Form-pruning to produce strong apical growth should be undertaken (see Agriculture Note AG0774: Guide to early age ‘form’ pruning).

These stands should also be thinned. Thinning would periodically remove trees of poor form and slow-growing trees, to reduce competition for water and nutrients and concentrate growth onto the best trees in the stand.

Sugar gum is often planted on a 3 m x 4 m grid in areas receiving less than 600 mm of rain annually, or in areas that have low soil fertility. At this spacing 833 trees are planted per hectare. In higher rainfall areas planting densities of 3 m x 3 m (1100 trees/ha) and 4 m x 2.5 m (1000 trees/ha) are possible. This spacing encourages the trees to grow straight and to develop light branches. Vehicles are able to drive between the planting lines, making access for pruning and weed control works easy. For site preparation information, refer to Agriculture Note AG0771: Eucalypt plantation establishment – site preparation.

Fencing off trees from stock is required, as the juvenile foliage has a sweet taste due to the presence of a chemical compound called glucoside. Guarding the trees will help to deter browsing and should be carried out with other pest control techniques such as shooting and baiting. Guarding also offers some protection from frost damage.

Managing for firewood production

Thinning operations can produce residues for sale as firewood, or a stand can be solely managed for firewood. Firewood woodlots do not require any pruning or thinning works, and are usually harvested after about 15 years. See Agriculture Note AG0814: Managing coppice in eucalypt plantations. After harvest, firewood stands may not have to be replanted. This species has an excellent ability to re-sprout (coppice) from the stump. The new shoots can be managed as new crop trees.

To encourage strong coppice production, stumps should be cut 5 to 10 cm from the ground (high stumps produce weak coppice which is unstable in strong winds) at a time of year when frost and moisture-stress is unlikely to affect the new shoots.

When the trees are 3-5 years old, two of the strongest and straightest shoots should be selected and the rest knocked off. These retained shoots can be grown on until they are 10-30 years old, when they can be cut for posts, poles or firewood.

If the shoots are being managed for sawlog production, the best stem on each stump should eventually be selected, and the others removed, to maximise growth on one stem only. Trees can be coppiced in this way many times.

Sugar gum leaves should not be used for fodder while leaves are wilting following coppicing or in their juvenile state, as cyanide is produced that is harmful to all types of grazing stock. Grazing stock on dead and adult leaves is acceptable.

Pests and disease risks

Extreme environmental conditions or competition among trees can cause trees to become weakened and therefore more susceptible to insect attack. Sugar gum is resilient to most insect pests, but is susceptible to small outbreaks of leaf feeders, or sap sucking insects and borers. These outbreaks are usually not damaging enough to require chemical spraying.

Insect attack needs to be carefully monitored in sugar gum coppice, as bark beetles can ringbark new stems, causing them to die. For more information on treatment of affected stands see Agriculture Notes:

• AG0801: Insect Pests-Control, and
• AG0799: Insect pests of young eucalypt plantations.
  

Uses

Sugar gum is fast becoming a recognised species for the high-quality sawn timber it produces. There are currently three furniture manufacturers in Victoria purchasing logs from existing plantations, paying around $100/m3 at the mill door. At least a further 6 small-scale millers and portable millers are sawing sugar gum in western Victoria alone.

Sugar gum has many on-farm uses including posts and poles, stock yard railings and due to its low electrical conductivity, electric fence droppers as well as the more conventional droppers. Sugar gum has wider uses for railway sleepers, telephone poles, house stumps, bearers under tile roofs, decking, and building frames.

Some of these more valuable products can be produced, provided that the timber is handled properly after felling (ie. stacked, covered and dried). The wood can be milled and used as excellent high-strength appearance-grade timber for indoor and outdoor furniture and flooring. The sawn timber can replace messmate for green framing timber but unless used immediately after milling requires drilling before nailing, because of its hardness and tendency to split.

When woodcutters come on to farms and harvest standing sugar gums for firewood, farmers currently receive $2-10/m3. Dry firewood fetches between $60-110/m3 depending on point of sale, while A-grade green sawn timber may sell for around $500/m3. Kiln-dried sawn timber may sell for $1800-2700/m3. Processing the timber on the farm may increase returns to the grower. There are several portable mills available on the market, or you might be able to employ a local miller to cut it for you. Remember to handle the timber properly after milling.

Sugar gum is popular with apiarists (bee keepers) and has also been used for charcoal production.

Wood quality

Sugar gum has a pale yellow-brown heartwood that when polished becomes a light ginger-brown color. The sawn timber is well featured, caused by an interlocking grain. The timber has a uniform texture and is extremely hard. The wood of sugar gum is prone to termite attack and the sapwood may be susceptible to the lyctus borer, although the heartwood displays good resistance. The heartwood is also resistant to chemical impregnation. Sugar gum timber is difficult to work, but polishes to an elegant finish. More research is still required into gluing techniques of sugar gum.

The green density is 1200 kg/m3 and air dried density (at 12% moisture content) is 1100 kg/m3. Sugar gum falls into CSIRO’s natural durability Class 1 rating (highly durable), and has an expected life both above and below the ground of at least 21 years.

CSIRO has recently completed a study on the potential of low rainfall species (including sugar gum) to be grown for high quality solid wood products. The study found that green and dead knots were a major factor preventing boards from receiving a high appearance-grade. Regular pruning to control the size of the knotty core is likely to improve product quality by producing clearwood.

Drying characteristics

Sugar gum is slow to dry and care must be taken to avoid significant degrade in the drying process. Drying degrade such as surface checking can be avoided by sealing the log ends with paint and ensuring appropriate drying facilities, (ie. timber is evenly stacked to allow free circulation around each piece). Air-drying will lower the moisture content of the timber to 15-17%. For high value products such as flooring and fine furniture, kiln drying is required to lower the moisture content to 8-12%. Slow kiln-drying schedules are required to minimise surface checking. Shrinkage caused by the loss of moisture from timber drying varies within species, but generally for sugar gum it is low, at 6% around the board and 10.5% along the board.

Acknowledgments

Melanie Waters, Rick Webb & Belinda Measki, Hamilton

The information in this Note, was obtained from a number of sources (listed below). For further information, refer to the following references:

• Bird, P.R. (2000) Farm Forestry in Southern Australia, a manual with a focus on clearwood production of specialty timber. Agriculture Victoria, Hamilton, Victoria.
• Boland, D.J., Brooker, M.I.H., Chippendale, G.M., Hall, N., Hyland, B.P.M., Johnston, R.D., Kleinig, D.A. and Turner, J.D. (1994) Forest Trees of Australia. CSIRO Publications, East Melbourne, Victoria.
• Doran, J. (2000) Electronic forestry compendium full data sheet for Eucalyptus cladocalyx CSIRO Forestry and Forest Products.
• Hamilton, L. (2000) Commercial sugar gum plantations for firewood and sawlog production. Department of Natural Resources and Environment, Victoria.
• Stewart, M., Binns, R. and Hamilton, B. (1999) Report on growth measurements of several tree species in the Wimmera region. Institute of Land and Food Resources, University of Melbourne.
• Washusen, R., Waugh, G. and Hudsen, I. (1999) Wood products from low rainfall farm forestry- final report Forest and Wood Products Research and Development Corporation.