Spotted gum

08.08.17

Spotted gum is a tall tree with a straight trunk, growing up to 45 metres in height, or sometimes taller. It has smooth powdery white, grey or pink bark, which sheds in patches, or spots, giving the tree its name. The common name spotted gum actually refers to four species of Corymbia that grow along the east coast of Australia from Victoria, through New South Wales and into Queensland.

Overview

Commercial farm trees, like Spotted gum, have the potential to offer a range of benefits to farmers and land managers by increasing Australia’s long-term timber supply while contributing social, economic and environmental benefits to regional areas.

Trees are often planted to provide windbreaks and shelterbelts for crops and livestock, to manage the watertable or to protect topsoil from erosion. However, many landholders, using existing infrastructure and integrating tree management practices into existing farm operations, may be able to earn an alternative income from planting farm trees that earn a commercial return. The risk inherent in all commercial tree operations is the long period between establishment and harvest. This can range from 30 to 50 years with no financial return in the interim.

Spotted gum has a wide range of uses including in heavy construction and engineering, but its attractive light to reddish brown timber means it is in demand for polished timber floors and cabinetry in residential and commercial buildings. It is also harvested for export woodchips and used in paper manufacturing.

Spotted gum is commercially harvested in New South Wales and Queensland, from native forests and plantations. It is gaining popularity as a plantation timber due to its early growth, narrow crowns (the upper part of the tree including branches and leaves), relatively good form and its good quality, general use timber. It is also considered a lower risk commercial tree, particularly in a farm forestry environment, where its thick bark means it can coexist with grazing animals and it is relatively tolerant to drought and fire events.

Facts and figures

  • The name spotted gum refers to four species of Corymbia that grow along the east coast of Australia
  • The bulk of spotted gum is harvested in Queensland
  • Spotted gum has a range of uses in heavy engineering, construction and outdoors
  • It is an attractive timber used in flooring and joinery
  • Plantation timbers can be harvested for export woodchips or paper production at around 12 years growth
  • Spotted gum is gaining popularity as a farm tree
  • It is considered a lower risk plantation tree due to its relative drought and fire tolerance
  • It is relatively pest and disease resistant
  • Genetic improvement is not advanced, therefore planting rates will need to be high to achieve quality trees

Production status

Sawn timber from spotted gum species is generally and widely available through timber merchants with commercial harvesting taking place in Queensland and New South Wales. In fact, spotted gum is the highest volume native hardwood harvested in Queensland.

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Map of current and potential growing regions

Uses

Spotted gum is used in wharf and bridge construction, railway sleepers, poles and as shoring timbers (providing support to structures).

Unseasoned, it is used in general house framing, decking, fencing, landscaping and retaining walls. Seasoned spotted gum is used for cladding, lining and joinery, quality indoor and outdoor furniture and parquetry. It is becoming a popular choice for internal and external flooring. Spotted gum is the preferred Australian species for use in tool handles that are subjected to high impact forces, such as axe handles.

Spotted gum in plantations can be harvested at 12 years for export woodchips and for use in paper manufacturing.

The species is often used for planting in parks and as a street tree.

Production Requirements

Growing regions

Four species of spotted gum grow naturally along the east coast of Australia.

  • Corymbia citriodora subsp. variegata occurs mainly in the coastal areas of northern New South Wales and southern Queensland, but also in western areas of southern Queensland.
  • C. citriodora subsp. citriodora grows from the mid-north coast of New South Wales to the Windsor Tableland, north Queensland.
  • C. maculata occurs from Bega in New South Wales to the mid-north coast of New South Wales, with some occurrence in eastern Victoria.
  • C. henryi grows in northern New South Wales.

Spotted gum is commercially harvested in New South Wales and Queensland, from native forests and plantations. It is thought that future supplies of plantation-grown spotted gum will be available from most regions in central and southern Queensland with suitable soils and annual rainfall over 600mm.

The species Corymbia maculata is becoming a popular farm tree in South Australia.

Soil type

Spotted gum grows on a wide range of soils types, but commonly in areas with poorer clay subsoils. The best growth has been indentified on moist, well-drained soils of moderately heavy texture with clay sub-soils, derived from shales, slates and granites.

Climate

Spotted gums grow in regions with an annual average rainfall of between 650–1300mm. However, some have been found growing, albeit slowly, in areas with lower rainfall, indicating a relative degree of drought tolerance. For optimum growth, spotted gums prefer an even rainfall distribution throughout the year or a summer maximum, but the species can be found growing well in areas of winter rainfall maximum.

The main limiting factor in terms of climate is the low frost tolerance of spotted gums. While the tree can tolerate at most a mild frost, a mild frost can still devastate young trees, therefore planting on low-lying, frost-prone sites is not recommended.

Varieties

Tree improvement of spotted gum through selection for regional variation for plantation timber production is only relatively new in Australia. Interested growers should contact the state department of primary industries or forestry to discuss improved seed selection and availability for their region.

Planting and crop management

Commercial trees can be integrated into existing agricultural systems to complement farm output or can be a separate use of available land, managed in plantations.

When selecting the planting site, take into account the accessibility of the trees for maintenance and harvesting. Being on flat, accessible land, with good access roads and room to manoeuvre heavy equipment will make management easier and keep costs down over time.

Once the site has been selected, the soil should be deep ripped and cultivated along rows spaced between 3-5–5m apart. The soil should be kept weed free for at least two months prior to planting in order to maintain subsoil moisture. Planting is recommended when adequate soil moisture has been achieved, for example after a rainfall event.

Because of the genetic variability of seedlings, initial stocking rates in spotted gum plantations will need to be fairly high (800–1000 stems/hectare) to ensure sufficient quality trees remain after thinning. These high stocking rates will eventually result in stagnant tree growth due to a lack of space, and like most species of Eucalypt and Corymbia, spotted gum tends to be intolerant of competition and responds well to thinning.

For high quality sawlogs, a long, straight, single stem is the objective and while spotted gums are considered to be self-pruning, occasionally there will be branches that need to be removed by pruning.

Spotted gums respond well to fertiliser application at establishment and after pruning, with best growth rates usually achieved with an application of nitrogen, sometimes phosphorus and possibly micronutrients.

Weeds, pests, and diseases

Weed control in plantations is particularly important before planting and for at least the first two years of growth, or until the trees reach 4m in height. Competition by weeds for sunlight, nutrients and moisture can reduce growth rates in young trees.

Spotted gum seedlings are susceptible to damage by hares and rabbits, and should be protected throughout their establishment phases.

Seedlings are generally insect resistant but occasionally grasshopper outbreaks can cause damage to trees in their first year. Outbreaks of mites and beetles can infest plantations following drought or dry periods, slowing growth in the following year. Termite activity can also be a defect in spotted gum; trees should be monitored for signs such as termite bumps with breather holes.

Disease outbreaks are rare, with good resistance to phytophthora and most root rot fungi, but high humidity or summer rainfall can lead to outbreaks of Quambalaria shoot blight.

Infrastructure Requirements

Land is the main infrastructure requirement for commercial tree growing.

If planting trees on a small scale and integrating them into the agricultural enterprise as windbreaks and shelterbelts, then manual maintenance, including pruning and thinning, can usually be managed with a chainsaw and farm equipment.

If planting a commercial plantation of high value sawlogs, then tree farm-scale, tractor-based forestry equipment may be necessary, including logging winches and chains. If the plantation is large it may be economical to purchase specialised harvesting equipment or modify existing farm equipment to undertake harvest. Contractors with specialised equipment and machinery may also be considered.

Harvesting & Processing

Initial site selection for commercial trees will have an influence on the harvest method used. Well-spaced trees near access roads will enable easier and cheaper harvesting.

Harvesting is generally undertaken in two ways. If timber production is the sole objective of the plantings, then clear felling is the logical method. If the trees form part of a shelterbelt, then harvesting of small volumes over a longer period of time is the more practical solution.

The timber is usually sold in two ways: either as ‘standing’ where a professional contractor pays a ‘stumpage rate’ to harvest and transport the logs, or ‘mill door price’ where logs are delivered to the timber mill having been harvested and transported privately or using contractors.

Unless the plantation is large and consists of high value logs, it will not be economical to purchase specialised harvesting equipment or modify existing farm equipment to undertake harvest. If the trees are large, pruned and well-spaced, specialised equipment may not be required at all. A trained operator using a chainsaw, farm tractor, loader and log trailer may be able to undertake the job in a cost-competitive way.

Markets & Marketing

It is important to consider current and/or future markets before choosing which commercial tree species to grow. However, with commercial trees taking around 30 years to provide a return on investment (in most cases) it is difficult to accurately predict future market opportunities at establishment.

Due to the lower wood density of younger trees, spotted gum grown in plantations may be suitable for export woodchips or paper manufacturing between 7-12 years of age. High quality sawlogs will take longer to develop, but prices will be higher.

Distance from markets will affect the profitability of a commercial timber enterprise, generally speaking, when the plantation is more than 100km from the processing centre (or road conditions are difficult), transport costs are higher. The closer the planting is to the processing centre, the greater the return for any given product, particularly if providing a high volume of a lower value timber, such as pulpwood and woodchips.

Planting and managing for high quality sawlogs can counter some of the market uncertainty and absorb transport costs, as it is often easier to find markets for high value timber, even with longer transport distances involved. High value timber requires ongoing maintenance of trees, ensuring they are managed for the specifications particular to high value markets, essentially: good form, diameter and limited defects.

Consideration of local markets for the timber provides options should future markets change as described in the Risk/challenges section. Local markets could include the building or construction industry in the region, farm use as fence posts, or domestic use as firewood.

Risks & Regulations

Risks/challenges

Compared with other plantation species, spotted gum is considered a relatively low risk option for investment due to its versatility, resilience and early harvest uses. However, uncertainty and risk are inherent in all commercial tree operations due to the long period of time between establishment and harvest which can be 30-50 years for some species.

Some of the risks include:

  • mills closing or relocating, leaving plantings at an uneconomical distance from processors
  • timber preferences or ‘fashions’ changing, resulting in decreased markets as certain colours or grades of timber fall out of favour with designers, builders and consumers
  • government policy changing, impacting on industry structure and profitability
  • alternative products developing, resulting in a falling demand for timber.

Alternative benefits from commercial tree plantings should be investigated to help reduce exposure to these risks. It is recommended that a short-, medium- and long-term return or benefit be identified when considering planting commercial trees. For example, a short-term benefit could relate to carbon sequestration or watertable abatement. A medium-term benefit may be derived from the trees providing a windbreak or shelterbelt for crops and livestock, and a long-term benefit might be income from harvested timber.

To further manage risk, it is recommended that tree selection take into account the potential of local use for the timber. A timber versatile enough to be processed and used locally will limit some of the uncertainty around future markets. For example, local markets could include the local building and construction industry, farm uses such as fence posts, and domestic uses such as firewood.

Regulatory considerations

If seeking to establish a commercial plantation (the definition differs by state, but is generally defined at around 30 or 40 hectares) liaison with the relevant state government agency will be necessary. Regulatory systems vary across states, with some requiring plantations to be authorised and others requiring plantations to meet legislative requirements and/or a management Code of Practice.

Publications

Publications/information

Natural Durability of Five Eucalypt Species Suitable for Low Rainfall Area: Sugar gum, spotted gum, red ironbark, yellow gum and swamp yate RIRDC Research Paper (2008)

Natural Durability of Six Eucalypt Species from Low Rainfall Farm Forestry RIRDC Research Paper (2009)

Spotted gum (plantations) Queensland Department of Agriculture, Fisheries and Forestry

Other resources

Private Forestry Service Queensland

Forestry NSW

Private Forests Tasmania

CSIRO’s Australian Tree Seed Centre – a national collection and research centre with seed available for purchase

Farm Forest Line – Free information service for on farm forestry practice

Image Gallery

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Mature forest of Spotted Gum

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Spotted gum plantation with grazing animals

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Taking nectar measurement in spotted gum

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Spotted gum flowers

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Spotted gum timber