Executive Summary
What the report is about
This study evaluated wood quality from older age stands of African mahogany Khaya senegalensis grown in Northern Territory, Australia. Potential users of the timber were also surveyed to determine the wood qualities most valued. Based on log qualities, wood properties and potential uses, the report provides early advice on drying and processing needs, and those traits to select for in managing newer plantations. It also evaluates the quality of the timber for high value uses. This information will assist potential investors, producers and processors in growing and producing high value timber, to underpin a saw log industry in the dry tropics of northern Australia. The opportunity to grow high value saw logs in Australia’s north provides alternatives to landholders and indigenous communities with large tracts of arable land who want to grow trees for commercial timber products.

Who is the report targeted at?
The results gained during this study provide potential investors, producers and processors with concise and accurate information on plantation-grown K. senegalensis log qualities and wood properties, and its potential to produce high value timber in northern Australia. The opportunity to grow sawlogs in northern Australia will provide alternatives to landholders and indigenous communities wanting to grow trees for a commercial return. The research will also inform ongoing tree breeding work on this species being undertaken collaboratively in northern Australia.

Background
There is a scarcity of information on the wood properties and utilisation potential of plantation grown African mahogany (Khaya senegalensis). As there is renewed interest in K. senegalensis as a plantation candidate for the dry tropics of Australia it has become important to learn more about the species’ potential to produce high-value products.

During 2003, the Northern Territory’s Department of Business Industry Resource Development (DBIRD), in partnership with the Queensland Department of Primary Industries and Fisheries (DPI&F), initiated a wood processing and utilisation study to characterise Australian plantation-grown K. senegalensis. These researchers have also established a genetic improvement program, with the aim of producing superior genetic stock to underpin the establishment of new plantations – this breeding program builds from a provenance trial planted by CSIRO near Darwin, Northern Territory in 1972. Thirty-eight of the best trees (32-year-old) from the CSIRO provenance trial, harvested from two sites near Darwin, Gunn Point and Howard Springs, were selected for the wood evaluation and use study. This information is intended to inform wood processors and users, as well as the genetic improvement program. Clonal material was taken from each of these trees in order to preserve the available pool of genetic material.

Aims/Objectives
The broad objectives of this study were to assess processing, seasoning, wood quality and utilisation characteristics of the sampled trees, and use this information to characterise the ‘potential’ properties of future K. senegalensis plantations and to make direct inferences about the individual trees that were selected for the genetic improvement program.

The specific objectives were:

1. Gain a better understanding of post harvest handling and treatment of African mahogany to optimise timber qualities and utilisation potential
2. Obtain fundamental wood properties, sawn recovery rates and arrange industry assessments to determine suitable applications to enhance marketing potential of the timber
3. To provide investors and growers with more information about rotation lengths best suited to the species/environment interaction and the wood qualities at various stages and /or log classes
4. Provide preliminary information on the drying behaviour of plantation grown African mahogany. This preliminary information will form the basis of a recommended drying schedule.

Thirty eight trees were selected for assessment and harvesting: 25 from Gunn Point and 13 from Howard Springs. Tree selection was based on diameter at breast height (DBH), clear bole length and form, as well as a general visual assessment of tree vigour (health). Those trees with obvious defect or sweep were excluded.

Log measurements, assessment of processing characteristics and wood properties were undertaken on the 38 trees. An industry assessment to determine the use of African mahogany, quality and perceived benefits of using Australian grown African mahogony was also undertaken.

Results/Key findings
Due to a lack of better-managed older-age stands, the trees were sourced from a stand planted in 1972 with little subsequent silvicultural management. These trees provide useful information on wood qualities, that can be improved upon in new and future plantations. Merchantable log volume harvested from the two sites was 17 m3, which equated to an average merchantable tree volume of 0.38 m3. Poor log form, typical from plantations planted with unimproved planting stock and little silvicultural management, typified the logs. The logs exhibited low end-splitting, but did have moderate pith eccentricity. On average, 50% of log volume was under heartwood, a relatively low proportion for plantation grown hardwoods.

A number of the logs were very unstable during sawing causing movement of the flitches and cant.

The logs were sawn into a wide variety of dimensioned boards, suitable for furniture type applications.

In total, a green-off-saw (GOS) recovery of 39% was achieved. The boards were air-dried over summer (Brisbane, Queensland) for approximately 2½ months from green to 12%. The timber was dried very conservatively to avoid degrade. Seasoning trials on small batches kiln dried timber indicated that the timber was relatively easy to dry over a short time-frame with very little drying degrade. The boards were subsequently dressed and graded according to appearance product standards (AS2796). Grade recoveries (based on total log volume) ranged from 8.1% for the highest grade (Select) in the most demanding product category (Joinery), to 29% for the lowest grade (High Feature) in the least demanding product category (Flooring). The main causes of downgrade were: distortion, knot related defects, wane; and stain. Mean distortion values for spring, bow and twist were 18, 10 and 3 mm, respectively.

Australian grown plantation K. senegalensis produces an aesthetically pleasing timber with marketable colour and figure attributes. The wood appears to be suitable for clear-finished or stained high-value applications, such as contemporary and reproduction furniture, cabinetry, windows, doors, and interior joinery. The timber could be described as having a heartwood colour that is pink to red when freshly sawn which generally darkens to pinkish-brown to red-brown after prolonged exposure. The sapwwod is distinctively paler. The texture was medium to course and the grain variable from straight to interlocking. Average basic density was 637 kg/m3. Unit shrinkage was 0.26% radially and 0.28% tangentially. The timber proved easy to glue with no glue bond failures being recorded. Average Modulus of Elasticity (MoE) and Modulus of Rupture (MoR) was 8.3 Gpa and 81.7 Mpa, respectively.

The timber proved to be ‘moderately hard’, in relation to resistance to indentation,with an average Janka hardness rating of 6.4 kN for seasoned timber. For Joint Strength Group the seasoned timber achieved Strength Group JD3 based on nail withdrawal and JD2 based on screw withdrawal. The timber proved naturally resistant to termite attack and highly resistant to decay when trialed in accelerated decay bioassays.

Based on the results of the industry assessment, the plantation trees proved to produce a good quality attractive timber that was judged equal to or better than the native African mahogany currently being imported into Australia (species unspecified). The timber was highly regarded by the various industry assessors who believed that there would be good prospects for the timber on the domestic market in the future, and could conservatively be retailed for between $3000 and $5500 per m3 for dried, dressed Medium feature to Select grade timber.

Implications for relevant stakeholders
Based on the findings of this study, Australian plantation grown K. senegalensis has proven to be a prime candidate species for plantation in the ‘dry’ northern tropics of Australia. The lack of stand management and the flow on effect to log quality was the main negative factor found during the study.

However, a K. senegalensis plantation planted with improved seed stock and managed according to best practice principles would significantly improve on the results published in this study.