The results of an important $1 million, five-year study that investigated approaches to help maximise the productivity and profitability of African mahogany plantations in northern Australia have been released. Source: Timberbiz
A report detailing the study findings aims to provide guidance for industry on the impact of climate and various silvicultural management practices including fertilisation, thinning, and pruning on plantation productivity. The study also examined processing options for the timber produced.
The project commenced in 2016 and was funded in partnership between African Mahogany Australia and Northern Tropical Timbers, with matching funding provided by the Australian Government, through FWPA.
The research was undertaken by a team including McGrath Forestry Services principal John McGrath; African Mahogany Australia CEO Frank Miller and Operations Manager Chris Oliver; Northern Territory Department of Industry, Tourism and Trade Director Plant Industries
Mila Bristow and Horticulture Extension Officer Dallas Anson, and Queensland Department of Agriculture, Fisheries and Forestry Research Facility and Project Manager Robbie McGavin.
The researchers observed a range of sites identified in the Douglas-Daly region of the Northern Territory. They set out to identify relationships between tree growth, thinning, and the nutrient status of soil and foliage, by using existing data and conducting a series of new trials that manipulated plantation nutrition and planting density.
“The ultimate goal of the project was to identify the optimum thinning and fertilisation regimes for productivity across existing African mahogany sites, with the intention to collect knowledge that would be transferable to a wide range of sites across northern Australia,” said Jodie Mason, FWPA’s Forest Research Manager.
Impact of water availability
A key finding was the strong relationship between water availability and productivity, with water constraints in the Northern Territory’s seasonally dry, monsoonal climate found to be the main barrier to productivity.
During the wet season, growth was rapid but ceased when pre-dawn leaf water potential declined to a relatively modest –0.7 megapascals (MPa). In drier months, trees had the capacity to mitigate the impacts of modest water stress by maintaining their minimum daily water potential, but under more severe and extended drought conditions they were unable to avoid permanent damage.
Fertiliser — nitrogen and phosphorus are the key
During fertiliser trials, growth improved where nitrogen and phosphorus were added to soil. Low fertility sites enjoyed double the usual growth rates, while additional growth of between 30% and 50% was observed at other sites. In contrast, sites with better agricultural fertiliser history showed limited or no response to the fertiliser.
Water availability was also found to have a strong influence on the response to fertiliser. In drier years, fertiliser response was limited, and higher leaf area created by fertilisation under wetter conditions was found to ultimately predispose plantations to severe water stress and drought-related mortality.
Across the board, response to fertiliser appeared to last for at least four growing seasons, with the impacts increasing over the first four years after fertilisation. One of the key recommendations made in the report therefore is the application of fertiliser at four-to-five-year intervals.
The optimum plantation density
In thinning trials, plantation density was found to influence productivity, with total production volume seen to rise by 10 to 20 per cent where density was increased from 300 stems per hectare (sph) to 600 sph. Conversely, further density increases to 800 sph led to a reduction in growth and an increase in mortality during dry years, due to intensified water stress.
Based on these findings, the researchers recommend African mahogany plantations should be maintained at or below 400 sph to optimise growth and reduce the likelihood of drought-related mortality.
Processing options
Because the African mahogany plantations in the Douglas-Daly region were established to provide material for the high-quality furniture market, it is critical they provide clear stems. However, stem shape also impacts recovery of usable wood. So, the team looked to determine the impact of silvicultural inputs on processing performance (for example, sawing or veneer production).
The effect of log geometry – including diameter, taper (diameter measured vertically) and sweep (curvature of the log) – on the performance of rotary veneer production was modelled against the current African mahogany resource, and the log geometries expected as the resource matures.
The main takeaway from these simulations was that log geometry substantially affected the performance of rotary veneer manufacture, and consequently log value was reduced due to in-creasing levels of taper, sweep and ovality. Analysis also found the financial viability of veneering short logs of up to 1.3 metres was uncertain, with researchers suggesting 2.6 metre logs should be prioritised instead.
The potential recovery of veneer and sawn wood was assessed. The recovery and quality of veneer was found to increase with age, with more than 30% of veneers achieving ‘C’ grade or better from the oldest trees. Veneer defects found to have the greatest impact on recovery included compression, surface roughness and grain breakout.
Meanwhile, sawn board recovery ranged from 35% to 49% of log volume, with boards from all sites demonstrating high levels of distortion. It is suspected the mechanism influencing board distortion and compression in veneers may be one and the same, but the cause is not yet understood. The team suggests further studies to inform appropriate management strategies to help overcome these issues in the future.
Market scoping stud
A market scoping study was added to the project during its final year to assist current and future African mahogany growers in fully understanding the commercialisation potential of the resource, both in Australia and the wider Asia–Pacific region. The outcomes of this study will support planning for value optimisation of current and future resources.
The core recommendation from the market scoping study was that, based on current market conditions, sawmilling, veneer, and plywood processing all look to be possible, subject to further research and considerations.
“I would like to thank and acknowledge the entire research team for lending their considerable talents to this research projects over an extended period of time,” said John McGrath, Principal at McGrath Forestry Services.
“Conducting a multi-year, field-based project in a remote area of northern Australia required considerable effort from all involved, the results of which will continue to be of benefit to African Mahogany growers in northern Australia for many years to come,” McGrath said.
