Optimal Rotation Age for Transmission Pole and Plywood Production in Ghana

14th April 2023

Gerald Taziwa recently successfully defended his MSc thesis in Forestry at the University of Stellenbosch, which was supervised by Margules Groome’s consultants Cori Ham and Dr Jonathan Dash. Gerald’s thesis explored the topic of calibrating rotation age for Eucalyptus pellita (E. pellita) and Eucalyptus grandis x urophylla (E. GU) to optimise economic returns for transmission pole and plywood production from greenfield plantations in Ghana.

Miro Forestry Limited (Miro) has established large-scale plantations of E. pellita and E. GU in Ghana and Sierra Leone, opening up significant new markets and investment opportunities in West Africa for the first time. Despite the early promise of these plantations, optimum silvicultural regimes are yet to be defined since the company is in its first full rotation. Working closely with Miro, Gerald’s research investigated the optimal rotation age for poles and plywood production in Ghana. A combination of market information, processing technology, silviculture regimes, and available growth and yield models were used to determine a suitable rotation age along with appropriate silvicultural treatments.

A market analysis for pole size distribution and a plymill experimental study for peeler log processing were the benchmarks used to establish the optimum pole and peeler log specifications to satisfy market demand and maximise peeler production based on Miro’s mill configuration. A biometric modelling approach was then used to calculate the tree sizes from which suitable log production could be ensured. Once favourable tree characteristics were ascertained, an investigation into the best regime to follow for pole and plywood ensued. This offered insight into suitable silviculture regimes for pole and plywood production in Miro’s plantations in Ghana.

Thinning scenarios were investigated within a forest growth simulator to better understand the stand response to different management interventions. Initial stocking, thinning intensity, and frequency of thinning were modified to test their impact on the growth dynamics of stand trees. Optimal rotation age for each species was defined by tree growth rates and the time taken to reach the target sizes for optimum pole or peeler log production.

The results from this study showed that the optimum pole tree should have a diameter at breast height (DBH) of 23 cm and a height of 23.3 m. The optimum plywood tree should have a DBH of 31 cm and a height of 30.8 m. The simulation results showed that a two-thinning regime with a final stocking of 600 stems per hectare gave promising results and that E. GU would achieve diameters of 23 cm for poles after 8.5 years following this regime. E. pellita did not achieve these dimensions for poles even over a much longer rotation.

Gerald also found that both species would not achieve the dimensions for optimum plywood production within the range of conditions tested. However, a detailed mill study found that the plymill recovery for smaller diameter class logs (15-20 cm) was not significantly different from larger logs (20-30 cm), meaning that smaller trees could be processed through Miro’s peeler mill without a significant loss in production. It was found that smaller trees (DBH = 21 cm, height = 21.5 m) were optimal for producing smaller class peeler logs and that using a two-thinning regime could optimise production to six years for E. GU and 11 years for E. pellita.

Gerald also calculated the biological and economic rotation ages for both species. It was found that E. GU had a biological and economic rotation of six and seven years respectively. The slower-growing E. pellita had a biological rotation of 7.3 years, and an economic rotation of seven years. This finding clearly illustrated that when the intended market demands very specific product dimensions, the use of biological and economic rotation ages must be used cautiously as the technical rotation age can be quite different once the market requirements are considered.

Congratulations to Gerald Taziwa from the team at Margules Groome on a job well done and for making a valuable contribution to the development of plantation forestry in West Africa.