The influence of species, tree improvement and cultural practices on rotation-end fibre production of Eucalyptus pulpwood plantations in South Africa

To remain competitive on a global scale it is necessary for forestry companies to increase the yield per hectare and work towards reducing the unit cost of timber. A three-trial series with a 2⁴ factorial design and two replications at each site within the temperate areas of KwaZulu-Natal in South Africa has reached rotation age (10 years). The four main treatments, each tested at a superior and inferior level, were species selection, genetic improvement, silvicultural intensity and level of site capture (planting density). The aim of this study was to investigate the effects of and interactions between the four treatments on fibre production over a rotation across a range of sites. The growth response associated with three of the four main treatments was maintained to rotation-age, although the relative gain from each decreased over time (from an additional 60.3% in fibre production per hectare at age 2.5 years to 27.1% at rotation age). The relative influence of species selection increased over time, whilst the relative influence of silvicultural intensity decreased slightly, but non-significantly. The selection of the best-suited species for a specific site was the most important to improve fibre production per hectare at rotation end by 13%. There was, in general, an absence of significant interactions between the four treatments. Thus, all four treatments must be optimised to achieve maximum productivity. After selecting the most appropriate genotype, further yield improvement is possible through genetic improvement (4%; not statistically significant), planting trees at the optimum density (5%; significant at the 10% level) and intensive silviculture (5%; significant at the 10% level). There was no indication that any of the treatments had a negative effect on wood quality. These results can be used to investigate the economic aspects to find the optimum between maximising productivity and minimising costs.